Modern cetaceans broadly form two distinct groups: baleen whales, which include humpbacks, blue whales and other krill-feeders, and toothed whales such as orcas, dolphins and sperm whales. There are several clear differences in their biology and lifestyles which are reflected in their anatomy. One example of this is that the skulls of toothed whales are asymmetrical. For many years this has been attributed to their use of echolocation in hunting and migrations, but it is now known that archaeocetes – ancestors to both groups of modern cetaceans – also had asymmetrical skulls. This means the asymmetry pre-dates the evolution of echolocation and the ability to produce the necessary high-frequency sounds. Instead, it has been suggested that asymmetry evolved in line with an increasingly aquatic lifestyle, enabling the directional detection of sound (but not echolocation, which requires the production and subsequent detection and processing of self-emitted sound) underwater. The ability to tell from which direction a sound originates is found in many terrestrial mammals using the difference between detection in each ear. This relies upon the difference in density between flesh and bone and the surrounding air (acoustic impedance). However, sound travels faster in water than in air, and density changes are decreased, so this difference is much more difficult to detect, particularly for high-frequency noise. One way to combat this could be cranial asymmetry, in order to increase the difference between ears and making complex sounds easier to interpret. Owls also use acoustic asymmetry to help accurately locate prey in the dark. Interestingly, this finding also suggests that baleen whales are likely to have secondarily evolved (recovered) cranial symmetry after it had been lost.
Archaeocetes: Ancestors to modern whales and dolphins which lived between 55 and 23 million years ago.
Cetaceans: A group of aquatic mammals including whales, dolphins and porpoises.