Callosity

Ischial callosity on Sulawesi crested macaque Macaca nigra

When occurring on an animal's buttocks, as with baboons, they are specifically called ischial callosities.[1] Ischial relates to the ischium: it forms the lower and back part of the hip bone.

The pads 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.[2]

Callosities on a northern right whale

In whales, the term callosity refers to the rough, calcified skin patches found on the heads of the three species of right whales. These callosities are a characteristic feature of the whale genus Eubalaena; 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, whale barnacles and parasitic worms which reside on them.[3][4] Young whales and diseased individuals are often infested with a different species of cyamid, which gives an orange hue rather than white on these whales.[5] 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.

Callosities are found on the upper surface of the whale's head, above the eyes, on the jawline and chin and surrounding the blowhole.[5] Callosities form a unique pattern on every right whale and though callosities which are overgrown break off, the patterns do not change over a lifetime. This makes them a very useful tool for the purposes of photo-identification and conservation.[3]

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 and it has been suggested by Payne & Dorsey (1983) that this is a sexually dimorphic feature used for intra-specific sexual aggression.[6] This explanation is not entirely satisfactory, as it does not account for the appearance of callosities in females. Alternative natural role of callosities has been considered as being optional tools against predators, and declines of right whales might have consequenced in declines of barnacles as well.[7]

• Callus

• Steudel, K (1981). "Functional Aspects of Primate Pelvic Structure: A Multivariate Approach" (PDF). American Journal of Physical Anthropology. 55 (3): 399–410. doi:10.1002/ajpa.1330550314. PMID 6791507.
• 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
• "On Butts and Baboons". Artsibasheva, A. http://monkeybuiznezz.wordpress.com/2012/09/27/on-butts-and-baboons/