ABSTRACT. As quantitative studies on primate positional behavior accumulate the lack of a standard positional mode terminology is becoming an increasingly serious deficiency. Inconsistent use of traditional terms and inappropriate conflation of mode categories hamper interspecific and interobserver comparisons. Some workers use common terms without definition, allowing at least the possibility of misunderstanding. Other researchers coin neologisms tailored to their study species and not clearly enough defined to allow application to other species. Such neologisms may overlap, may completely encompass, or may conflate previously defined labels. The result is, at best, the proliferation of synonyms and, at worst, the creation of confusion where clarity had existed. Historical precedents have sometimes resulted in "catch-all" terms that conflate any number of kinematically different behaviors (e.g. "brachiation," "climbing," and "quadrumanous climbing"). We recognize three areas where distinction of positional modes has some current importance: (1) Modes that require humeral abduction should be distinguished from adducted behaviors; (2) locomotor modes that involve ascent or descent should be distinguished from horizontal locomotor modes; and (3) suspensory modes should be distinguished from supported modes. We recommend a nomenclature that is not dedicated to or derived from any one taxonomic subset of the primates. Here we define 32 primate positional modes, divided more finely into 52 postural sub-modes and 74 locomotor sub-modes.

Observational data were collected on the positional behavior of habituated adult female orangutans in the rain forest of the Kutai National Park, East Kalimantan, Indonesia. Results revealed the following about locomotion during travel: movement was concentrated in the understory and lower main canopy; and brachiation (without grasping by the feet) accounted for 11% of travel distance, quadrupedalism for 12%, vertical climbing for 18%, tree-swaying for 7%, and clambering for 51%. In climbing and clambering, the animal was orthograde and employed forelimb suspension with a mixture of hindlimb suspension and support. Thus suspension by the forelimbs occurred in at least 80% of travel. Locomotion in feeding trees resembled that during travel but with more climbing and less brachiation. Feeding was distributed more evenly among canopy levels than was travel, and use of postures (by time) included sitting 50%, suspension with the body vertical 11%, and suspension by hand and foot with the body horizontal 36%. The traditional explanation of the evolution of the distinctive hominoid postcranium stresses brachiation. More recently it has been proposed that climbing, broadly defined and partly equivalent to clambering in this study, is the most significant behavior selecting for morphology. The biomechanical similarity of brachiation and the orthograde clambering of orangutans precludes the present results from resolving the issue for the evolution of Pongo. The orangutan is by far the largest mammal that travels in forest canopy, and a consideration of the ways that its positional behavior solves problems posed by habitat structure, particularly the tapering of branches and gaps between trees, indicates that suspensory capacities have been essential in permitting the evolution and maintenance of its great body size.

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.

Field observations demonstrate clear differences in locomotion and feeding postures between spider monkeys (Ateles) and howling monkeys (Alouatta). When feeding, Ateles employs sitting postures approximately half the time, and a variety of suspensory postures using the tail the other half. Ateles moves quadrupedally during 52% of locomotion, by tail-arm suspension 25 %, and various mixed support-suspensory modes the remainder. Tail-arm suspension is practiced more rapidly on thinner supports, and on more negatively inclined supports than is quadrupedal movement. Howlers do not locomote by tail-arm suspension: movement is almost entirely quadrupedal and is slower than that of spider monkeys. The positional behavior of spider monkeys fits closely recent views of major adaptive changes in hominoid evolution emphasizing brachiation and speed during travel. Howler locomotion and also tissue composition appear related to diet and digestive mechanisms.

A fundamentally new psychology related to the ability to conceive of limited aspects of the self may have evolved in the ancestor of the great ape/human clade. Existing models of the evolution of primate intelligence do not provide an adequate explanation of the apparent restriction of this phenomenon. We propose that the capacity for self-conception evolved as a psychological mechanism enabling large-bodied, highly arboreal apes to cope with problems posed by the need to negotiate their way through a habitat that was fragile due to their body size. First, we briefly outline the case for believing that a new psychology related to self-conception evolved during the Miocene in the ancestor of the great ape/human clade. Next, we examine the existing models of the evolution of primate intelligence and assess their ability to account for the evolution of self-conception. Finally, we offer details of our alternative model, along with a series of predictions that can be derived from it.