The ontogeny of chimpanzee and pygmy chimpanzee locomotor behavior: a case study of paedomorphism and its behavioral correlates

Abstract

Pygmy chimpanzees ( Pan paniscus) have been described as “paedomorphic” in comparison with common chimpanzees ( Pan troglodytes) because some of their adult proportions and features are present in immature chimpanzees. However, interpretations of what this morphological paedomorphism means behaviorally have varied widely. This paper documents the changes in positional behavior that occur during the ontogeny of pygmy and common chimpanzees so as to assess whether the adult pygmy chimpanzee is more similar in its positional behavior to immature rather than adult chimpanzees. Results of field studies indicate that both pygmy and common chimpanzees show significant and similar patterns of change in locomotor behavior during ontogeny. With increasing age, in both species, there is an increase in the frequency of quadrupedalism. In early infancy, chimpanzee positional behavior is clearly forelimb dominated, and clinging, climbing and armhanging are most common. Aided bipedalism is the earliest form of frequent hindlimb use. Only later is the infant able to coordinate forelimb and hindlimb use in quadrupedalism. As the frequency of quadrupedalism increases, the frequency of bipedalism decreases. By 2 years of age, common chimpanzee locomotion is primarily quadrupedal. In addition to changes in the type and frequency of locomotor activities performed, there are also ontogenetic changes in the type of substrates used and the locomotor activities performed on them. Results also indicate that adult pygmy chimpanzee arboreal locomotor behavior differs from that of adult common chimpanzees and most closely resembles that of infant chimpanzees. Adult pygmy chimpanzees, like immature common chimpanzees, use more suspensory and quadrupedal behavior than adult common chimpanzees. Shea (1981) has demonstrated that heterochrony in African ape evolution has resulted in substantial morphological differentiation. The results of this study complement Shea's findings by demonstrating that behavioral differences are associated with the morphological patterns.