Ischial callosities on a baboon

A callosity is another name for callus, a piece of skin that has become thickened as a result of repeated contact and friction.

Monkeys [link]

When occurring on an animal's buttocks, as with baboons, they are specifically called ischial callosities. They enable the monkeys to sleep sitting upright on thin branches, beyond reach of predators, without falling.

The ischial callosities are one of the most distinctive pelvic features which separates Old World monkeys from New World monkeys.^[1]

Right whales [link]

Callosities on a Southern Right Whale

Otherwise, the word callosity is generally reserved for describing the calluses found on the head of the three species of right whales.

Callosities are a characteristic feature of the Eubalaena genus of whales; because they are found on the head of the whale and appear white against the dark background of the whale's skin, they make it very easy to identify these species. The callosities themselves are grey; the white appearance is due to large colonies of whale lice around them. Callosities arise naturally and are present even in late-term whale fetuses, although the work of lice digging into the surface of the skin may make them more jagged and hard over time.

The evolutionary purpose of callosities is unknown. Male right whales have a higher density of callosities than females. Males have been observed scratching one another with their callosities, so they may play a role in sexual selection. This explanation is not entirely satisfactory, as it does not account for the appearance of callosities in females.^{[citation needed]}

Callosities form a unique pattern on every right whale. This makes them an extremely useful tool for the purposes of photo-identification and conservation.

See also [link]

• Callus

Notes [link]

References [link]

• Steudel, K (1981). "Functional Aspects of Primate Pelvic Structure: A Multivariate Approach" (PDF). American Journal of Physical Anthropology 55 (55): 399–410. DOI:10.1002/ajpa.1330550314. PMID 6791507. https://www.zoology.wisc.edu/labs/hll/pdf/Steudel%201981%20functional%20aspects%20of%20pelvic%20structure.pdf. 
• Callosities by Mason T. Weinrich in the Encyclopedia of Marine Mammals. ISBN 0-12-551340-2.
• A Dictionary of Zoology 1999, Oxford University Press 1999

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