Body mass, sexual dimorphism and femoral proportions of Proconsul from Rusinga and Mfangano Islands, Kenya

The criteria traditionally used to distinguish between species of Proconsul have been largely size-related and based primarily on the dentition. We present here new estimates of body mass for seven Proconsul from early Miocene (17·8 mybp) sites on Rusinga and Mfangano Islands, Kenya, derived from cross-sectional diaphyseal and articular dimensions of the femur, structural features that bear a close functional relationship to body mass in living species. The fossil specimens cluster into two major groups: one at just over 9 kg (including the well-known KNM-RU 2036), and the other between about 26 and 38 kg. Since a 3:1 to 4:1 body mass ratio between males and females of the same species is unknown among living primates or indeed land mammals, it seems most probable, contrary to some recent hypotheses, that at least two species of Proconsul are represented at these sites-the larger P. nyanzae and the smaller a new species formerly identified as P. africanus. The P. nyanzae specimens can be further divided into subgroups of about 35-38 kg and 26-28 kg, which may represent males and females of this species. In terms of size and general proportions, the P. nyanzae femora are most similar to those of pygmy chimpanzees, while the smaller Proconsul femora are similar to those of some living cercopithecines. The P. nyanzae femora also show evidence of a relatively mobile (ape-like) hip joint. However, in certain respects the general hindlimb bone proportions of the largest living cercopithecines and smallest great apes are similar, overlapping within the body size range of P. nyanzae, i.e., near 30 kg. The relatively shorter and thus more robust appearing femora of the larger living African apes may be a result of mechanical constraints associated with climbing in an anthropoid with a body mass significantly above this size range.