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The Dim Dolphin Controversy

The subject of this week’s special episode is a controversial article by Professor Paul Manger of Johannesburg’s University of the Witwatersrand concerning the dolphin brain and dolphin intelligence. This article has caused quite a stir in the scientific world and in the media, producing headlines like: “Scientist says dolphins are dimwits”. In an effort to better understand the science behind this controversial viewpoint, The Dolphin Pod interviewed Prof. Manger about his article. In addition, we interviewed Dr. Lori Marino, Senior Lecturer in Neuroscience and Behavioral Biology at Emory University in Atlanta Georgia. A full transcript of the interviews with Prof. Manger and Dr. Marino can be found on The Dolphin Pod website: www.thedolphinpod.com

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 The Dolphin Pod has attempted to provide an unbiased view of this controversial subject by examining the evidence supporting and contradicting Prof. Manger’s hypothesis. This episode is longer than our usual podcasts (stretching to 26 minutes) and contains a fair helping of scientific jargon, which has been explained throughout. The Dolphin Pod wishes to thank Prof. Manger and Dr. Marino for participating in this discussion – they have both been very generous in taking the time to respond to our questions.

Note: to help keep things clear, you will hear a distinctive ‘bleep-bleep’ sound at the beginning and end of a quotation read from the written responses of Prof. Manger or Dr. Marino.

 

 (FOR A COMPLETE TRANSCRIPT OF THE INTERVIEW SCROLL DOWN TO THE END OF THE PODCAST TRANSCRIPT)

A recent scientific article claiming that dolphins are not as intelligent as previously thought has been seized upon by the media, and has gained much attention in recent weeks. News outlets like MSNBC, YahooNews, the Times of India, and Aljazeera have produced a series of headlines including:

Scientists say dolphins are dimwits
Ignorance is bliss for dolphins
Dumb Dolphins

And my personal favorite:

Dolphins are flippin' idiots

The author of the article in question, Prof. Paul Manger of Johannesburg's University of the Witwatersrand, has fanned the flames of this 'dim dolphin' controversy with his comments to the press. In a widely distributed interview with Reuters' reporter Ed Stoddard, Prof. Manger is quoted as stating:

"You put an animal in a box, even a lab rat or gerbil, and the first thing it wants to do is climb out of it. If you don't put a lid on top of the bowl a goldfish will eventually jump out to enlarge the environment it is living in. But a dolphin will never do that. In the marine parks, the dividers to keep the dolphins apart are only a foot or two above the water between the different pools."

Concerning the trouble that dolphins have with tuna nets, Prof. Manger stated: "If they were really intelligent they would just jump over the net because it doesn't come out of the water"

Both the article itself and these statements have drawn a storm of criticism from the scientific community, and created a buzz of commentary on the internet. One online editorial proclaims: 'Dolphins Dumb - Scientist is Dumber'. But, behind this controversy is a peer-reviewed scientific paper that has not been adequately subjected to detailed discussion in the media. The paper is titled "An examination of cetacean brain structure with a novel hypothesis correlating thermogenesis to the evolution of a big brain". This paper, authored by Prof. Manger appeared in the May, 2006, issue of Biological Reviews of the Cambridge Philosophical Society. Concerning his paper, Prof. Manger told The Dolphin Pod that 'most reporters have taken small sound bites and missed the point'.

In an effort to better understand the science behind this controversial viewpoint, The Dolphin Pod interviewed Prof. Manger about his article. In addition, we interviewed Dr. Lori Marino, Senior Lecturer in Neuroscience and Behavioral Biology at Emory University in Atlanta Georgia. Dr. Marino has published many scientific articles on the evolution of the dolphin brain and dolphin cognition.


If you are interested in reading the questions posed to Prof. Manger and Dr. Marino by The Dolphin Pod, together with their answers, a full transcript of these interviews can be found on The Dolphin Pod website: www.thedolphinpod.com

To get started, let's examine what Prof. Manger's article is all about:

Prof. Manger's novel hypothesis, which he introduces in his article, is that the size of the cetacean brain (that is, the brain of whales, dolphins and porpoises) evolved as a means of regulating temperature, and not for processing information. It is important to point out that this is a novel, controversial hypothesis - scientists generally assume that all animal brains have evolved first and foremost for information processing, and not heat production. But, Prof. Manger hypothesizes that the unique evolutionary history of cetaceans led to the development of a different kind of brain built for a different purpose. Prof. Manger presents many arguments to support his hypothesis, including the following main points:

1) Cetaceans evolved large brains designed for thermogenesis (that is, heat production) because of a decrease in ocean water temperature that coincides with the increase in brain size approximately 30 million years ago.

2) The structure of the cetacean brain appears to be designed for thermogenesis and not information processing, correlating with the well known sleep physiology of cetaceans.

3) The research results claiming that cetaceans (dolphins, in particular) are intelligent are based on unproven assumptions and, in actuality, cetaceans are not particularly intelligent, as intelligent behavior is the result of complex neural processing for which there is no evidence in the brains of cetaceans.

In terms of causing controversy in the scientific world by introducing a new hypothesis, Prof. Manger is in good company - in 1925, Raymond Dart, a former head of the Department of Anatomy (now the School of Anatomical Sciences) at the University of the Witwatersrand where Prof. Manger currently lectures, introduced a novel hypothesis claiming that modern humans evolved from an early African hominid that he termed Australopithecus africanus. At the time, this idea was heavily criticized by Dart's colleagues who would simply not accept that modern humans came from Africa. This hypothesis is of course widely accepted today. Will the same hold true for Prof. Manger's controversial ideas concerning the cetacean brain? Only time will tell. In the meantime, let's look in detail at each of the three arguments presented by Prof. Manger to see what evidence there is to support or contradict them.

First, let's discuss the hypothesis that cetaceans evolved large brains designed for heat production:

The early aquatic ancestors of cetaceans had relatively small brains. However, about 30 million years ago something remarkable happened: cetaceans started to evolve much larger brains, soon reaching the size we see in modern cetaceans. Prof. Manger argues that this sudden increase in brain size is related to a major cooling of the earth's oceans that took place at about the same time that we see large-brained cetaceans cropping up. The brains of cetaceans, argues Manger, likely grew larger in order for these animals to stay warm in the cooler waters. According to this hypothesis, cetacean brains have a large cerebral cortex with an abundance of heat generating cells that the brain uses to keep warm while awake and, more importantly, while asleep. For most mammals, when we go to sleep, our body temperature drops quite significantly - a little over 1 degree Celsius. Prof. Manger argues that this is something very dangerous for an aquatic mammal living in cold water. According to Prof. Manger, in water, a resting mammalian body loses heat at a rate 90 times faster than in air. The cetacean brain compensates for this heat loss by initiating a strange form of sleep that lacks REM (rapid eye movement), and where only one half of the brain sleeps at a time. This allows the cetacean brain to continue producing heat and keeps the body's muscles moving, preventing excessive heat loss when resting. This type of sleep will also increase production of a neuro-chemical called noradrenaline that will increase the metabolism (and thus, heat production) of the non-neural cells of the brain. Prof. Manger believes that it was the need for thermogenesis alone that triggered the evolution of the cetacean brain into its modern form and told The Dolphin Pod that "there is no need to posit at all a need for complex information processing as an evolutionary pressure leading to larger brain size in cetaceans."

Looking for evidence of a relationship between current oceanic water temperatures and the brains of living cetaceans, Prof. Manger compared high and low water temperatures from the ocean habitats for various cetacean species, as well as temperature range to cetacean body size, brain size, and EQ (encephalization quotient). EQ is measurement of expected brain size relative to body size. Prof. Manger found a significant correlation between EQ and temperature range, which suggested that the scaling of brain-to-body size found in cetaceans must somehow be linked to oceanic temperature; something you may expect to find if cetacean brains are indeed designed for producing heat.

Dr. Lori Marino, however, does not agree with this interpretation of the data. She points out that the link between cooler water temperatures and the emergence of large brains could just as easily have been connected with a change in the way cetaceans hunted their prey. Perhaps cooler water temperatures meant that there were fewer fish available, forcing cetaceans to evolve larger and smarter brains so as to better find food. Furthermore, she states: "although EQ increased dramatically in cetaceans around 30 million years ago, brain size did not increase all that much. The dramatic increase in EQ about 30 million years ago that Manger touts as support for his hypothesis is due to a significant but modest increase in average brain size from 749 g (archaoecetes) to 782 g (Oligocene odontocetes) and a much more dramatic decrease in average body size from 1,654 Kg to 207 kg. Manger's hypothesis relies upon a much larger increase in brain size than what actually occurred. By focusing on EQ, Manger is being misleading."

Dr. Marino's research into the evolution of cetacean brains offers a very different explanation as to the reason cetaceans (particularly dolphins) evolved large brains - an explanation that is commonly accepted in the scientific community. Dr. Marino says "My ideas about why cetaceans became so encephalized and intelligent focus on behavioral ecology, sociality, and communication. Many cetaceans show convergent capacities with some other mammal groups, such as primates and elephants". In other words, dolphins evolved large brains to meet the demands of their environment, and it was their environment that required them to produce more complicated and 'intelligent' behavior - primarily social behavior.

The two hypotheses offered by Dr. Marino and Prof. Manger offer alternative explanations as to why cetacean evolved large brains. I thought perhaps these two explanations could meet somewhere in the middle, and asked Prof. Manger if a brain that evolved for thermoregulation might, as a secondary effect, also be a brain better equipped for processing information, to which he answered: "This may be a possibility, but not in the case of the cetaceans."


