Ontogeny of body size variation in African apes

Abstract

Size variation in African apes (Gorilla gorilla [gorilla], Pan paniscus [pygmy chimpanzee], and Pan troglodytes ["common" chimpanzee]) is substantial, both within and between species. We investigate the possible evolutionary significance of this variation through an analysis of the ontogeny of size variation in this group. In addition, we highlight possible areas of future endocrinological research, and evaluate recently proposed alternative models that attempt to account for ontogenetic variation in apes. The present study shows that intergeneric variation in size is largely a consequence of differences among species in the rate of body weight growth. Interspecific size variation in Pan is a product of both rate and duration differences in growth. The ontogenetic bases of sexual dimorphism vary in this group. Dimorphism is largely a result of sex differences in the duration of body weight growth in gorillas and pygmy chimpanzees, but results from differences in the rate of growth in common chimpanzees. Ontogenetic analyses largely confirm earlier interpretations, but with better data and methods. The great degree of ontogenetic variation within and among these species, especially in the timing and magnitude of "pubertal" growth spurts, implies that studies of endocrine growth control in African apes could be a productive line of future research. We also suggest that ontogenetic variation can be understood with respect to ecological risks. Growth rates seem to be negatively correlated with ecological risk in African apes, suggesting links between ontogenetic patterns and social and ecological variables. High growth rates in gorillas compared to Pan are most consistent with this model. Variation between chimpanzees and pygmy chimpanzees (especially females) also seem to fit predictions of this model.

A shift toward birthing relatively large infants early in human evolution

“…Fortyseven of these included mass data for both the infant (within 2 wk of birth) and mother. The average chimpanzee neonatal mass (1,733 g) is near the value found in other studies (17,18,70). Although adult female wild chimpanzees are not as heavy as those reported in this study (34), immature wild chimpanzees are also smaller than laboratory-born chimpanzees of a similar developmental age (71), consistent with findings that birth weight is correlated with mother's mass in humans (72) and nonhuman primates (73).…”

“…Second, Australopithecus females were smaller than female chimpanzees. Australopithecus females are estimated to have been between 29 and 33 kg (14,15), below the 34 to 46 kg range reported for wild female chimpanzees (34) and the 44 to 61 kg range of captive female chimpanzees (18) (Table S1). Direct comparisons of adult cranial capacity to femoral head and ankle dimensions reveal significant differences between female chimpanzees and Australopithecus (Fig.…”

Paleobiological Implications of the Ardipithecus ramidus Dentition

“…Sexually distinct patterns of canine eruption in relation to body size growth have yet to be well documented in modern great apes but appear to broadly share the cercopithecoid pattern described above (28,(32)(33)(34). Initiation of canine eruption in P. troglodytes differs by about 1.5 to 2 years between the sexes (35).…”

Life history costs and benefits of encephalization: a comparative test using data from long-term studies of primates in the wild

“…An examination of somatic growth trajectories supports this notion. In primates, longer duration of growth is associated with prolongation of early stages of the growth trajectory, which are characterized by slower growth rates, and a reduction in the relative length of later stages that have high growth rates (Leigh 1996(Leigh , 2001Leigh and Park 1998; see also Vinicius 2006). In addition, the long juvenile periods and slow growth rates of primates has been suggested as a strategy to reduce the risk of metabolic deficiency, particularly in the face of elevated feeding competition stemming from living in large social groups (Janson and van Schaik 1993).…”