Sexual Selection and Lemur Social Systems

Abstract

The lack of sexual size dimorphism among the lemurs of Madagascar affects agonistic relations between males and females, and therefore contributes to the most salient feature of lemur social systems: female dominance over males. The purpose of this chapter is to examine the nature and mechanisms of intrasexual selection in polygynous lemurs to illuminate the relationship between sexual selection and sex differences in dominance. First, I test the hypothesis that the lack of sexual size dimorphism in polygynous lemurs is a result of high viability costs. This hypothesis predicts that males in polygynous lemur species should have significantly larger canines than females, if male combat contributes importantly to variance in male reproductive success. Male-biased sexual canine dimorphism occurs in only 4 polygynous species, however. The reverse is true in Propithecus diadema and significant sex differences in canine size are absent in 8 other species. Moreover, there is no heterogeneity in the average degree of sexual canine dimorphism among lemurs with different mating systems. The general lack of sexual size dimorphism in polygynous lemurs is therefore not the result of high viability costs. More likely, it reflects weak selection on characters associated with male combat. The widespread absence of sexual dimorphism in body and canine size may still be reconcilable with sexual selection theory if male-male competition in polygynous lemurs is primarily post-copulatory, i.e., if it takes the form of sperm competition. In contrast to the prediction of this hypothesis, however, I found that males of solitary, pair- and group-living lemurs do not differ significantly in average relative testes size during the breeding season. Together, these comparative analyses suggest that intrasexual selection is of similar intensity in both monogamous and polygynous lemurs. Possible reasons for reduced variance in male reproductive success in non-monogamous species include scramble polygyny competition in some solitary species, as well as the existence of male-female pair-bonds in some group-living species. Female dominance occurs in species where males have superior weapons, but it does not characterize all polygynous lemurs. Thus, reduced intrasexual selection may have facilitated female dominance, but sexual selection theory alone cannot provide a sufficient explanation for the evolution of female dominance.