Positional behavior and body size of arboreal primates: A theoretical framework for field studies and an illustration of its application

Abstract

The rationale for most field studies of the positional behavior of arboreal primates has been the need to document natural behaviors quantitatively in order to infer the functional significance of morphological configurations. This focus on interactions of morphology with behavior is justifiable, but there exists another important level of biological relationships, that of the animal with its structural habitat, which it must negotiate to find food and avoid being preyed on. Recently it has become apparent that body size is likely to affect relationships of positional behavior with habitat structure, as well as with morphology. Here I offer a framework for research on functional relationships of positional behavior, body size, and habitat structure, with the ultimate objective of elucidating the aptive significance of the great diversity exhibited by arboreal primates. This approach specifies several distinct problems that animals solve, and indicates how research might be directed at revealing the relative effectiveness with which different primates solve them. A preliminary application of the framework examines sympatric north Sumatran primates.

Orangutans employ unique strategies to control branch flexibility

“…frequencies of orangutan suspensory locomotion also fail to follow predictions from body mass, which may reflect the existence of arboreal pathways that all individuals attempt to follow (10,12). In this study, although each age-sex group utilizes a significantly different mean SS (Fig.…”

“…Classic predictions regarding suspension also suggest that larger animals and larger species should suspend more than their smaller counterparts when support size is controlled and should otherwise use larger supports (5,10). In the other apes, the prediction is supported in gibbons and siamangs (27) but not in intraspecific studies of lowland gorillas (20) or interspecific studies of gibbons and chimpanzees (27).…”

“…This allows orangutans to maximize stability by using their long prehensile toes to grip multiple small branches and a hand to further aid balance, while freeing the remaining hand to reach out for feeding or weight transfer. Because the Sumatran orangutan (Pongo abelii) is the largest habitually arboreal primate (10) and has one of the broadest repertoires of positional behavior of all of the primates [consisting of Ͼ100 biomechanically distinct postural and locomotor behaviors (11)], we hypothesize that this repertoire includes unique locomotor strategies for dealing with the flexibility of supports during arboreal travel. However, we expect that thresholds must exist below which supports become too compliant for safe locomotion.…”

Functional Anatomy and Adaptation of Male Gorillas (Gorilla gorilla gorilla) With Comparison to Male Orangutans (Pongo pygmaeus)

“…Adult male orangutans, the largest-bodied primate to inhabit the canopy, orient their heavy torso in a variety of angles above or below supports (Figure 9). Their limbs can be recruited to act in nonstereotyped positions relative to the body and to each other (Cant, 1992;Povinelli and Cant, 1995). Joint flexibility is crucial to the variety of postures and modes in feeding, nesting, and moving in trees, including vertically climbing, and suspending (Sugardjito and van Hooff, 1986;Cant, 1987;Thorpe and Crompton, 2006).…”

“…Using their bulk, they sway branches and form bridges to adjacent trees, and use their long and powerful arms and hands to pull in flexible branches to cross gaps. They recruit their prehensile feet to grasp nearby branches and propel themselves through the canopy (Chevalier-Skolnikoff et al, 1982;Sugardjito, 1982;Cant, 1987Cant, , 1992Thorpe and Crompton, 2006). Their cognitive skills assist in assessing the biomechanics of branch flexibility and in assuming the most effective body positions to make their way through the canopy (Povinelli and Cant, 1995).…”