Evolutionary Study of Ear Structures of Whales                  return to Zhe-Xi Luo's main page

The spectacular evolutionary radiation of whales can be attributed, at least in part, to their specialization in underwater hearing. Because underwater environments can greatly limit vision (eyes) and olfaction (nose), whales rely more on hearing (ears) for communication, navigation, and feeding. The two groups of modern whales have very different adaptations to underwater hearing.

Toothed whales (Odontocetes) have echolocation (ultrasonic sonar). This involves generating a high-frequency clicking sound from the nasal passage, beaming the sound through the melon in the forehead, and then receiving the echo of the high frequency sound from the environment. Echolocation and high-frequency hearing are crucial for sensory perception in toothed whales. As a part of this adaptation, numerous structural specializations are developed in the inner ear and in the petrosal and the tympanic bone.

By contrast, baleen whales (Mysticetes) have no echolocation but can hear very low-frequency sounds. Low frequency sound has greater penetrating power underwater and wider scatter, allowing the baleen whales to communicate over long distances and wider geographic areas.

Figure 9: Inner-ear endocast of a pygmy sperm whale generated by 3-D graphic reconstruction from serial sections (Luo and Marsh, 1996). Abbv.: co—cochlea (light and dark blue); end—endolymphatic duct (green); fr—fenestra rotunda (white with green outline); fv—fenestra vestibuli (white with green outline); lsc—lateral semicircular canal (yellow); v vestibule (red).

To understand the evolutionary history of such different hearing specializations in Odontocetes and Mysticetes, it is necessary to establish the phylogenetic transformation of the ear structures in early odontocetes and early Mysticetes, as well as in the archaic archaeocete whales. This research attempts to reconstruct the anatomical structures of the inner ear (Fig. 9) and to map the systematic distribution of characteristics of the petrosal and the tympanic. The objectives are to establish the pattern of evolution of the inner ear structures, both for high-frequency hearing in toothed whales, and for the low-frequency hearing in baleen whales, and to use the morphological information of the middle ear for inference of phylogeny.

CMNH Research | Carnegie's Dinosaurs | CMNH Home