The Dolphin’s Vestibular Sense

Space. The final frontier. Only the bravest of us dare to venture into the cold darkness of space. Those intrepid, courageous men and women, who have voyaged beyond earth’s atmosphere, have doubtless been subjected to a surge of powerful emotions as they gaze upon planet earth from such a great height. These emotions can only be described as…. nausea, dizziness, queasiness, disorientation, sickness, wobbliness, lightheadedness, wooziness, and are often accompanied by … vomiting!

I am talking about what astronauts often refer to as Space Sickness. NASA occasionally trains its astronauts in special aircraft that free fall toward the earth, providing the occupants of the aircraft with a brief, zero gravity environment. This type of aircraft is often referred to as The Vomit Comet. Being in a weightless environment can cause some people an awful lot of distress – even with just brief exposure. Roller coasters and similar amusement park rides have been known to produce spectacularly disturbing stories of hapless guests who lost their lunch while in mid-ride.

What could be the reason for all of this nausea? And, what does it have to do with dolphins? Well, this is something we should look at more carefully. (Picard audio – obviously you think this is something we should look at more carefully) Yes… didn’t I just say that? (Picard audio – proceed) yes, thank you Captain Picard. Space Sickness and the other kinds of motion sickness that I have described are caused by disruption to one of the most important, but often overlooked, senses that all animals posses: the vestibular sense. Have you ever heard of it? Maybe not, but I guarantee that it is of utmost importance to your everyday life. Understanding what the vestibular sense is will help to explain some of the most important evolutionary changes that took place in the anatomy of dolphins-changes that have been vital to their ability to adapt to an aquatic, essentially zero gravity lifestyle.

The vestibular sense is basically your sense of balance, although that is a rather simple description – the vestibular system is responsible for a whole variety of important vital functions. The tissues and organs that comprise the vestibular sense are located in the inner ear. Within the inner ear are a series of tube like organs called the semi-circular canals and a second set of organs called the otoliths. These organs are filled with sensory cells that respond to movement. The semi-circular canals are able to sense the direction in which your head rotates, whereas the otoltiths sense movement in a linear direction in relation to gravity. As these organs sense movement, they are constantly sending signals to your brain and muscles throughout your body that allow you to make instantaneous, subconscious corrections to your body position so that you stay balanced and upright in 3D space. Your brain is also able to sense where different parts of your body are located, an ability termed proprioception, by working in conjunction with the vestibular sense; thus. This allows humans to perform death-defying high wire acts, as well as accomplish very basic things like walking or standing. Essentially, the vestibular sense makes your body very sensitive to where your head is positioned in relation to gravity, and therefore allows you to be aware of important and potentially deadly objects like the ground. Having a well developed vestibular sense is what allows many primates to climb around in trees with such ease; being awkward and off-balance when 50 feet up in a tree could lead to a nasty fall.

Another important job of the vestibular sense is to coordinate the movement of your head with the movement of your eyes. Without a close connection between eye and head movements, you would get a view of the world that was a lot like a shaky camera – a Blair Witch view of the world. Try bobbing your head back and forth as fast as you can. You will notice that your eyes are very able to sense these movements and to keep themselves focused on a single point if required. This is thanks to the vestibulo-ocular reflex. To see what happens when your eyes are subject to movement that is not involving your head, in order to give yourself that Blair Witch feeling, try the following experiment. Close one eye. Now very gently touch the side of your open eye – not on the eyeball, but on the side of your eye where your eyelid is. Very very gently push on your eyeball a few times. You will see the world shake back and forth – a weird feeling. If you keep this up for too long, you will likely make yourself sick. So, don’t do that. The reason the world jumps around like this is because the movements of your eyeball are no longer in synch with your head movements, and your brain isn’t able to make sense of the visual input. It is receiving conflicting information: your eyes are being moved, but your head is staying still.

If you have damage to your vestibular sense – like if you have an inner ear infection – you may experience dizziness and nausea. This is because the vestibulo-ocular reflex may be broken or malfunctioning, or because the other parts of the vestibular sense are no longer receiving useful information about the position of your head in relation to motion and gravity. This is the exact same kind of problem that leads to Space Sickness, and the reason that people throw up on the Vomit Comet or on roller coasters. Without the normal body-initiated movements in relation to gravity that humans usually make, the vestibular system is receiving motion information that might not be similar to the visual information it is receiving from the eyes. With this simple mismatch, the body and the brain get confused, and you get dizzy. This is the same kind of thing that makes people seasick.

Dolphins spend all of their lives in an aquatic environment. Since floating in water eliminates much of the influence of gravity, this is very similar to spending their whole lives on the space shuttle. We know that NASA astronauts will get sick both when they first start spending time in zero gravity and after they come back to earth. The body takes time to adjust to weightlessness. But the dolphin vestibular system has evolved to cope with a 100% aquatic, and virtually weightless, existence. In 2002, Fred Spoor and his colleagues wrote a scientific article that appeared in the journal Nature and offered a theory as to how the land mammal ancestors of dolphins may have evolved into aquatic mammals -it was all about the vestibular sense. They discuss how changes in the size of the semi-circular canals made it possible for land animals to spend a greater amount of time in water without making themselves seasick. Current dolphin species have semi circular canals that are 3 times smaller than the average land mammal, meaning that they are much less reliant on gravitational information to tell them where their bodies are in space. This allows them to execute stomach churning barrel rolls, twists and turns under water, without making themselves sick. Unlike a human, who pretty much needs to know where the ground is 100% of the time, dolphins are free to float around with impunity.

There is much to be said about the vestibular sense and there are some great articles online that can provide you with more information. I hope this presentation satisfies your initial curiosity!(Picard audio – well I’m not satisfied). OK…(Picard audio – I do have some questions for you, however) Ok – well hold on – let me sum it up again. The vestibular sense has evolved in dolphins, leaving them less susceptible to the kind of dizziness that affects humans in zero gravity. When the vestibular sense of humans malfunctions, either because of damage, disease or conflicting information from the environment (like when we are in zero gravity), we get sick. (Picard – and you’re telling me that there are no explanations for these malfunctions). Well, like I said, the sense can malfunction when the information the brain is getting from the eyes and the vestibular sense doesn’t match up. (Picard audio – that was not my question – what caused the malfunctions) As I just explained, either damage to the organs of the vestibular sense, disease, or conflicting environmental information. (Picard audio – and you don’t intend to back off from that position). Well – no, I mean… that’s just the way it is Captain Picard. . (Picard audio – hmmm but it all seems so vague

 A fantastic website with a detailed overview of the vestibular sense: The vestibular sense