Support for this statement can be found in Prof. Manger's second argument, which suggests that the structure of the cetacean brain appears to be designed for thermogenesis and not information processing. He cites more than 20 examples as to why the cetacean brain could not be the brain of an intelligent animal, including:

1. Poorly differentiated neuronal morphology (this means that there are not a lot of different kinds of cells in the dolphin brain. And, the cells that are there are not designed in a way to increase their accessibility to large amounts of information and thus complex processing)

2. Low number of neurons and cortical areas (Neurons are the kind of brain cells that are thought to carry out the bulk of information processing and organize themselves in areas of the cortex that carry out a specific range of functions)

3. Lack of a distinct pre-frontal cortex (the pre-frontal cortex is understood to be the part of the mammalian brain (in humans especially) that carries out the most complex information processing)

4. A large ratio of glial cells to neurons (more on this later)


In my email interviews, Dr. Marino makes some rather strong objections to these claims, which Prof. Manger counters with some equally tough rebuttals. It was, to say the least, a colorful exchange! You can read these remarks in their entirety on The Dolphin Pod website. To whet your appetite, here is a sample of one exchange concerning Prof. Manger's discussion of the pre-frontal cortex. Dr. Marino suggests that "there is no reason to expect that if the dolphin brain has an analogous region to the prefrontal cortex that it would be, in fact, in the front of the brain!" Prof. Manger states in response that "In every single study of the organization of the cerebral cortex of mammals, the prefrontal cortex has been located at the anterior pole of the cerebral hemisphere. All studies, including the thalamic connectivity studies, support the parcellation of the cerebral cortex I have made in the paper."

It should be apparent from these statements that something as seemingly straightforward as a description of the structural organization of the cetacean brain is anything but! And this debate, mind you, is not yet the most controversial in Prof. Manger's paper. Let's continue by expanding a bit on Prof. Manger's point concerning the high ratio of glial cells to neurons found in the dolphin brain.

If neurons are the kinds of brain cells that process information, then glial cells are traditionally considered cells that support the function of neurons by providing nutrition and structure to the brain. Prof. Manger cites in his paper the results of studies done by other scientists that demonstrate that cetaceans brains have a very high ratio of glial cells to neurons. He contends that this fact likely reveals that the cetacean brain is, in his words, 'not optimally wired, presumably impacting negatively on processing efficacy and power of the cetacean cerebral cortex". In other words, cetaceans simply do not have the right type of cell structures to do anything complex with their brains. According to Prof. Manger, it is this overabundance of glial cells that is helping the dolphin brain to generate heat, not process information.

Again, Dr. Marino criticizes this interpretation of glial cell function. She states the following: "One of Manger's most ironic flaws is his outdated view of the role of glial cells in the brain. The older view was that glia were there to provide nutritional and structural support for neurons and that they were basically inert units - "brain glue". However, recent research has shown that, in addition to these previously-known features, glial cells also play an important role in the modulation and coordination of neuronal activity in the brain. They do not just support but actually participate in information-processing and we have only begun to understand the complexity of the role glia may play."

Prof. Manger responds to this however with the following statement "I am well aware of the evidence suggesting the possible role of glia in neuronal activity, and would never deny this. I have forwarded the possibility that this large amount of glia is related to thermogenesis, others, such as Dr. Marino may prefer to think of these as related to intellectual processes, but given the number of coincidences I have gathered and outlined in my paper, it is difficult to support an alternative position."

Prof. Manger argues that it is the combination of all of these brain structure shortcomings that reveal that something is amiss with the cetacean brain. He told The Dolphin Pod: "there are just too many features at several structural levels of the dolphin brain that are not organized in a manner that indicates normal neural processing of information. There is too much comparative neuro-anatomical data that indicates that the brain of the cetacean is not built for complex information processing."

It is obvious by listening to the diametrically opposed points of view expressed by Prof. Manger and Dr. Marino that the relationship between brain structure and intelligence is highly controversial and by no means settled at this point. It is likely that many more decades of research will be required before there is any consensus!

Prof. Manger's final argument, and the one that has landed him in the most hot water with both the general public and other scientists, is the following: The research results claiming that cetaceans (dolphins, in particular) are intelligent are based on unproven assumptions and in actuality cetaceans are not particularly intelligent.

Prof. Manger's evidence from his paper that dolphin behavior studies do not convincingly argue that dolphins are intelligent contain two main arguments:

Argument 1) Dolphins don't have a language

He states in his paper: "The vocalisations of cetaceans have been the subject of speculation ever since Lilly (1962) entertained the possibility of a dolphin language, or 'dolphinese '. Despite extensive efforts to either impose a language upon cetaceans, or decipher their vocalisations, it is clear that no such language exists." I should point out here that this view is very much in step with the modern scientific view of the structure and function of the natural communication system of dolphins.


Argument 2) The claims that dolphins and primates have complex brains capable of similar cognitive and computational abilities are questionable.

Prof. Manger briefly discusses why three abilities cited by Dr. Marino in a 2002 article appearing in the journal Brain Behavior and Evolution -- social group behavior, language comprehension, and self-recognition ability -- do not provide convincing evidence of cognitive complexity in the dolphin brain.

These arguments consist of only a small portion of Prof. Manger's paper, and many scientists have protested that Prof. Manger has not adequately addressed the large body of behavioral evidence that exists stating that dolphins live cognitively complex lives. Dr. Louis Herman of the Kewalo Basin Marine Mammal Laboratory in Honolulu Hawaii, who has spent decades researching the cognitive abilities of dolphins has stated in a posting to the MARMAM email discussion list: "A problem is that Manger's review of the cognitive literature is limited and includes little of the large body of contemporary laboratory and field data showing advanced cognitive skills in dolphins."

In a book chapter titled 'Intelligence and rational behavior in the bottlenose dolphin', appearing in Rational Animals? published by Oxford Press in 2006, Dr. Herman describes his own work with dolphins as follows: "Behavioural studies we carried out during the years subsequent to 1980, as well as many completed earlier, have catalogued a wealth of data on dolphin cognitive traits and abilities that together provide compelling evidence for a level of intelligence commensurate with expectations based on the size, structure, and development of the brain."

I asked Prof. Manger if he believed there is any convincing behavioral evidence from experimental research with cetacean species (particularly dolphins), that might suggest that they indeed have cognitive abilities on par with or exceeding that of primates, citing Dr. Herman's work as an example. Prof. Manger answered very simply: "no."

Dr. Marino has voiced tough criticism of this stance: "Manger blatantly dismisses a whole generation of experimental and observational work on dolphin cognition, behavior, and sociality by hand-waving. This is simply not how science works. The data on dolphin cognitive complexity is there regardless of whether Manger wants it to be there or not. No amount of protesting or hand-waving will make it disappear. Manger neglects most of the large contemporary body of published, peer-reviewed data coming from Herman's lab demonstrating impressive and varied cognitive abilities in dolphins that is fully in keeping with expectations from the large size of the brain and its complex structure."

Concerning Prof. Manger's criticism of her 2002 paper, Dr. Marino states, "there are three indisputable findings that Manger simply cannot make go away without completely dismissing the validity of the entire peer-reviewed scientific process. First, bottlenose dolphins recognize themselves in mirrors. In 2001, Diana Reiss and I provided irrefutable evidence of this in our paper in PNAS (Proceedings of the National Academy of Sciences). Manger claims that we relied upon latency measures to infer cognitive activity. In that paper, we tested several hypothesis based on a number of parameters, including latency. But, the critical outcome measure was behavior at the mirror with respect to the mark. So he is simply incorrect. Second, years of research by Louis Herman and his colleagues have shown that bottlenose dolphins can comprehend syntactic sentences and abstract concepts. Short of reviewing the entire literature here I can only suggest that Manger go back and do that for himself and then articulate in a more cogent way just what it is about this research that he doesn't accept. Third, again, work by people like Richard Connor, Hal Whitehead, and many others have documented complex behaviors such as alloparenting, division of labor, coalition formation, cooperation, and intergenerational transmission of learned behaviors in cetaceans. If Manger wants to suggest that these characteristics are not complex, well, I would like to see what would pass as complex for him. In general, what Manger does is dismiss everything in the literature but provides no alternative criteria for what would be acceptable evidence of cognitive complexity".

The Dolphin Pod asked Prof. Manger what behaviors are evidence of intelligence in animal species, he stated: "The evolution of intelligence and comparing intelligence across species is a very difficult task. However, much depends on your definition of intelligence. Personally, I look at two factors:
How fast is information being processed?
How much information is being processed simultaneously?"

Prof. Manger provides more detail as to how to go about testing intelligence in animals, which you can read in the copy of my interview with him on The Dolphin Pod website. Prof. Manger believes that, given the structural problems with the dolphin brain that were described earlier, dolphins would be unlikely to perform well on the kinds of tasks he outlines. He states: "It is my feeling that due to the structure of the brain cetaceans would have great difficulty in achieving a task set up as described above, a task which both rats and goldfish can achieve."

But, the evidence presented by Prof. Manger that suggests that the structure of the cetacean brain is an important factor in determining a dolphin's level of intelligence on the whole does not sway many other scientists. Dr. Lance Barrett-Lennard of the Vancouver Aquarium Marine Science Centre provided a rather concise summary of this viewpoint in an interview with The Globe and Mail with the following statement: ''A dolphin could have a brain the size of a walnut and it wouldn't affect the observations that they live very complex and social lives.''

Concerning the observations of complex social behaviors, Prof. Manger states in his paper: "Other social behaviours listed by Marino (2002), such as alloparental care, cultural transmission, fission-fusion societies, interpod warfare, and intrapod alliances, all rely on behavioural observations and have not been sufficiently documented to allow independent scrutiny."

