Dolphins, whales and porpoises have extraordinarily small balance organs, and scientists have long wondered why.
Now a study at Washington University School of Medicine in St. Louis has contradicted a leading theory, which held that the animals moved their heads so vigorously that they had to have smaller, less responsive balance organs to avoid overwhelming their senses.
Working with a Midwestern zoo and a local rancher, the researchers, led by Timothy E. Hullar, MD, a Washington University ear, nose and throat specialist at Barnes-Jewish and St. Louis Children's hospitals, directly measured the head movements of dolphins and compared them with those of a closely related land animal a rodeo bull. Cattle have much larger balance organs than dolphins, yet the tests showed that both species had similar head motions.
The findings will be published in the April issue of the Journal of Experimental Biology. Hullar says the results deepen our understanding of the role of balance systems, including those of people.
Much of an animal's or person's balance is controlled by the semicircular canals located in the inner ear. Even though a bottlenose dolphin is about 8 feet long, its semicircular canals are as tiny as those of the average mouse, an animal that could comfortably ride on the tip of the dolphin's nose.
"About 35 million years ago, the ancestors of whales and dolphins went from a terrestrial habitat to an aquatic habitat," says Hullar, assistant professor of otolaryngology and of anatomy and neurobiology. "During this evolutionary process, their semicircular canals got smaller and smaller. The scientific thinking has been that since the canals measure head motion, something must have changed a lot in how these animals move their heads."
Hullar points out that the general trend is for vertebrate semicircular canals to be proportional to body size. Since dolphin canals are so much smaller than th
|Contact: Judy Martin|
Washington University in St. Louis