As you may be aware, dolphins are able to use a special kind of sonar called echolocation or biosonar. In fact, all toothed cetaceans, that is – all of the whales, dolphins and porpoises that have teeth – are able to echolocate. Echolocation is the primary sense for most of these species; more important even than vision. And, if you think about it, that makes a lot of sense. You don't have to dive very deep in the ocean until light levels all but disappear. Many cetaceans live and hunt for food in a pitch-black environment. But, how does echolocation work? Well, would you be shocked to learn that dolphins echolocate by slapping their nostrils together? I thought so. However, I think this statement needs a bit of clarification.
Here's a quick overview of the echolocation process for dolphins. A dolphin is able to produce click sounds, which are sent out into the water. Once these sounds hit an object, echoes are created; the dolphin then listens to these echoes and is able to form a kind of mental image of the object. But, how do nostrils fit into this process?
Well to answer that question, I'll provide a not-so-quick overview of the echolocation process: A dolphin produces these click sounds using a structure in its head called the phonic or sonic lips. Humans, like nearly all mammals, produce sounds using their vocal cords. Dolphins do not have functional vocal cords; what's left of their vocal cords, called vocal folds, lost their ability to produce sound millions of years ago during their evolution from land animals. Instead, these phonic lips were evolved from what was once the dolphin's nose. Evolution has provided dolphins with a single opening at the top of its head through which it breathes – this opening is called a blowhole. The phonic lips (the former nostrils) are tucked just underneath the blowhole in the nasal cavity. By sending pressurized air past these lip-like structures, they are sent into vibration, and click sounds are produced. There are a series of nasal sacs in the dolphin's head that allows them to shuttle air back and forth across the phonic lips. Scientists studying dolphin echolocation were, for many decades, completely baffled as to how a dolphin managed to produce these clicks. No-one was sure exactly where in the dolphin's head these clicks were originating. The scientists thought possibly the clicks came from down in the larynx, in the nasal cavity, or maybe even from their blowhole. Thanks to a few relatively recent studies, scientists are now reasonably sure that the phonic lips are the source of clicks, although it is still a mystery as to exactly how pushing air across these lips results in the clicks themselves. Our best guess is that the lips (the former nostrils) slap against other fatty bodies in the dolphin's nasal cavity, which then transfer the sounds through the dolphin's head and out into the water. Since dolphins have two sets of phonic lips (having evolved from each of the two nostrils), they are able to produce two sets of click sounds simultaneously. This means that they can produce two sets of click sounds simultaneously, as well as whistle sounds which are produced in the larynx. Dolphins are great multi-taskers when it comes to sound production!
Click sounds are very short in duration – between 40 and 70 microseconds, but they can be very very loud; around 220 decibels for bottlenose dolphins. Dolphins usually produce clicks in a rapid series called a 'click train'. These click trains can consist of hundreds, sometimes even thousands of clicks per second. Here is an example of a dolphin's echolocation click train. (Play Sound) These click sounds contain very high frequencies – some of them well above the range of human hearing, above 120 kHz. You can learn more about dolphin hearing range in a previous episode of The Dolphin Pod called So High it Hertz. Although high frequencies don't travel as far as low frequencies, these high frequencies with very short wavelengths allow a dolphin to echolocate on small objects and pick out fine detail – the higher the frequencies, the better the detail. This allows a dolphin to locate and track tiny prey species. The sperm whale, a toothed cetacean that is also able to echolocate, relies on its echolocation during deep dives into pitch black waters in order to locate and track much larger prey. The sperm whale can use its louder, lower frequency echolocation clicks to locate giant squid and other prey over long distances – possibly even several kilometers.
But, back to dolphin echolocation: click sounds produced by a dolphin are directed out through the front of the dolphin's head. They first pass through special fatty tissue called the melon. This is that lump you see at the front of a dolphin's head that looks like a big rounded forehead. The melon is filled with a kind of lipid called acoustic fat, which has the same density as seawater. The dolphin can change the shape of her melon as the click sounds pass through it – in this manner, the melon acts as an acoustic lens: the click sounds are formed into a kind of cone-shaped beam that extends out in front of the dolphin. This is very loosely a bit like a flashlight beam. The dolphin can direct this beam of sound toward objects that it is investigating, like a human diver, for instance. As each of the clicks hits the diver and bounces off, a click echo is produced, which then heads back toward the dolphin. A dolphin actually receives sound through its lower jaw. A dolphin's jaw is filled with the same kind of acoustic fat that is found in the melon; this allows for sounds to be transmitted up the jaw and toward the dolphin's middle ear.
The echolocation process – sending out clicks and listening to the click echoes – is what produces a kind of mental image of the object that a dolphin is investigating with clicks. We know that the changes in the structure of the click echoes are what a dolphin uses to form this mental image, although it is still an unsolved mystery exactly how they manage to accomplish it. This echolocation 'image' is unlikely to be something that a human being could imagine simply because people can't echolocate. But, this "mental image" is currently the best analogy we've got. Scientists have learned from experiments with dolphin echolocation that their acoustic image is quite detailed, and allows a dolphin to do some pretty amazing things.
Some experiments into what is called cross-modal matching have revealed that dolphins are able to identify an object using vision that they had previously only been able to learn about using echolocation, and vice-versa. Cross-modal matching is something you can test for in humans, too – you can try it yourself, here's how. Blindfold your friend and give them an object to inspect with their hands, like an orange. Your friend will then be able to form a kind of mental image of the orange using the tactile sensory information sent to their brain from their hands. Now, take the orange away from them and remove the blindfold. Hold up both the orange, and another object – like a spoon, and ask your friend which object they were just holding. Your friend will likely be able to say that it was the orange. Even though they never saw the orange, they formed a kind of mental image of the orange using the information from another sense or modality (touch, in the case of the orange). Dolphins can do something just like this, but across the senses of vision and echolocation.
What's unique and handy about echolocation is that a dolphin can use it to sense the density of objects, as well as discriminate between objects of differing compositions. Echolocation clicks can penetrate soft structures like the sand … and maybe even the diver's body! This is about as close to X-ray vision as any animal is going to get!
If you have ever had the chance to swim with dolphins, you might have been able to feel a dolphin's echolocation on your skin. For species like bottlenose dolphins, you can usually hear their echolocation underwater – but not always. Sometimes the clicks they use are too high in frequency to hear. You can usually tell when a dolphin is echolocating, however. They often move their heads slowly back and forth as they scan with their echolocation. This is called 'head-scanning' – as they change their head position, the click echoes also change structure, which helps the dolphin to get an image of what it is looking at. Or, do I mean listening at?
So, there you have it, A not so quick overview of dolphin echolocation. There is, however, much more to be said about this subject, and we will explore this amazing dolphin sense in more detail in upcoming episodes of The Dolphin Pod. In the meantime, you can impress your friends by telling them that dolphins produce echolocation clicks with their nostrils. Surely that is a useful bit of information for your next cocktail party.