This view, however, is not widely accepted. Two decades of research on the behavior of wild bottlenose dolphins in Shark Bay, Australia, have produced a series of studies describing in great detail the social complexity of this population. In a book chapter titled 'Social cognition in the wild: Machiavellian dolphins?' in Rational Animals?, Dr. Richard Connor and Dr. Janet Mann review field studies of this dolphin population. They offer compelling evidence for the evolution of a cognitively complex dolphin brain able to cope with a complicated social environment. Connor and Mann state in this chapter: "our Shark Bay discoveries suggest strongly that, apart from primates, no animal group offers more potential for productive exploration of the relationship between brain size, cognitive skills, and behaviour than cetaceans."


I asked Prof. Manger if he envisions future papers where he may address these criticisms and offer an alternative interpretation of the current behavioral data in more detail. He responded by saying: "I am uncertain as to the extent and usefulness of future papers dealing with the reported behaviour in dolphins and whales, as without the appropriate brain, there is no intelligent behaviour."

This seems to be where the largest gap exists between Prof. Manger's arguments and the current scientific mainstream opinion. Whereas Prof. Manger states: "we cannot divorce structure from function in the way that many who study dolphin behaviour have been doing currently and for the last 50 years", Dr. Herman has written that 'it is behavior, not structure, that measures the intellectual dimensions and range of the species". Those studying dolphin behavior say that the proof of dolphin intelligence is in their behavior, but Prof. Manger believes that the proof of a lack of intelligence can be found in the structure of the dolphin brain. These two viewpoints do not meet in the middle.

I believe I am correct in stating that Prof. Manger's ideas are very much outside of the main-stream of thinking when it comes to our understanding of dolphins as intelligent animals. Prof. Manger's ideas have received rather strong criticism from both scientists and the general public. Concerning this flood of criticism, Prof. Manger told The Dolphin Pod: "It has also been said that I am funded/paid off by the tuna industry and the whaling countries, which is complete nonsense. I am a scientist who has examined the data and forwarded a hypothesis that was extensively peer-reviewed. There is no conspiracy theory here. … several people have suggested that I "hate" dolphins and condone a "mass slaughter" of the remaining marine wildlife. Again, this is nonsense. My science is directed at finding the truth of the matter. I study the brains of various animals in an attempt to understand the animals themselves in a more realistic manner. With a proper understanding, we can devise appropriate management strategies of animals that will, in the long-term, maintain and protect biodiversity against the human factor in the environment."

Prof. Manger's hypothesis and his interpretation of the behavioral data concerning dolphin intelligence are controversial, and have certainly shaken things up in the dolphin world. But, science is science, and if a hypothesis should fail, it should do so based on solid evidence against it, and not simply because it is an unpopular point of view. How the behavioral evidence that dolphins are cognitively complex animals is to be reconciled with Prof. Manger's observations concerning the oddly-structured dolphin brain, and how this all relates to dolphin intelligence is likely something that science will be busy working out for years to come.


Read a complete transcript of the interviews with Prof. Manger and Dr. Marino

 

Interviews with Prof. Paul Manger and Dr. Lori Marino: the dim dolphin controversy

Table of Contents (click on link to navigate)
Round 1 questions Prof. Manger
Round 1 questions Dr. Marino
Round 2 questions Prof. Manger
Round 2 questions Dr. Marino

Questions Round 1 Dr. Manger

The Dolphin Pod: What kind of research/evidence is necessary to lend support to the thermogenetic hypothesis of cetacean brain evolution?

Prof Manger: To support such a hypothesis, data on four fronts must be supported. First, there must be a point in the course of cetacean evolution when temperature would become a significant environmental selection pressure. Second, the fine structure of the extant cetacean brain must show features that are in accord with temperature being an influence. Third, the altered brain-body weight scaling of extant cetaceans must be shown to be influenced by temperature, and fourth, the actual size of the extant cetacean brain must be shown to be the result of water temperature acting as a selective pressure. Currently information relating thermogenesis to cetacean brain evolution is summarized in my paper and involves:

1) Unihemispheric slow wave sleep (USWS)
2) Excess glia and its function
3) The potential interaction of noradrenaline with glia and USWS
4) The startling historical coincidence of increased brain size with drops in water temperature in cetacean evolution
5) The strong relationship between EQ and water temperature range in extant cetaceans
6) The birth weight of young cetaceans

Additional information that is currently published includes the fact that the newborn cetacean swims continuously in early life (Lyamin's studied published in Nature).

Future experimentation could include studies that look at:
1) The effect of changes in water temperature on the physiological nature of sleep in the cetaceans. Would increased water temperature lead to more pronounced USWS? If water temperature were raised enough could clear signs of REM sleep be found? This must be done with physiological recording of the EEG of the two hemispheres of the sleeping cetaceans.
2) How does the locus coeruleus of the cetacean compare to other mammals? Does it have more neurons than would be expected for a mammal of its brain weight? If yes, then perhaps the cetacean brain is producing a great deal more noradrenaline that even currently anticipated.
3) What are the specific thermal mechanisms at a molecular level that are generating heat in the mammalian brain and what form do these take in the dolphin brain?

The Dolphin Pod: In this paper, as in your 2003 paper in J. Sleep Res., you hypothesize that unihemispheric slow-wave sleep and lack of REM sleep in cetaceans functions to compensate for heat loss to the water during sleep, and that a need for thermoregulation was the selective pressure leading to the evolution of USWS. Do any of the data you describe in this paper concerning variations in ocean temperatures for core habitats of and blubber thickness for various species (both cetaceans and other marine mammals) lend support to this hypothesis?

Prof Manger: Yes. The relationship between brain weight and body weight (EQ) and the range of water temperature (TR) are strongly correlated. Blubber thickness is known to correlate with body size in all marine mammals (paper by RYG, M., LYDERSEN, C., KNUTSEN, L. Ø., BJORGE, A., SMITH, T. G. & ØRITSLAND, N. A. (1993). Scaling of insulation in seals andwhales. Journal of Zoology (London) 230, 193-206.), thus I would expect some correlation to exist between ocean temperatures and blubber thickness, but in cetaceans the clear and strong correlation is between EQ and TR.

The Dolphin Pod: Are there, for example, correlations between differences in structure or firing rates of locus coeruleus neurons and ocean temperatures ranges across species?

Prof Manger: This unfortunately has not been studied, and may be very difficult to study. Recording discharge rates of locus neurons in freely moving animals requires extensive invasive surgery to implant the electrodes, and then requires the sacrifice of the animal at the completion of the experiment to verify electrode placement. I seriously doubt whether permission to undertake such a technically difficult study would ever be granted, although the results would be extremely interesting. A very recent study by Ridgway and colleagues (Functional imaging of dolphin brain metabolism and blood flow, J Exp Biol 209:2902-2910) does indicate that what I have hypothesized regarding firing rates of locus neurons does coincide with USWS. Perhaps more imaging data of this sort may provide further support for the ideas I propose.

The Dolphin Pod: Does the thermogenetic hypothesis of cetacean brain evolution assume that the need for more complex information processing was not an evolutionary pressure for a large EQ in cetacean species,

Prof Manger: Yes.


The Dolphin Pod: or simply that it had a secondary or minor influence?

Prof Manger: There is no need to posit at all a need for complex information processing as an evolutionary pressure leading to larger brain size in cetaceans. The microstructure of the cetacean brain indicates that the brain is not a complex information processor.

The Dolphin Pod: Could greater 'information processing' power have been an emergent property of a brain that evolved for thermoregulation?

Prof Manger: This may be a possibility, but not in the case of the cetaceans. There is too much comparative neuroanatomical data that indicates that the brain of the cetacean is not built for complex information processing (see comments re: Elston's work on primates below).

The Dolphin Pod: If thermogenesis is the primary reason for the evolution of large brains for cetaceans, why is the difference both in EQ and CI for mysticetes vis-à-vis odontocetes so large?

Prof Manger: This is not the case. The largest odontocete, Physeter catadon, has an EQ of 0.44 which is just above the EQ found for two species of Balaenopteridae (B. borealis and B. physalus both of which have EQs of 0.43). The difference you are perceiving here is one that is related to size, the baleen whales in general are much larger than the toothed cetaceans, and thus due to negative allometric scaling (the body weight increases more rapidly than brain weight, i.e. for every doubling in body weight of a cetacean the brain only increases by a factor of 1.3) may be interpreted as a difference, but this isn't the case.
In terms of the CI there is only one available data point for the mysticetes. More research on the volumetric divisions of the brains of the mysticetes and odontocetes (such as the work by Stephan, Baron and colleagues on primates, insectivores and bats) is required to determine the accuracy of this single data point (which I obtained from the literature).

The Dolphin Pod: Were these two sub-orders subject to different evolutionary pressures in terms of thermogenesis?

Prof Manger:
No. The microstructure of the mysticete brain appears to be the same as that of the odontocete, indicating similar evolutionary pressures creating a brain that worked (and continues to work) in the environment of the most common recent ancestor of the two sub-orders.

The Dolphin Pod: You discuss how acoustic specialization of the odontocete brain for processing biosonar is likely not a driving force behind the evolution of large brain size for this suborder. Is there a fundamental difference in the neuroarchitecture of odontocete and mysticete brains due to the presence of areas dedicated to biosonar processing that may explain the difference in CI and EQ between these two suborders?

Prof Manger: It really isn't known if there is a difference in the way the brains of the two groups process acoustic information, the experiments just haven't been done. I would imagine there would be some differences due to the echolocation in odontocetes, and these might be most easily identified in a structure like the inferior colliculus (which creates an auditory space map). Unfortunately, well-preserved brains of mysticetes are not readily available for scientific observation. Much more information regarding the structure of the mysticete brain is needed to answer your question fully.

