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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.


Ischial callosity on Sulawesi crested macaque Macaca nigra

All Old World monkeys, gibbons, and some chimpanzees have pads on their rears known as ischial callosities.[1][2] 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.[3]

Right whales[edit]

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.[4][5] 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.[6] 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.[citation needed]

Callosities are found on the upper surface of the whale's head, above the eyes, on the jawline and chin and surrounding the blowholes.[6] 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.[4]

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 they are a sexually dimorphic feature, used for intra-specific sexual aggression.[7] That explanation is not entirely satisfactory, because it does not account for the appearance of callosities in females.[citation needed] It has been proposed that the barnacles attached to callosities are important in helping fend off attacks by orcas.[8]

See also[edit]


  1. ^ "Ischial callosities". MonkeyBuiznezz. Retrieved 2014-02-12.
  2. ^ Fleagle, John G. (2013). Primate adaptation and evolution (3rd ed.). Amsterdam: Elsevier/Academic Press. ISBN 9780123786326. OCLC 820107187.
  3. ^ Steudel (1981), p 399
  4. ^ a b "Callosities". New England Aquarium. Archived from the original on 2014-11-22. Retrieved 24 August 2014.
  5. ^ Ward, Paul (2001). "Right whales". Cool Antarctica. Retrieved 24 August 2014.
  6. ^ a b "Right Whale Research". Center for Coastal Studies, Provincetown, MA. Retrieved 24 August 2014.
  7. ^ Payne, Roger & Eleanor M. Dorsey (1983). "Sexual dimorphism and aggressive use of callosities in right whales (Eubalaena australis)". Communication and Behaviour of Whales (PDF). pp. 295–329. Retrieved 24 August 2014.
  8. ^ Hayashi, Ryota. "Past biodiversity: Historical Japanese illustrations document the distribution of whales and their epibiotic barnacles" (PDF). University of Tokyo. Archived from the original (PDF) on 2016-02-14. Retrieved 2021-06-28.