The function of ischial callosities

Abstract

Ischial callosities are generally related to the sitting positions of cercopithecoid monkeys, gibbons, and siamangs, but the selective advantage of ischial callosities to the sitting postures of these animals is uncertain. It is suggested that ischial callosities are adaptations that evolved for comfortable and stable sitting on thin branches during feeding in the peripheral branch zone. It is further suggested that prehensile tails in larger New World monkeys also developed as mechanisms for exploitation of food resources found among terminal branches. Small platyrrhine monkeys do not possess any structural adaptations to peripheral branch feeding because their body size permits them to efficiently exploit small or large branches for food. Pongids are in the process of losing their ischial callosities because their large body size precludes sitting on thin branches.