The Dolphin Pod: Some authors have described a large area of 'nonprojectional cortex' in the odontocete brain. According to your findings, is this an accurate description for any of the areas of the dolphin brain that you describe, and how does this area of the cortex relate to either a discussion of thermogenesis or information processing?

Prof Manger:
I am not aware of any region of the cetacean cortex that may be termed "non-projectional cortex". For example, the paper by Revishchin and Garey (1990) The thalamic projection to the sensory neocortex of the porpoise, Phocoena phocoena. J Anat 169:85-102. indicates that all regions of cortex receive direct thalamic projections. I am not aware of any region of the cetacean cortex that might be described as non-projectional. This is perhaps an old-fashioned term for regions of the neocortex that are not considered primary sensory areas (these being primary motor, somatosensory, visual and auditory areas). We now know that these "associational" "secondary" or "tertiary" regions of cortex receive direct thalamic projections in all mammalian species. So there is no "unexplainable" region of cortex in cetaceans.

The Dolphin Pod: In your opinion, is there any convincing behavioral evidence from experimental research with cetacean species (particularly odontocetes), that might suggest that they indeed have cognitive abilities on par with or exceeding that of primates? As examples:

a.. Herman's work with symbol manipulation
b.. Xitco's work with pointing, reference and joint attention

Prof Manger: No.

The Dolphin Pod: A follow-up question: if, for example, the results of these experiments are due to a 'basic form of learning', why have the results of these and similar experiments not been replicated more often in species other than dolphins and primates?

Prof Manger: One reason is due to a lack of trying. If you examine the literature on the cognitive abilities of birds you might be astounded at what a bird can achieve. Such features as cognitive dissonance in parrots, following of human eye gaze in corvids, tool use and manufacture, and even deliberate deception have been well documented (reviewed in Butler A, Manger P, Lindahl B, Arhem P. Evolution of the neural basis of consciousness: a bird-mammal comparison. Bioessays 27: 923-936 (2005)).

Also, I might ask at this point, why have many of the behavioural tests done in other animals not been achieved with dolphins? For example, I am not aware of a radial arm maze test (that investigates spatial memory) ever having been performed on dolphins? I feel your question here should be reversed, and we should be asking why have many of the standard behavioural paradigms used in many other species not been used on dolphins?


The Dolphin Pod: In your opinion, what are the strongest neurological correlates of intelligence for brains in general?

Prof Manger: The brain is a hierarchically organized processor of information. This spans from the molecular level, through receptor complexes, synapses, parts of neurons (such as dendrites), single neurons, clusters of neurons, nuclei and cortical areas, systems, with behaviour being the expression of this processing. Dolphin brains are mammalian brains, and as such, follow the patterns established early in mammalian evolution. For a brain to be a complex processor of neuronal information all levels of this organizational hierarchy must be functional, otherwise the brain fails. For example, in recent work with colleagues (Elston GN, Benavides-Piccione R, Elston A, Zietsch, B., Defelipe J, Manger P, Casagrande V, Kaas JH. Specializations of the granular prefrontal cortex of primates: implications for cognitive processing. Anat Rec A Discov Mol Cell Evol Biol. 2006 Jan;288(1):26-35.) we have shown that with increases in brain size in primates, the complexity of the neuron increases dramatically. This increase in complexity, and thus functionality, will have a flow on effect to higher levels of brain organization, and thus result ultimately in more complex behaviour. There are too many examples within the brains of cetaceans that violate the evolutionary patterns found in other mammals. These are listed in the paper, but include:

Low cell density
Lack of layer 4
Thin thickness for brain size
Low number of cortical areas
No/inordinately small pre-frontal cortex
High glia:neuron ratio
Low corticalization index
Small and poorly differentiated hippocampus
Unbalanced proportion of elements of the neuropil
A majority of poorly differentiated and aspinous neurons

There are just too many features at several structural levels of the dolphin brain that are not organized in a manner that indicates normal neural processing of information, thus the hierarchy typical to mammals fails. Any level of this hierarchy can become more or less complex during evolution. Increases in complexity will lead to increased behavioural sophistication and vice versa. All levels of neural organization must be investigated. Size can be important, as allometric increases in overall brain size may cause spandrels in complexity at lower levels of organization and thus increased complexity in neural processing, and this may indeed be found in primates and other species, but there is no evidence for this in cetacean species. A somewhat convoluted answer, but in brief, the whole shebang must be working, a weak link causes failure.

The Dolphin Pod: You state that the evidence in favor of significant intellectual capacities of dolphins is tenuous. In your opinion, what behaviors are evidence of intelligence in animal species, and which animals display these behaviors? How do dolphins measure up when compared to the overall intelligence of other animal species?

Prof Manger: The evolution of intelligence and comparing intelligence across species is a very difficult task. However, much depends on your definition of intelligence. Personally, I look at two factors:

1) How fast is information being processed?
2) How much information is being processed simultaneously?

The crucial demands for any behavioral model used to test intelligence are: (1) The instrumental response of the animal has to be a complex behavior, i.e. consisting of a sequence of locomotor actions rather than just a simple behavioral response to a stimulus. This should reflect the animal's understanding of the cognitive task (the application of knowledge). The experimental environment should thus allow more than one equivalent behavioral response. (2) The task semantics (rules of behavior) must contain a complex logic (if - then); only in this case can an animal learning the task be compared with human-like intellectual activity. In order to solve this kind of logic problem the animal should demonstrate substantial cognitive abilities (i.e. be capable to forecast the result of its actions). (3) The rule governing behavior should be set up in a general form so that learning a task would require the generation of several working hypotheses. Moreover, an animal can interpret the task rule in its own manner depending on its cognitive abilities and experience. (4) Learning should not be completed after one trial. To meet this criterion the amount of information in the experimental environment should exceed the amount of information that an animal is capable of processing simultaneously.
In aiming to study behavioral self-organization in animals (spontaneous vs. trained), the learning procedure should be such that it allows the animal to control the experimental situation - a free-choice learning procedure satisfies this condition. The main feature of the free-choice method is that it presents to the animal all of the information about the task from the beginning of the learning period. Another characteristic feature of this method is that the experimenter does not interfere in the animal's behavior in any way. The different prompted signals used by an experimenter during training or during an autoshaping procedure that guide an animal to a task goal, help to form the desired habit and make solving the problem easier. In the free-choice learning paradigm, the session time is the only condition limited by the experimenter.

Dolphins have not been tested in this manner in any of the reported behavioural studies, therefore, they cannot be "ranked". It is my feeling that due to the structure of the brain the cetaceans would have great difficulty in achieving a task set up as described above, a task which both rats and goldfish can achieve (papers documenting this have beeb published by Kira Nikolskaya - but in Russian).


The Dolphin Pod: Many scientists researching dolphin cognition might argue that examination of neuroarchitecture at this stage in our knowledge of how brain structure relates to behavior, is unlikely to reveal anything useful about the cognitive abilities of an animal. The bulk of claims that dolphins are intelligent animals in recent decades have come from behavioral studies. For example, Schusterman's work with both dolphins (with a high EQ) and sea lions (with a low EQ) has revealed that both species are capable of learning cognitively complex tasks - tasks that other (presumably 'less intelligent') animal species have difficulty learning. Do these kinds of results reveal that complex cognition functions independently of structural measurements like EQ and neuroarchitecture? How would you respond to criticisms of your own claims in this paper that state that intelligence is a measure of intelligent behavior, and not brain structure, thus an argument for the intelligence of an animal based on neuronal-structure is 'old fashioned'?

Prof Manger: These would be rather facile claims, as they indicate that if anything "looks" like intelligent behaviour to the observer, then it must be intelligent - even if no nervous system at all exists! For example, a hypothetical bacteria to which water is lethal may avoid water through a simple sensory mechanism. This may be considered intelligent behaviour, as the bacteria avoided a lethal situation. Do we really consider this intelligent behaviour?

The Dolphin Pod: Is there anything else that you would like to add, clarify or comment on regarding your hypothesis, dolphin intelligence or criticisms of your ideas that have appeared in the media?

Prof Manger: There has been a great deal of criticism of the ideas I have forwarded, however, I am yet to find any criticisms from people who have fully understood the hypothesis in it's entirety - most have taken small sound bites and missed the point. I would encourage people to thoroughly read what I have written and when understood then find fault. A healthy, open, and honest scientific debate is appropriate. Statements calling to have me "muzzled", "dismissed", "ostracized", and so on will do no good to the central point of research, which is the revelation of the truth - even if it is uncomfortable and threatens heartfelt beliefs.

It has also been said that I am funded/paid off by the tuna industry and the whaling countries, which is complete nonsense. I am a scientist who has examined the data and forwarded a hypothesis that was extensively peer-reviewed. There is no conspiracy theory here.

Lastly, several people have intimated that I "hate" dolphins and condone a "mass slaughter" of the remaining marine wildlife. Again this is nonsense. My science is directed at finding the truth of the matter. I study the brains of various animals in an attempt to understand the animals themselves in a more realistic manner. With a proper understanding we can devise appropriate management strategies of animals that will in the long-term maintain and protect biodiversity against the human factor in the environment.

 

Questions Round 1 Dr. Marino


The Dolphin Pod: Do you believe that ocean temperatures could have been the primary or even sole evolutionary pressure responsible for the large brain size (i.e. EQ) of odontocetes?

Dr. Marino: We currently do not have enough resolution on the paleoclimatic data for the Eocene-Oligocene transition to determine whether changes in oceanic temperatures played a role in selecting for larger brains in cetaceans back then. However, even if we were to obtain that level of resolution the most we would be able to say is that there is a temporal correlation between oceanic cooling and increases in encephalization in cetaceans. Obviously some factors did play a role in shaping the evolution of the cetacean brain but selection does not always work in a direct manner. It is highly plausible that oceanic cooling could have produced a change in prey availability that would have provided an impetus for changes in behavioral ecology in cetaceans that led to increased encephalization. Any number of scenarios are plausible at this point.

