The Waterside Ape by Peter H. Rhys Evans

Author:Peter H. Rhys Evans
Language: eng
Format: epub
Publisher: CRC Press

FIGURE 11.4 Toothed whale forehead melon for echolocation. (Courtesy of Miles Away Photography/Shutterstock.)

By contrast, the baleen whale can vocalise 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.

Semi-aquatic mammals, on the other hand, still need air-transmitted sound reception on land but also require protection of the tympanic apparatus when underwater. The hippopotamus can stay underwater for about 15 minutes, and closure of its ear canal is achieved by contraction and angulating the ear canal backwards. The platypus achieves this with a skin fold, and the desman, by a glandular swelling.

Hooded seals show an interesting model of evolutionary adaptation in that they have broad-based exostoses in the floor of the external canal lateral to the tympanic membrane. These exostoses, along with cavernous tissue in the middle ear, allow the seal to dive to depths of greater than 1000 metres at pressures of 100 atmospheres without damage to the middle ear or tympanic membrane.28 Clearly, the presence of external auditory canal exostoses confers a selective advantage for hooded seals in the marine environment. And so it might have done in hominids.

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