Craniometric differentiation and systematics in the genus Pan

The traditional classification of the common chimpanzee, Pan troglodytes, into three geographical subspecies has never been examined in detail using large samples of craniometric data. In this paper, we report the results of such a study of samples of P. t. verus, P. t. troglodytes, and P. t. schweinfurthii, plus the pygmy chimpanzee, P. paniscus. Univariate and multivariate discriminant analyses were utilized to determine whether there exist significant morphometric distinctions among the three purported subspecies of common chimpanzees. Multivariate analyses revealed significant discrimination among samples assigned to the three subspecies of P. troglodytes on the basis of known provenience, thus supporting the traditional separation derived from geographical distribution and external soft tissue features. However, the morphometric differentiation among the subspecies of Pan is markedly less than that observed in the other great apes. This may reflect important microevolutionary phenomena, and relative to the other great apes, chimpanzees may have differentiated more recently, remained adapted to broader and more generalized ecological niches, and/or maintained higher levels of intergroup contact and genetic exchange due to contiguous areas of inhabitation and their high mobility, large home ranges, and social structure. These studies support the separation of the pygmy chimpanzee as a distinct species of Pan.