Behavioral implications of ontogenetic changes in intrinsic hand and foot proportions in olive baboons (Papio Anubis)

Relatively long digits are considered to enhance grasping performance in primates. We tested whether growth-related changes in intrinsic hand and foot proportions may have behavioral implications for growing animals, by examining whether ontogenetic changes in digital proportions are related to variation in voluntary grasping behaviors in baboons.

Longitudinal morphological and behavioral data were collected on 6 captive olive baboons (Papio anubis) as they aged from 5 to 22 months. The length of digits and metapodials, measured from radiographs, were used to calculate phalangeal indices (i.e., PIs: summed length of non-distal phalanges relative to corresponding metapodial length). We also examined the allometric scaling of digital bones relative to body mass. We observed baboon positional behaviors over a 15-day period following the radiographic sessions, quantifying the frequency of forelimb and hindlimb grasping behaviors.

PIs for all digits declined during growth, a result of the differential scaling of metapodials (which scaled to body mass with isometry) versus phalanges (which scaled with negative allometry). The incidence of forelimb and hindlimb grasping behaviors declined with age. Though we found no relationship between forelimb grasping and hand proportions, the incidence of hindlimb grasping was directly correlated with postaxial digit PIs.

Only changes in the intrinsic proportions of the pedal digits are associated with variation in grasping activity in growing baboons. This finding accords previous biomechanical and neuroanatomical studies showing distinct functional roles for the hands and feet during primate locomotion, and has important implications for reconstructing primate locomotor evolution.