Abstract
Factors influencing the hominoid life histories are poorly understood, and little is known about how ecological conditions modulate the pace of their development. Yet our limited understanding of these interactions underpins life history interpretations in extinct hominins. Here we determined the synchronisation of dental mineralization/eruption with brain size in a 20th century museum collection of sympatric Gorilla gorilla and Pan troglodytes from Central Cameroon. Using δ13C and δ15N of individuals’ hair, we assessed whether and how differences in diet and habitat use may have impacted on ape development. The results show that, overall, gorilla hair δ13C and δ15N values are more variable than those of chimpanzees, and that gorillas are consistently lower in δ13C and δ15N compared to chimpanzees. Within a restricted, isotopically-constrained area, gorilla brain development appears delayed relative to dental mineralization/eruption [or dental development is accelerated relative to brains]: only about 87.8% of adult brain size is attained by the time first permanent molars come into occlusion, whereas it is 92.3% in chimpanzees. Even when M1s are already in full functional occlusion, gorilla brains lag behind those of chimpanzee (91% versus 96.4%), relative to tooth development. Both bootstrap analyses and stable isotope results confirm that these results are unlikely due to sampling error. Rather, δ15N values imply that gorillas are not fully weaned (physiologically mature) until well after M1 are in full functional occlusion. In chimpanzees the transition from infant to adult feeding appears (a) more gradual and (b) earlier relative to somatic development. Taken together, the findings are consistent with life history theory that predicts delayed development when non-density dependent mortality is low, i.e. in closed habitats, and with the “risk aversion” hypothesis for frugivorous species as a means to avert starvation. Furthermore, the results highlight the complexity and plasticity of hominoid/hominin development.
Highlights
Resource limitation and mortality are key factors that determine life history strategies in mammals [1], including hominoids [2]
Such approaches are rooted in the observation that first permanent molar eruption is correlated with adult brain size [5,6] and, other life history variables [7,8]
The body of anthropological and neontological evidence cautions against an overemphasis of the absolute timing of dental development as a means to determine whether or not hominin life histories have advanced relative to the great ape condition [21]
Summary
Resource limitation and mortality are key factors that determine life history strategies in mammals [1], including hominoids [2]. Dental eruption is regarded as informative, especially when combined with histological analyses that determine the absolute timing of events [3,4]. Such approaches are rooted in the observation that first permanent molar eruption is correlated with adult brain size [5,6] and, other life history variables [7,8]. Analysis of an historical skeletal sample of great apes from Cameroon, West Africa, affords the opportunity to test this possibility while controlling for the effects of habitat and ecology
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