S ound is a way of life for whales. Since these great mammals left the land and took to the seas millions of years ago, their survival has been dependent on the evolution of ear structures that are highly specialized for underwater hearing. The dark abyss of the ocean limits eyesight, and water dilutes scent. But sound travels faster and covers greater distances in water than it does in air. So whales must rely primarily on sound for sensory perception, as well as for communication. Sound, therefore, has a profound influence on navigation, feeding, socialization, breeding and other whale behavior.
Extant whales, or cetaceans, fall into two groups—toothed whales, or odontocetes, and baleen whales, or mysticetes. Toothed whales are fast swimmers and hunters, and include porpoises, dolphins, killer whales and sperm whales. Baleen whales are filter-feeders that live on zooplanktons and include the blue whale, the humpback whale, the Gray whale and the bowhead whale.
Among the many differences between toothed and baleen whales is their specialization for underwater hearing.
Long before humans began using sonar for navigation, toothed whales were using this method to "see" their environment. Also known as echolocation, the toothed whale's biological sonar involves vocalization from the nose, and the reception of the echo in the ear. The whale recycles the air in its complex nasal passage, and produces a high-pitched sound that is beamed out to the environment through the melon, an oil body in its bulging forehead. The sound beamed from the melon into the environment is bounced back to the whale, providing information that helps the animal to accurately decipher its environment.
The high-pitched, or ultra-sonic, sound emitted by the whale offers a finer resolution for the echolocation and results in a more accurate perception of its environment. For the ultra-sonic sound to serve toothed whales, their ears developed many structures specialized for hearing high-pitched sounds.
By contrast, the baleen whale can vocalize and hear very low-pitched, or infrasonic sound, which can travel great distances and scatter to large areas in water. With infrasonic sound, baleen whales can communicate with each other over geographic areas as large as an ocean basin.
These whales are famous for their rich repertoire of songs. The eerie melodies are an integral part of the social fabric of baleen whales, allowing the groups to stay in contact with each other over vast expanses of ocean.
How did toothed and baleen whales evolve these entirely different hearing adaptations? The extensive fossil record of extinct whales could have the answer to this intriguing question.
The ear bones of all whales are extremely hard and dense, and they are common in whale fossils found in marine sediments. The internal bony structures related to hearing functions are well protected by the dense and hard ear bones, and are often preserved in exquisite condition. The fossilized bony ear structures can provide evidence for the underwater hearing in the extinct whales and offer a window for paleontologists to look into the evolutionary history of whales' ears. The knowledge about the evolutionary origins of their remarkable hearing adaptations would enrich our understanding of why sound is so crucial for the whale's navigation, feeding and social behavior.
Computer Graphics Aid Research in Whale Evolution
The advent of new technology, such as CT-scanning and computer graphics, has opened up new possibilities to study the ears of fossil whales. The internal structures of the ear bones can be revealed by CT-scanning (computerized tomography, or x-ray) or by physical sectioning, which involves cutting the fossil for study. The anatomical features observed by scanning or sectioning can be converted to computer graphics. By way of computer graphics, the vital statistics of some of the ear structures can be more easily established. Once we have more accurate information on some of their bony structures, such as size and proportion, we can estimate the range of hearing for the extinct whales.
Using these techniques, my research collaborators and I are able to show that the earliest fossil toothed whales (present 28 million years ago in Late Oligocene but extinct by the Late Miocene 13 million years later) already had some bony structures for hearing high-pitched sound. This discovery showed that whales were capable of echolocation, as least to some extent. Yet the degree of development of the specialized structure in early and archaic toothed whales was far less advanced than in the modern families of toothed whales that originated in the Miocene and are still around today. This suggests that, although echolocation may have originated with the early toothed whales, it was not nearly as advanced or sophisticated as it is in modern toothed whales. The extremely sensitive sonar of modern whales most likely developed when the radiation, or spreading, of the modern families of toothed whales occurred in the Miocene.
On the other hand, the early fossil baleen whales have retained the primitive ear structures of their distant and archaic relatives. It is possible that the baleen whale's capacity for hearing infrasonic sound is inherited from archaic whales or hoofed mammals.
In light of what we have learned from the ear structures of fossil whales thus far, it is likely that the divergence of ultrasonic echolocation of toothed whales and the infrasonic hearing and communication of baleen whales occurred after the toothed and baleen whales split from each other in their evolution.
Although this research is still in progress, we already have learned some interesting things about how ancient whales heard. With the ongoing accumulation of more scientific information from fossil whales, we will get a better picture of the evolution that has given rise to the spectacular and intriguing hearing abilities of modern whales.
Zhexi Luo is assistant curator of Vertebrate Paleontology at Carnegie
Museum of Natural History.