Posture, Ischial Tuberosities, and Tree Zone Use in West African Cercopithecids

Abstract

Ischial callosities are present in all cercopithecoid primates and hylobatids while callosity-like structures are occasionally found in gorillas, chimpanzees and orangutans. The size and shape of calloused sitting pads and the underlying tuberosities vary among cercopithecoids and multiple explanations for this variation have been proposed. We examined the functional context of ischial callosity use in three colobine and four cercopithecine species ranging in the Ivory Coast’s Taï forest. Innominates of 96 individuals were collected opportunistically and tuberosity size was determined by digitizing the outline of each bone and calculating the circumscribed area. The square root of this dimension was scaled with acetabular diameter to create a species index which was compared to several behavioral variables. Data from the field and laboratory indicate that the largest tuberosities are found in monkeys that feed most often from seated postures in the small branch milieu of tree crowns. Our results provide strong support for Vilensky’s (1978) hypothesis that ischial callosities are adaptations for exploiting the terminal branch niche.