You know I like the lizard analogies for human evolution -- I wrote about limb length and predation last time around -- and now we have another paper from Jonathan Losos' group looking at ecological differentiation and sexual dimorphism:
Sexual dimorphism is widespread and substantial throughout the animal world (1, 2). It is surprising, then, that such a pervasive source of biological diversity has not been integrated into studies of adaptive radiation, despite extensive and growing attention to both phenomena (1, 3, 4, 5, 6, 7). Rather, most studies of adaptive radiation either group individuals without regard to sex or focus solely on one sex. Here we show that sexual differences contribute substantially to the ecomorphological diversity produced by the adaptive radiations of West Indian Anolis lizards: within anole species, males and females occupy mostly non-overlapping parts of morphological space; the overall extent of sexual variation is large relative to interspecific variation; and the degree of variation depends on ecological type. Thus, when sexual dimorphism in ecologically relevant traits is substantial, ignoring its contribution may significantly underestimate the adaptive component of evolutionary radiation. Conversely, if sexual dimorphism and interspecific divergence are alternative means of ecological diversification, then the degree of sexual dimorphism may be negatively related to the extent of adaptive radiation.
These anoles have evolved into four different ecomorphs repeatedly on different islands of the Greater Antilles, and the sexes differentiate not only in their morphology but also their habitat use and diet.
Primates are generally group foragers, and because they forage together, males and females eat the same foods a lot of the time. The major components of sexual dimorphism across primates have mostly been considered in relation to body size and canine dimorphism, both of which have a strong social import, but less obvious ecological import. That is an apparent contrast to the anoles, whose dimorphism allows males and females to specialize to slightly different niches.
But even though body size and canine size are the main elements of dimorphism that can be compared across all primates, both these features and others may take on ecological importance within primate species. For one thing, sexual dimorphism leads to ecological differentiation even within foraging groups -- not necessarily because different sized individuals can exploit different foods, but because large individuals have preferential access. This has clear dietary and behavioral import -- for example, hunting is a social activity in chimpanzees; males hunt and females don't, and if a female did hunt (with males around), the males would probably take away the kill. That's not entirely because males are larger, but sexual dimorphism helps to determine the social ecology.
What about hominids? In the Plio-Pleistocene, there were at least three sympatric species of hominids in East Africa (and possibly more) and at least two in South Africa (and possibly more). These species were differentiated by body size, relative brain size, and masticatory adaptations. In other words, they occupied different ecologies involving different foods, and natural selection reinforced their ecological differences (even if the average diet involved much overlap, as I reviewed earlier).
The robust species in East Africa (A. boisei) appears to have had substantial body size dimorphism. The habiline species (H. habilis) was either substantially dimorphic, or was actually composed of two species. The large-bodied Homo may have had reduced dimorphism comparable to that in recent humans. Yet, many people have suggested that this least dimorphic species should have been the one where males and females had the greatest ecological differentiation. This is based on analogy with recent hunter-gatherers, assuming that the introduction of meat in substantial quantities requires a sexual division of labor.
Male and female lions have substantial body size dimorphism, and they are ecologically differentiated by prey size. Just thinking out loud...
Butler MA, Sawyer SA, Losos JB. 2007. Sexual dimorphism and adaptive radiation in Anolis lizards. Nature 447:202-205. doi:10.1038/nature05774