Social rank and body size as determinants of positional behavior inPan troglodytes

A yearlong study of the positional behavior ofPan troglodytes at the Mahale Mountains National Park yielded 571 hr of observation.Cant (1987) articulated four predictions concerning the relationship between body weight/branch diameter and positional behavior based on the classic suspensory-ape paradigm. He noted that only two were supported by orangutan data. Three of these four hypothesis were not supported by chimpanzee data, as follows: there was no significant difference between the three largest males and the four smallest males in (1) branch diameters (5.8 cm vs 5.2 cm) nor (2) in the percentage of arm-hanging (13.6% vs 12.1%); and (3) large males did not arm-hang significantly more often than small males in any of three support diameter categories. The fourth hypothesis, that arm-hanging should be more common among smaller branches, was supported: arm-hanging as a percentage of all posture rose from 2.5% to 8.3% to 24% as stratum size decreased from >10 cm to <3 cm. The possibility that the first three hypotheses failed because of confounding effects of a correlation between body size and social rank was examined. Multiple regressions were done on 6600 2-min instantaneous observations on focal individuals. With social rank effects factored out, larger individuals preferentially utilized smaller, rather than larger supports. When positional mode frequencies were compared between large and small males matched for social rank, large males exhibited a lower frequency of arm-hanging than small males. An unexpected result was that social rank more consistently predicted branch diameter choice than body size. The most profound trend was for high ranking males to use larger supports, even though they spent more time in the terminal branches. These results suggest that (1) suspensory behavior is functionally related to small branch diameters; (2) chimpanzees do not prefer smaller branches, rather they are forced into them by food choice limitations; and (3) social rank more profoundly affects chimpanzee behavior than body weight.