Of all of our six senses, balance is possibly the most underestimated. Lose it and you're in a bad way. Every terrestrial animal depends on three fluid-filled semicircular canals – part of the vestibular system that measures head rotation – buried in their skulls to keep them upright. Timothy Hullar, from the Washington University School of Medicine, explains that the size of an animal's vestibular system is closely related to its body size. However, whales' vestibular systems are much tinier than they should be. According to Hullar, a clinician who specialises in human balance disorders, there are several possible explanations for the whale's tiny balance system. These include Fred Spoor's suggestion, from University College London, that whales may have scaled down their semicircular canals to reduce their sensitivity. Hullar explains that this would allow whales and other cetaceans to extend the performance range of their vestibular systems and allow them to move their heads over large ranges where more sensitive larger vestibular systems would fail. Intrigued by this possibility, Hullar decided to test whether cetaceans move their heads more than similarly sized terrestrial mammals (p. 1175).
Knowing that the nearest modern relatives of cetaceans are two-toed mammals, Hullar realised that cattle would make a good comparison with dolphins. And living in Missouri, he could have access to cattle that are happy to buck their heads on demand: rodeo bulls. Discussing the possibility of getting access to rodeo bulls with a rancher who happened to be one of his patients, the rancher offered to put Hullar in touch with a neighbour who breeds the animals. Having explained to the bull trainer that he wanted to measure how much bulls move their heads, Hullar and his undergraduate student, Benjamin Kandel, drove down to the ranch in deepest Missouri to put a bull through its paces. ‘Working with these animals I grew to have an appreciation for how big and ornery they are,’ remembers Hullar. Carefully strapping three gyroscopes to the bull's horns while it was in the bucking-chute, the ranchers sent it bucking into the arena while Hullar and Kandel filmed the animal from the safety of a tower.
Having recorded the bull's head movements, Hullar had to find some dolphins, which seemed easier said than done in the landlocked Midwest, until he made contact with the Indianapolis Zoo where the dolphin team were very happy to help. Training the dolphins to swim up and down carrying the gyroscope array in their mouths and filming them as they did barrel rolls and fast turns, Hullar and Kandel were ready to find out whether the dolphins' head movements were more extreme than the bulls'.
Back in the lab, Kandel analysed the movies and gyroscope data and realised that instead of being more extreme, the dolphins' head movements were slightly less vigorous than the movements of the bucking bulls. So it seems unlikely that the dolphins scaled down their vestibular systems to decrease their sensitivity and increase the performance range to accommodate large head movements.
Whatever drove cetaceans to downsize their vestibular systems is still a mystery that Hullar is keen to solve. ‘This is such a dramatic change in size that something important must have caused it,’ says Hullar.