Positional behavior in Saimiri boliviensis and Ateles geoffroyi

Positional behavior, associated context, and substrate use in Ateles geoffroyi and Saimiri boliviensis were quantified using instantaneous focal animal sampling and a hierarchically organized nomenclature to describe posture and locomotion. This paper then examines behavioral predictions derived from pseudo-behavioral locomotor classifications, multivariate morphometric studies, and studies of the behavioral consequences of body size. The principal interspecific contrasts include (1) the superiority of Ateles with respect to locomotor and postural use of the forelimbs in tension, brachiation, arm swinging, dropping, quadrupedal climbing, forelimb dominated leaping/dropping take-offs and landings, suspensory posture, forelimb suspension, and use of larger and more complex supports; and (2) the superiority of Saimiri with respect to most forms of quadrupedal-cursorial locomotion and leaping, quadrupedal leaping/dropping take-off and landing patterns, locomotor and postural use of the limbs under compression, nonsuspensory posture, sitting, quadrupedal standing, and the use of twigs and single supports. In both animals, however, nonsuspensory postures and quadrupedal locomotion prevailed. Although behavioral findings were generally consistent with predictions derived from morphological accounts, the fit was loose. Discrepancies arose, especially concerning climbing and leaping. These were attributed to inadequacies in current nomenclature descriptive of positional behavior arising from the indiscriminate use of overinclusive vernacular and pseudo-behavioral terms.