But it is critical to point out that when we test for correlations between EQ and current maximum and minimum temperature for oceans in which modern cetaceans live we find that only body size is important. The colder the water the bigger the body. But brain size has nothing to do with it once you control for body size.

The Dolphin Pod: Could the cetacean cortex (containing an unusually high number of glia cells) have evolved for thermogenesis and not information processing? Are there other hypotheses as to why cetaceans have an unusually high glia:neuron index?

Dr. Marino: One of Manger's most ironic flaws is his outdated view of the role of glial cells in the brain. The older view was that glia were there to provide nutritional and structural support for neurons and that they were basically inert units - "brain glue". However, recent research has shown that, in addition to these previously-known features, glial cells also play an important role in the modulation and coordination of neuronal activity in the brain. They do not just support but actually participate in information-processing and we have only begun to understand the complexity of the role glia may play.

The Dolphin Pod: Is Dr. Manger correct in stating that there are "no neurological correlates for the purported intellectual abilities of cetaceans"?

Dr. Marino: No, he is not correct in any meaningful way. We know, for instance, that the life history periods typical of highly cognitive lifestyles are correlated with residual brain size in cetaceans, just as they are in primates and birds. Long lives, long juvenile periods and long periods of adult reproductive time are thought to be favored by what we call "cognitive buffering", the ability to cope with life threatening situations (famine, predation, fluctuations in climate) with flexible behavior.

The Dolphin Pod: Is Dr. Manger correct that the following features of the cetacean brain have negative impacts on cortical processing:

1.. poorly differentiated neuronal morphology
2.. low number of neurons and cortical areas
3.. lack of a distinct pre-frontal cortex
4.. small hippocampal formation


The main substantive point that Manger misses is this. Cetacean brains have been on an independent evolutionary trajectory for close to 55 million years. During all that time the dolphin brain evolved a unique combination of features. The result of this process is that one cannot readily make one-to-one comparisons between cetacean brains and the brains of other mammals without taking this different evolutionary history into account. Manger accepts that cetacean brains are highly different from other mammal brains but his arguments belie the fact that he understands what this means. Here are my specific comments:

Poorly differentiated neuronal morphology - the view of the dolphin cortex as a primitive homogeneous simplified mass of tissue is a very outdated interpretation of dolphin neurobiology. Manger's acceptance of this outdated view is revealed by the fact that he states on page 11 of his paper that the dolphin cortex can best be described by terminology originating from 1909 and claims that essentially no new conclusions about the dolphin cortex have been reached since 1879. This is simply not true. In fact, one of his most stunning omissions is a widely-read 2005 paper on cetacean cortical complexity by Patrick Hof, Rebecca Chanis, and myself in which we describe the cortical cytoarchitecture of the bottlenose dolphin brain and show that the dolphin cortex is highly differentiated and contains a wide variety of different neuronal morphotypes.

Low number of neurons and cortical areas - This is a puzzling claim to make because no one knows how many neurons are in cetacean brains and we do not have adequate data on the number of cortical areas either.

Lack of a distinct pre-frontal cortex - This argument relies upon a very concrete way of looking at brain anatomy. Manger is absolutely correct in that there is little tissue anterior to the motor and somatosensory regions of the dolphin neocortex. He is also correct in that what little tissue there is does not resemble primate prefrontal cortex. However, he does not seem to understand that when we speak of identifying "prefrontal cortex" in any meaningful functional way what is meant is that we are seeking the functional and architectural analog to the primate prefrontal cortex in the dolphin brain. There is no reason to expect that if the dolphin brain has an analogous region to the prefrontal cortex that it would be, in fact, in the front of the brain!

Small hippocampal formation - The hippocampal formation in cetacean brains is reduced. But only certain components of it are small. Other components, such as the amygdala, are well developed. Moreover, Manger is not correct when he states that the cetacean limbic lobe is under-developed. In fact, the limbic lobe and the insular cortex are extremely elaborated in cetacean brains and many authors have remarked on this feature of cetacean brains in the literature. Finally, Manger neglects to mention that cetacean brains contain a paralimbic lobe that is not found in other mammal brains. We do not know the function of the paralimbic lobe in dolphins but it is fair to mention it in a comparative context.

The Dolphin Pod: Dr. Manger singles out your 2002 paper from Brain, Behavior and Evolution, discussing how three cited examples of complex cognitive behaviors in cetaceans (social group behavior, language comprehension, and mirror self recognition) are actually poor indicators of cognitive complexity in cetaceans as these are based on 'unreliable assumptions'. Are his criticisms accurate?

Dr. Marino: I'm not sure what kind of 'unreliable assumptions' Manger thinks these studies are based on. But there are three indisputable findings that Manger simply cannot make go away without completely dismissing the validity of the entire peer-reviewed scientific process. First, bottlenose dolphins recognize themselves in mirrors. In 2001 Diana Reiss and I provided irrefutable evidence of this in our paper in PNAS. Manger claims that we relied upon latency measures to infer cognitive activity. In that paper we tested several hypothesis based on a number of parameters, including latency. But the critical outcome measure was behavior at the mirror with respect to the mark. So he is simply incorrect. Second, years of research by Louis Herman and his colleagues have shown that bottlenose dolphins can comprehend syntactic sentences and abstract concepts. Short of reviewing the entire literature here I can only suggest that Manger go back and do that for himself and then articulate in a more cogent way just what it is about this research that he doesn't accept. Third, again, work by people like Richard Connor, Hal Whitehead, and many others have documented complex behaviors such as alloparenting, division of labor, coalition formation, cooperation, and intergenerational transmission of learned behaviors in cetaceans. If Manger wants to suggest that these characteristics are not complex, well, I would like to see what would pass as complex for him. In general, what Manger does is dismiss everything in the literature but provides no alternative criteria for what would be acceptable evidence of cognitive complexity.

The Dolphin Pod: Do you believe that Dr. Manger's discussion of vocalizations and language comprehension in cetaceans convincingly argues that cetaceans (particularly odontocetes) do not have an atypical capacity for artificial language usage and comprehension for a non-human animal species?

Dr. Marino: As he does with the corpus of literature showing dolphins are cognitively and socially complex, Manger refuses to acknowledge the prodigious body of work by Louis Herman and his colleagues showing that bottlenose dolphins can mentally represent and understand abstract concepts and comprehend an artificial syntactically-based communication system containing thousands of unique sentences composed of symbolic gestural and auditory "words". The only other species to show this rare capacity are bonobos and humans.
Manger fails to understand that the key feature of a language that gives it power is not the word but the sentence. Herman's work has shown that bottlenose dolphins are able to process instructions given to them in sequences of symbols (sentences) as long as five words in length, and in which word order, a syntactic device, as well as word meaning or reference, determines the meaning of the sentence as a whole. Manger neglects most of the large contemporary body of published, peer-reviewed data coming from Herman's lab demonstrating impressive and varied cognitive abilities in dolphins that is fully in keeping with expectations from the large size of the brain and its complex structure.

The Dolphin Pod: Do you think that the current behavioral evidence suggesting that dolphins are capable of cognitively complex behaviors is, as Dr. Manger states, 'tenuous, and based upon untested, unproven, unquestioned, and anthropomorphic assumptions"?


Dr. Marino: Absolutely false. This is Manger's most egregious claim and one that deserves very little attention. Manger blatantly dismisses a whole generation of experimental and observational work on dolphin cognition, behavior, and sociality by hand-waiving. This is simply not how science works. The expression "The proof is in eating the pudding" is apropos' here. The data on dolphin cognitive complexity is there regardless of whether Manger wants it to be there or not. No amount of protesting or hand-waiving will make it disappear.


The Dolphin Pod: Dr. Manger offers a new method for calculating corticalisation index (CI) by separating out gray and white matter. This new method places the CI of odontocetes well below the average of most other mammals. In your opinion, is this new method for calculating CI superior to the method first used by Glezer et al. (1988)? Does this reveal anything significant concerning the organization of the dolphin cortex?

Dr. Marino: This index per se reveals nothing about cortical complexity because the large quantity of white matter in the cetacean brain is exactly what one would predict on the basis of scaling of a brain of large absolute size. In other words, when brains get large the amount of white matter increases simply to maintain processing speed and efficiency. Furthermore, recent comparative MRI studies show that the human brain has a higher relative amount of white matter in the prefrontal cortex than other primates. More connections are presumed to contribute to faster information processing, contrary to what Manger concludes for cetaceans. In fact. Manger's own data, presented in his Table 2, shows that when computing the volume of grey matter as a percentage of total brain volume, humans fall below the other primate species he lists.

The Dolphin Pod: According to your research, has Dr. Manger correctly described the neuroarchitecture of the cetacean brain, and is he correct in stating that cetacean brains "do not indicate a structure supportive of high levels of intellectual capacities"?

Dr. Marino: As I mentioned previously Manger's view of dolphin neuroarchitecture is grossly outdated and he fails to appreciate the fundamental consequences of evolutionary divergence on brains.

The Dolphin Pod: Do you believe that Dr. Manger's hypothesis that "unihemispheric slow-wave sleep and lack of REM sleep in cetaceans functions to compensate for heat loss to the water during sleep" could explain the evolution of unihemispheric slow-wave sleep in cetaceans?

