DIFFERENCES IN TERRESTRIAL VELOCITY IN MACACA AND PRESBYTIS

Over 150 sequences of terrestrial movement were timed and measured in the toque macaque [Macaca sinica], the gray langur [Presbytis entellus], and the purple-faced langur [P. senex] to determine the relation between velocity, gait and psychosocial context. Species differences were found in mean velocity, favored gait and surface preference. All 3 spp. used the walk and the gallop at the slowest and fastest speeds respectively. The macaques and gray langurs walked frequently, but the purple-faced langurs were never seen to do so. At intermediate speeds, the macaques used either the fast walk or the slow gallop, whereas the gray langurs used irregular patterns of walk-gallop-walk-gallop. The purple-faced langurs were faster (about 20 ft/s) and less variable than the other species, regardless of distance. These data suggest that motor expression varies among the cercopithecoid monkeys; the correlation between locomotion and anatomy is not as close as it is among wholly terrestrial or aboreal forms; slowness in the macaque is an expression of social confidence, not of biomechanical inability; high speed in the purple-faced langur is due to psychosocial factors rather than to terrestrial adaptability.