Female dominance in two basal primates, Microcebus murinus and Microcebus lehilahytsara: variation and determinants

Female social dominance over males can only be found in a small number of mammalian species, but is unusually concentrated among Malagasy lemurs. Three major hypotheses are currently discussed to explain this phenomenon, namely the energy conservation hypothesis, sleeping site hypothesis and ancestral lemur condition hypothesis. Since proximate determinants of female dominance have rarely been studied, this study aimed to investigate the influence of factors connected to these hypotheses, focusing on sex-specific differences in energy requirements and group composition in two closely related nocturnal lemur species. More specifically, the effects of species, season and age on agonistic behaviour and the expression of female dominance were tested, while the effects of individuality, body mass, breeding experience and habituation were controlled. Seasonal variations in intersexual dominance relationships were investigated in captivity in grey mouse lemurs, Microcebus murinus, a prominent female-dominant species, and Goodman's mouse lemurs, Microcebus lehilahytsara, whose agonistic behaviour was studied for the first time. Data were collected during a series of encounter experiments between one male and one female (15 dyads of M. murinus and nine of M. lehilahytsara) during the reproductive and nonreproductive seasons. Moderate female dominance was expressed in both species with females winning the majority of conflicts year round, independently of context. However, effects of season and species were found in conflict rates, in the probability for females to win conflicts, and in the number of dominant females. Moreover, the age difference between dyad partners and the breeding experience of females influenced whether conflicts were decided in favour of males or females. It can be concluded that female dominance probably represents an ancestral trait in mouse lemurs and it is proposed that its evolution might have been driven by sex-specific energetic constraints prevailing in the species-specific ancestral habitats, and may moreover be influenced by the species-specific social organization.