Dr. Marino: I don't immediately accept his argument for one simple reason - there is no evidence for it. Manger relies upon comparisons of cetaceans with other aquatic mammals to bolster his argument. But it is this very comparison that weakens his hypothesis. Despite his claim to the contrary, the brains of cetaceans, pinnipeds, and sirenia are all extremely different on a number of dimensions and there are no commonalities across them that point to selection for thermoregulation in an aquatic environment.

The Dolphin Pod: Following from your own research, what do you believe to be the major evolutionary pressures that facilitated the evolution of large brain sizes in cetaceans?

Dr. Marino: My hypotheses about evolutionary pressure for large brain size in cetaceans are data-based. Dolphins and other cetaceans are extremely cognitively and socially complex. They also possess sophisticated echolocatory, cross-modal perceptual, and vocal learning abilities. My ideas about why cetaceans became so encephalized and intelligence focus on behavioral ecology, sociality, and communication. Many cetaceans show convergent capacities with some other mammal groups, such as primates and elephants. My colleagues and I are focusing on identifying the common selection pressures across these groups that might have led to convergent cognition.

The Dolphin Pod: Do you have any further remarks on this paper (its claims and methods), or comments relating to the criticisms of your own work that were brought up in this paper?

Dr. Marino: No further comments.

Questions Round 2 Prof. Manger


The Dolphin Pod: Do you agree with Dr. Marino's comments:
(Qouted from Dr. Marino): "We currently do not have enough resolution on the paleoclimatic data for the Eocene-Oligocene transition to determine whether changes in oceanic temperatures played a role in selecting for larger brains in cetaceans back then. However, even if we were to obtain that level of resolution the most we would be able to say is that there is a temporal correlation between oceanic cooling and increases in encephalization in cetaceans. Obviously some factors did play a role in shaping the evolution of the cetacean brain but selection does not always work in a direct manner. It is highly plausible that oceanic cooling could have produced a change in prey availability that would have provided an impetus for changes in behavioral ecology in cetaceans that led to increased encephalization. Any number of scenarios are plausible at this point."

Prof Manger: Of course several scenarios can be put forward, and indeed what Dr. Marino proposes may be correct. But given the data I have put forward, isn't the simplest explanation the most preferred one - Ockham's Razor? Why convolute the issue with speculations regarding information that we don't have? I have based my conclusions on the available evidence - that published by experts in the field. There does appear to be a good temporal correlation between water temperature and encephalization in cetaceans as outlined in my paper, and given that they are mammals living in water and the high rate of heat loss to the water, surely it is logical to conclude that this will be a significant environmental pressure? In terms of changes in diet, later evolution in the baleen whales saw the genesis of the baleen oral apparatus, however, these were not present in the earlier members of this suborder, thus dietary changes have occurred, and these are correlated to anatomical changes, but this does not appear to correlate with the period when temperature would have been a significant environmental pressure nor when there were significant alterations in the anatomy, size and scaling of the brain.

(Qouted from Dr. Marino): "But it is critical to point out that when we test for correlations between EQ and current maximum and minimum temperature for oceans in which modern cetaceans live we find that only body size is important. The colder the water the bigger the body. But brain size has nothing to do with it once you control for body size."

I am not privy to these calculations, so it is difficult to speculate upon them - how does one control for body size when body size is one of the variables examined? Not only this, EQ is already a control for body size. I can only point out that I did indeed publish the statistically significant correlations between water temperature and body size in my paper, and showed the same with brain size, but none of these correlations indicated a high predictive value (see figure 15), however, the correlation between EQ and water temperature range showed the strongest correlation and highest predictive value. This last correlation is therefore the one I focused upon.

The Dolphin Pod: One of your strongest claims is that the glia:neuron ratio is quite high in cetacean brains, and that this is evidence that this kind of brain could not be a good information processor. Is it correct to say that glia cells play a very limited if any role in information processing in the brain? See Marino's comments below:
(Qouted from Dr. Marino): "One of Manger's most ironic flaws is his outdated view of the role of glial cells in the brain. The older view was that glia were there to provide nutritional and structural support for neurons and that they were basically inert units - "brain glue". However, recent research has shown that, in addition to these previously-known features, glial cells also play an important role in the modulation and coordination of neuronal activity in the brain. They do not just support but actually participate in information-processing and we have only begun to understand the complexity of the role glia may play."

Prof Manger: I am well aware of the evidence suggesting the possible role of glia in neuronal activity, and would never deny this. This is why I state in the paper that (pg 314): "It has been suggested that one role of glia in the vertebrate brain is thermogenesis (Donhoffer, 1980; Szelenyi, 1998)." and go on to discuss the evidence supporting thermogenesis as a functional attribute of glia. In fact in different vertebrates, only the mammals and the birds have highly expressed numbers of glia, which is associated with being warm-blooded. Ascribing the above expressed "outdated" and ironically flawed (sic) view of glia to my way of thinking is not correct. In the cetaceans we see an extraordinary abundance of glia, which requires an explanation. I have forwarded the possibility that this large amount of glia is related to thermogenesis, others, such as Dr. Marino may prefer to think of these as related to intellectual processes, but given the number of coincidences I have gathered and outlined in my paper, it is difficult to support an alternative position.

The Dolphin Pod: A follow up question: it has been suggested that Einstein was 'more intelligent' than other humans because his brain had a larger number of glia cells than the average human brain.

Prof. Manger: This is not the case. The reference and abstract of this paper is given below (taken directly from that provided on PubMed):

"Diamond MC, Scheibel AB, Murphy GM Jr, Harvey T. (1985) On the brain of a scientist: Albert Einstein. Exp Neurol.88:198-204. Neuron:glial ratios were determined in specific regions of Albert Einstein's cerebral cortex to compare with samples from 11 human male cortices. Cell counts were made on either 6- or 20-micron sections from areas 9 and 39 from each hemisphere. All sections were stained with the Kluver-Barrera stain to differentiate neurons from glia, both astrocytes and oligodendrocytes. Cell counts were made under oil immersion from the crown of the gyrus to the white matter by following a red line drawn on the coverslip. The average number of neurons and glial cells was determined per microscopic field. The results of the analysis suggest that in left area 39, the neuronal: glial ratio for the Einstein brain is significantly smaller than the mean for the control population (t = 2.62, df 9, p less than 0.05, two-tailed). Einstein's brain did not differ significantly in the neuronal:glial ratio from the controls in any of the other three areas studied."

The Dolphin Pod
: Some studies have shown that glia cell growth in the rat brain increases when rats are exposed to an enriched environment. Does this not suggest that a larger number of glia in any brain might correspond to more information processing and not less?

Prof. Manger: These studies also show significant differences in neuronal morphology (increased dendritic branching, greater spine density etc. which will more directly lead to greater information processing) with enriched environments, thus it is impossible with present knowledge to really support the specific conclusion you indicate above.


The Dolphin Pod: Concerning your 4 features of the cetacean brain that have a negative impact on cortical processing, Dr. Marino made the following comments - do you have a response to these criticisms?
(Qouted from Dr. Marino): "Poorly differentiated neuronal morphology - the view of the dolphin cortex as a primitive homogeneous simplified mass of tissue is a very outdated interpretation of dolphin neurobiology. Manger's acceptance of this outdated view is revealed by the fact that he states on page 11 of his paper that the dolphin cortex can best be described by terminology originating from 1909 and claims that essentially no new conclusions about the dolphin cortex have been reached since 1879. This is simply not true. In fact, one of his most stunning omissions is a widely-read 2005 paper on cetacean cortical complexity by Patrick Hof, Rebecca Chanis, and myself in which we describe the cortical cytoarchitecture of the bottlenose dolphin brain and show that the dolphin cortex is highly differentiated and contains a wide variety of different neuronal morphotypes."

Prof Manger: This paper wasn't cited in mine as my paper was "in press" when this paper was published, therefore it wasn't a "stunning omission" but something that occurred due to timing of the publication process. For the most part, this paper really doesn't add anything new to the literature from what was described in earlier studies. There is no part of the paper that demonstrates neuronal morphotypes previously undescribed (defined by actually anatomy of the neurons themselves and not done with basic stains such as cresyl violet which labels Nissl bodies to reveal the location of the cell body but not the dendritic branching, axonal morphology or chemical nature of the neuron), and on the whole reflects everything that has previously been described - e.g. the location of the giganto-pyramidal cells (which was described in 1951!). There are several claims for additional cortical areas, however, these are not documented in a manner that allows independent scrutiny - for example, no photomicrographs of transitions between cortical areas are given. Moreover, no architectonic map, as provided by Kesarev et al (1977) is provided. On the whole, this study does little except assert statements that are not supported by documentation, and the documented photomicrographs are not different from those previously published.

What appears to be missing in Dr. Marino's above statement is an understanding of the hierarchical nature of neuronal processing of the brain. The hierarchy moves from molecules to molecule complexes to synapses, part of neurons, individual neurons, neuronal clusters, cortical areas/nuclei, and systems resulting finally in behaviour. Changes in morphology can occur at any level of this hierarchy, if for example the neurons were to become more complex, the end result of processing, or behavioural expression, would be more complex without the need for changes, say at the organizational level of cortical areas. If one part of the hierarchy is rather simple in organization, such as the neurons in the dolphin brain (simple in terms of the dendritic arborization and synaptic connectivity), then the hierarchy will be processing less complex information.

The 1909 terminology used in my paper comes from Brodmann's seminal publication on cortical architectonics, and his architectonic map is still widely used today in neurology and the neurosciences. Brodmann is perhaps the most widely respected figure in the field of cortical architectonics, and his work is exemplary. His terminology is used by most neuroscientists, so to indicate that the terminology I have used is outdated is ludicrous.

(Qouted from Dr. Marino): "Low number of neurons and cortical areas - This is a puzzling claim to make because no one knows how many neurons are in cetacean brains and we do not have adequate data on the number of cortical areas either."

