Ontogenetic Changes in Intrinsic Hand and Foot Proportions in Chimpanzees, Pan troglodytes

Ontogenetic trajectories in intrinsic hand and foot proportions are likely to vary in primates according to their locomotor adaptations. We analyzed the developmental trajectories of chimpanzees (Pan troglodytes), which exhibit particular locomotor adaptations related to suspensory and climbing abilities, testing whether autopodial growth reflects biomechanical and ecological constraints associated with ontogenetic changes in locomotor behavior.

We analyzed 157 radiographs of 7 female and 198 of 8 male P. troglodytes individuals, from 1.5 to 13 years old. Lengths of metapodials and proximal/intermediate phalanges were measured to compute phalangeal indices (PIs: ratio of summed non-distal phalangeal length to corresponding metapodial length). Ontogenetic allometry was assessed by regressing bone length against body mass, with sex included as a factor. Non-linear Gompertz growth models were fitted to estimate bone-specific growth rates and durations.

The results show a positively allometric relationship between all bone lengths and body mass. However, sex-related differences are observed in the allometry of phalanges and metapodials. PIs decline during growth for all digits, with this ontogenetic decline remaining highly similar between sexes. Gompertz models further indicate distinct growth rates between manual and pedal elements.

Ontogenetic changes in chimpanzees indicate canalized autopodial growth, with reduced intrinsic proportional changes at ~5 years, aligning with their major positional shift. Comparative analyses show species-specific phalangeal trajectories, possibly more pronounced in the foot than in the hand and likely reflecting adaptations to ecological and locomotor demands, such as terrestrial knuckle-walking in large-bodied chimpanzees.