Sperm Whales and Killer Whales with the Largest Brains on Earth Show Extreme Differences in Cerebellum
Whales intrigue people. Killer whales draw crowds to marine parks. They are often featured in nature films that extol their intelligence, sound repertoire, and hunting skills. Large sperm whales are champion divers in ocean depths. Due to depictions such as in Melville's Moby Dick, sperm whales are regarded as large, fearsome, powerful, and intelligent creatures. These two toothed whale species have the widest distribution in the world's ocean. Both use echolocation. They both are long lived and have the longest periods of gestation among whales. Sperm whales dive much deeper and much longer than killer whales. It has long been thought that sperm whales have the largest brains of all living things. Our brain mass evidence, from published sources and our own specimens, shows that big males of these two species share the distinction of having the largest brains on earth. However, we also find that cerebellum size is very different between killer whales and sperm whales. The sperm whale cerebellum is only about 7% of total brain mass, while the killer whale cerebellum is almost 14%. These results are significant because they contradict claims that the cerebellum scales proportionally with the rest of the brain in all mammals. It also corrects the generalization that all cetaceans have enlarged cerebella. We also suggest reasons that such a large cerebellar size difference exists in these two species. Cerebellar function is not fully understood, and comparing the abilities of animals with different size cerebella can help uncover functional roles of the cerebellum in humans and animals. Here we show that the large cerebellar difference likely relates to evolutionary history, diving, sensory capability, and ecology.
We thank Raymond Tarpley for discussions and help with initial measurements of some specimens. Kevin Carlin helped with statistical analysis. Anonymous reviewers made helpful suggestions to improve the manuscript. We were aided by several people in obtaining specimens for measurements including Lanny Cornell, Brad Andrews, James McBain, Tom Reidarson, Les Dalton, Alexander Costidis, and Tag Gornall. We thank Brent Whittaker of the National Aquarium and the family of Dr. Myron Jacobs for giving us archival pictures from Dr. Jacobs' work so that we could measure unpublished sperm whale photographs containing scale markers. This work received support from the U.S. Navy Marine Mammal Program, the Office of Naval Research and Sea World.
* Presenting author