Prof Manger: In the paper, on page 310, I have listed the available data indicating the exact published numbers of cortical neuronal density in 3 species of cetaceans and 12 other mammalian species. Cetacean neuronal density is far below that seen in other mammalian species, this is not a puzzling claim, it is published data. Also, see the recently published paper by Poth et al (2005) Brain Res. Bull. 66:357-360.

Regarding the number of cortical areas, I used the only published complete architectonic map of cetacean cerebral cortex, that of Kesarev and colleagues, as a baseline. This study clearly indicates a low number of cortical areas in comparison to many other mammalian species. The results of this study are backed up by the available physiology, also described in the paper. Moreover, a recent study using computer assisted architectonic methods [Fung et al., (2005) Brain Res Bull. 66:353-356] provide evidence for two auditory cortical areas they call PAC (primary auditory cortex) and SAC (secondary auditory cortex) in Pontoporia that are almost identical in location and extent to the regions demarcated Pi and Pli by Kesarev et al (1977) in Tursiops. Thus there is absolutely no reason to question the conclusions I have derived regarding the number of cortical areas in the dolphin - more than one previous study supports my parcellation of cetacean cerebral cortex, the only one claiming to go against it is that of Marino and colleagues.

(Qouted from Dr. Marino): "Lack of a distinct pre-frontal cortex - This argument relies upon a very concrete way of looking at brain anatomy. Manger is absolutely correct in that there is little tissue anterior to the motor and somatosensory regions of the dolphin neocortex. He is also correct in that what little tissue there is does not resemble primate prefrontal cortex. However, he does not seem to understand that when we speak of identifying "prefrontal cortex" in any meaningful functional way what is meant is that we are seeking the functional and architectural analog to the primate prefrontal cortex in the dolphin brain. There is no reason to expect that if the dolphin brain has an analogous region to the prefrontal cortex that it would be, in fact, in the front of the brain!"

Prof Manger: In every single study of the organization of the cerebral cortex of mammals, the prefrontal cortex has been located at the anterior pole of the cerebral hemisphere. This includes studies in monotremes, marsupials, insectivores, chiropterans, primates, ungulates, carnivores, etc. etc.! The observed physiology of the cetacean cerebral cortex indicates that it is organized in the mammalian pattern, with visual cortex posteriorly, auditory laterally, and somato-motor anteriorly. Is Dr. Marino seriously suggesting that the dolphins are an exception to the observed rule of areal organization of mammalian cerebral cortex? To support this claim Dr Marino must known something no-one else does, or has done as yet unreported experiments. As reviewed in my paper, there is no region of the cetacean cerebral cortex that can be readily described as pre-frontal. All studies, including the thalamic connectivity studies, support the parcellation of cerebral cortex I have made in the paper. Dr. Marino appears to be "inventing" a new part of cerebral cortex, one that doesn't exist - see comments below on the "paralimbic lobe" of cetaceans.

(Qouted from Dr. Marino): "Small hippocampal formation - The hippocampal formation in cetacean brains is reduced. But only certain components of it are small. Other components, such as the amygdala, are well developed."

Prof Manger: The hippocampal formation consists of the hippocampus proper, the dentate gyrus, the entorhinal cortex and the subiculum. The amygdala is not part of the hippocampal formation. The hippocampal formation is the portion of the brain responsible for the formation and recall of long-term or enduring memories, and in conjunction with the pre-frontal cortex enables short-term or working memory (both having been demonstrated conclusively in goldfish and are absolutely essential for production of language in humans). The fact that components of it are reduced and poorly organized indicates a compromise in the functioning of this system that is very important for memory and hence all the cognitive processes that require memory.

In my paper I clearly indicate that the amygdala, apart from a loss of the nuclei involved in olfaction, is normally organized (pg. 312). The amygdala is that part of the brain that processes species relevant environmental stimuli and generates the overall state of the brain that we call emotion. It has nothing to do with memory, but rather serves as an interface between the environment and species relevant emotional behaviour.

(Qouted from Dr. Marino): "Moreover, Manger is not correct when he states that the cetacean limbic lobe is under-developed."

Prof Manger: Here Dr. Marino is now questioning the studies and conclusions of several recognized experts of dolphin neuroanatomy. The studies are referenced on page 312 of my paper. She is incorrect in her assertion here. For example, Kruger (1966) indicates that the relative volume of the thalamic portion of the limbic system is almost half that seen in other mammals.

(Qouted from Dr. Marino): "In fact, the limbic lobe and the insular cortex are extremely elaborated in cetacean brains and many authors have remarked on this feature of cetacean brains in the literature."

Prof Manger: The insular cortex is not part of the limbic system. It is that part of the cortex that overlies the claustrum, and is found in all mammals. I have not read anywhere of this regions of the cetacean brain being "extremely elaborated" except in descriptions of the number of gyri and sulci, which applies to the cetacean brain as a whole.


(Qouted from Dr. Marino): "Finally, Manger neglects to mention that cetacean brains contain a paralimbic lobe that is not found in other mammal brains. We do not know the function of the paralimbic lobe in dolphins but it is fair to mention it in a comparative context."

Prof Manger: This is a very unusual assertion. I assume it relates to Dr. Marino's attempt to find an analogue to pre-frontal cortex in cetaceans. The term "paralimbic lobe" has two definitions in the scientific literature, one used for mammals in general, and the other for cetaceans. For mammals, the term paralimbic lobe is referred to as those structures that directly project upon the limbic system, such as the posterior orbital gyrus, insula (or claustro-cortex), nucleus accumbens, caudate nucleus, habenula, interpedunclar nucleus, various hypothalamic nuclei, temporal cortex, and claustrum. These are structures found in all mammals, thus in this sense, the above statement of Dr. Marino is incorrect.

What Dr. Marino appears to be referring to though, is the "paralimbic lobe" of the cetacean cerebral cortex described by Morgane et al., (1980) Brain Res Bull 5 (suppl. 3). In this extensive description of the surface anatomy of the cetacean cerebral cortex Morgane and colleagues describe a region of cortex lateral to the cingulate gyrus (part of the limbic system) that they term, using an anatomical nomenclature (nothing to do with function), the paralimbic (or "around the limbic") lobe. Consequent studies (physiological and connectional) have shown this "paralimbic lobe" to be the primary and secondary visual cortex, as described in my paper (the regions designated Os, Om, and Olm by Kesarev). In later studies of this region of the brain, Morgane and colleagues (1988, J Comp. Neurol. 273:3-25) do not use the term paralimbic lobe, but rather refer to it as visual cortex. Thus, we do know the function of the apparently mysterious paralimbic lobe - it is visual cortex.

Yes, I did not refer to the "paralimbic lobe" in my paper, but used the correct assignation of visual cortex - nothing mysterious in this as all mammals have a visual cortex.

The Dolphin Pod: Concerning your new method of calculating CI and its implications, Dr. Marino made the following comments - do you have a response?
(Qouted from Dr. Marino): "This index per se reveals nothing about cortical complexity because the large quantity of white matter in the cetacean brain is exactly what one would predict on the basis of scaling of a brain of large absolute size. In other words, when brains get large the amount of white matter increases simply to maintain processing speed and efficiency. Furthermore, recent comparative MRI studies show that the human brain has a higher relative amount of white matter in the prefrontal cortex than other primates. More connections are presumed to contribute to faster information processing, contrary to what Manger concludes for cetaceans. In fact. Manger's own data, presented in his Table 2, shows that when computing the volume of grey matter as a percentage of total brain volume, humans fall below the other primate species he lists."

Prof Manger: The CI index reveals the amount of the brain devoted to neural processing, i.e. the proportion of the brain where synapses are found - this being the neuropil. The neuropil is incredibly important in determining the potential processing power of a brain, as it is in this region of the brain that neurons communicate. By showing that for their brain size the amount of space potentially dedicated to the neuropil is lower in cetaceans than would be expected is a further indication of a brain not built for complex information processing. On top of this I demonstrated that the proportions of the cetacean neuropil are probably quite unbalanced in comparison with other mammals, being occupied more by glial processes and myelin rather than dendritic aborizations and axonal branching - another indication of a poor processor. Yes, the human is slightly lower than other primates, but the cetaceans are far lower than would be expected for their brain size, which is radically different for most mammalian species.

The Dolphin Pod: The strongest (scientific) objections to your paper seem to be coming from scientists who have collected or written about laboratory and field data concerning dolphin cognition. Although you critique some of these laboratory experiments in your paper, many argue that you failed to fully address the bulk of evidence for intelligent dolphin behavior from the current literature. You have outlined in a previous answer some ideas concerning what intelligent behavior is and how to test for it. Do you envision future papers where you may address these criticisms and offer an alternative interpretation of the current behavioral data in more detail?

I think the major problem here, and the threat perceived by many scientists (not only scientifically, but also financially), including Dr. Marino, is the fact that intelligence, measured by intelligent behaviour, is the observable expression (or lack of expression in terms of behavioural inhibition) of neural activity and processing, and of course, if the processor is not built appropriately, as indicated in my paper and the studies of many previously, then the actual ability for the production of intelligent behaviour must be missing. We cannot divorce structure from function in the way that many who study dolphin behaviour have been doing currently and for the last 50 years. In fact, even with a 50 year search for intelligence in dolphins no "smoking-gun" has appeared. I am uncertain as to the extent and usefulness of future papers dealing with the reported behaviour in dolphins and whales, as without the appropriate brain, there is no intelligent behaviour. I think the onus is now on the cetacean behaviourists to prove beyond doubt that what they are doing cannot be explained by a simpler mechanism rather than jumping to the cognitive conclusion.

The Dolphin Pod: To take one example that you mention: you pointed out that the intelligent behavior exhibited by avian species may be surprising to some, and mentioned eye-gaze following in corvids. Many would argue that eye gaze following in corvids is indeed evidence of complex cognition in these animals, as it is similarly argued for some of the primate species capable of following eye gaze. This kind of behavior has not been observed in some of the conventionally acknowledged 'less intelligent' animals like rats, or even some of the more 'intelligent' species like horses. It has, however, been shown that dolphin species exhibit gaze following behaviors similar to those observed in corvids and primates. Following from your ideas concerning the dolphin brain, what do you believe are the reasons for this specific discrepancy?

Prof Manger: My first reaction here would be to examine contextual ambiguities - those parts of any experiment that do not gel with the sensory or motor capabilities of the animals being tested. For example, given the poor eyesight and nocturnal habits of the rat, would we expect it to be an animal that follows the gaze of others? It has been shown that this type of comparison as a method of ranking intelligence is rather a poor way of doing it. In fact, the "learning sets" type of behaviour based on visual stimuli first showed that rodents were less intelligent that primates and cats, but when tested on learning sets involving olfactory stimuli, rodents performed equally well. Thus, the experimental context of these comparisons must be considered prior to making any value judgments of the relative cognitive abilities of the various species.

 

Questions Round 2 Dr. Marino

The Dolphin Pod: Could greater 'information processing' power have been an emergent property of a brain that evolved for thermoregulation?

Dr. Marino: Greater information processing power could, in principle, emerge from any number of evolutionary developments. Given what we know about brain evolution and ontogeny, however, it is difficult for me to accept that a significant proportion of the variance in intelligence in cetaceans could be due to thermoregulatory changes in the brain.

The Dolphin Pod: Is it possible that an increase in glial cells in the brain may actually indicate a brain that is better and faster at processing information? I refer to studies on rats that show an increase in the growth of glial cells for rats exposed to an enriched environment. Does this contradict Dr. Manger's claims that a high glia:neuron ratio indicates less information processing?

Dr. Marino: What we know about the role of glia contradicts Manger's claims that a high glia-neuron ratio indicates less information processing. In fact, all the evidence indicates that an increase in glial cells is correlated with more efficient and faster information processing. Here is what the scientific literature says:

1) Glia are not inert elements taking up space in the brain. Rather, they participate in the modulation of neural activity. They contribute to the complexity of neural processing in this way.

2) Glia provide myelin (otherwise known as 'white matter') for efficient neural transmission. Myelin maintains information processing rate. When brains enlarge the amount of white matter increases in order to maintain processing speed and efficiency.
Examples of what happens to neural activity when glia are lost are the de-myelinating disorders such as multiple sclerosis and many of the "dystrophies". These disorders show that glia are critical for efficient neural activity.

3) The more glia there are the larger the proportion of neuropil. Neuropil is the material outside the cell body - dendrites and fibers. The more dendrites the more complex the synaptic connectivity of the brain. Therefore, contrary to Manger's claim, a high proportion of glia is consistent with a high level of information processing complexity.

5) Recent comparative MRI studies show that the human brain has a higher relative amount of white matter in the prefrontal cortex than other primates. Is Manger prepared to assert that humans are less intelligent than monkeys?

The Dolphin Pod: Do you have any comments on Dr. Manger's response to the following question that I posed:
(The Dolphin Pod to Prof. Manger) In your opinion, what are the strongest neurological correlates of intelligence for brains in general?
(Quoting Prof. Manger): "The brain is a hierarchically organized processor of information. This spans from the molecular level, through receptor complexes, synapses, parts of neurons (such as dendrites), single neurons, clusters of neurons, nuclei and cortical areas, systems, with behaviour being the expression of this processing. Dolphin brains are mammalian brains, and as such, follow the patterns established early in mammalian evolution. For a brain to be a complex processor of neuronal information all levels of this organizational hierarchy must be functional, otherwise the brain fails. For example, in recent work with colleagues (Elston GN, Benavides-Piccione R, Elston A, Zietsch, B., Defelipe J, Manger P, Casagrande V, Kaas JH. Specializations of the granular prefrontal cortex of primates: implications for cognitive processing. Anat Rec A Discov Mol Cell Evol Biol. 2006 Jan;288(1):26-35.) we have shown that with increases in brain size in primates, the complexity of the neuron increases dramatically. This increase in complexity, and thus functionality, will have a flow on effect to higher levels of brain organization, and thus result ultimately in more complex behaviour. There are too many examples within the brains of cetaceans that violate the evolutionary patterns found in other mammals. These are listed in the paper, but include:

Low cell density
Lack of layer 4
Thin thickness for brain size
Low number of cortical areas
No/inordinately small pre-frontal cortex
High glia:neuron ratio
Low corticalization index
Small and poorly differentiated hippocampus
Unbalanced proportion of elements of the neuropil
A majority of poorly differentiated and aspinous neurons

There are just too many features at several structural levels of the dolphin brain that are not organized in a manner that indicates normal neural processing of information, thus the hierarchy typical to mammals fails. Any level of this hierarchy can become more or less complex during evolution. Increases in complexity will lead to increased behavioural sophistication and vice versa. All levels of neural organization must be investigated. Size can be important, as allometric increases in overall brain size may cause spandrels in complexity at lower levels of organization and thus increased complexity in neural processing, and this may indeed be found in primates and other species, but there is no evidence for this in cetacean species. A somewhat convoluted answer, but in brief, the whole shebang must be working, a weak link causes failure."

Dr. Marino: The main point that Manger misses is this - and it's a big one. Cetacean brains have been on an independent evolutionary trajectory for close to 55 million years. During that period the dolphin brain evolved a unique combination of features. The result of this process is that one cannot readily make simple comparisons between cetacean brains and the brains of other mammals without taking this divergent evolutionary history into account.


With that said, let's visit each of the features of the cetacean brain that Manger asserts is an indication of low information processing complexity.

1) Low cell density - the cell density in the dolphin brain is exactly what is expected for a brain of such a large size. Cell density decreases with absolute brain size in mammals. It must in order to maintain other properties of neural transmission. So cell density tells you nothing.

2) Lack of layer 4 - the presence of a granularized layer 4 in primates does not imply that this is the way cortical complexity is achieved in all mammals. The ancestral evolutionary lines that eventually produced primates and cetaceans were separated by about 95 million years. The closest cetacean relative is an ungulate but the ancestors of ungulates and cetaceans split off over 55 million years ago. So what are the chances that cetacean brains - so isolated in time and ecology from other mammals - would become complex in exactly the same way primate or other mammal brains would.
Here's an example. Dolphins echolocate by sending high frequency clicks out of their forehead and receiving the echoes with their lower jaw which mechanically conducts the sound to the middle ear. So dolphins have evolved a completely different way to receive and conduct acoustical input from humans and other mammals (including other echolocating mammals like bats). This shows that extremely different and complex ways of processing information can indeed occur - given enough time.

3) Thin cortex(?) for brain size - the dolphin neocortex may be thin but it possesses a surface area that far exceeds the human brain. So, without any way to functionally anchor this characteristic to computational complexity - it isn't clear at all that a thin highly convoluted cortex processes less information than a thicker less convoluted cortex. The opposite argument can just as readily be made. Take your pick!

4) Low number of cortical areas - This claim is based on nonexistent data. No one (and that includes Paul Manger!) knows how many functional cortical areas exist in the dolphin brain.

5) Small prefrontal cortex - This point is so mired in a concrete way of thinking about neuroanatomy that one barely knows where to begin. Manger surely cannot seriously claim that the frontal lobe analog in the dolphin brain needs to be - literally - in the front!

6) High glia-neuron ratio - Please see my more detailed response above. If anything, high glia:neuron ratio is more indicative of information processing complexity than not.

7) Low CI - Manger's analyses of CI are confounded with body size, which he does not take into account. Therefore, his claims about low CI are based on flawed analyses.

8) Small hippocampus - Only certain parts of the hippocampus in cetacean brains are reduced. Other components, such as the amygdala, are well developed. Other areas that are related to the hippocampal formation, such as the limbic lobe and insular cortex, are extremely elaborated in cetacean brains. Cetacean brains also possess a paralimbic lobe that is not found in other mammal brains.

More importantly, despite the fact that Manger wants to claim that dolphin memory is weak because of a small hippocampus, he cannot account for the prodigious body of empirical literature showing that dolphins possess quite exquisite and sophisticated memory capacities. Studies by Herman and colleagues have shown that both auditory memory (memory for things heard) and visual memory (memory for things seen) are well developed and robust in dolphins. These studies deal with short-term or "working memory"-processing new information and retaining it in conscious memory. Manger overlooks work by Xitco and colleagues that has revealed the dolphin's superb ability to remember the spatial locations of named structures in its extensive habitat. And mysticetes display migration patterns that can best, if not only, be explained by their possession of a highly sophisticated spatial long-term memory capacity.

The bottom line is that cetaceans have excellent memory capacities despite a small hippocampus. The behavioral literature cannot be dismissed because the neuroanatomical substrate of memory in cetaceans is not yet understood. Obviously there is another part of the cetacean brain that has taken over the function of memory.

9) Unbalanced elements of neuropil - If Manger is referring to the high proportion of glial cells then that claim has already been responded to. There is no evidence for an "unbalanced neuropil" in cetaceans.

10) Poorly differentiated cortex - Manger's view of the cetacean cortex is so out of date that he cites studies from 1879. Yet he ignores recent work (that he should be well aware of) showing that the cetacean neocortex is, in fact, highly differentiated and complex.
What is so puzzling is that some of his own work has shown that dolphin cortex is differentiated.

 


 
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