Female dominance in mouse lemurs

Female social dominance over males is relatively rare in mammals, but can be found more frequently in Malagasy lemurs, although not consistently expressed in all species. Female dominance in lemurs has been studied over the last 30 years, but proximate and ultimate reasons for this phenomenon are still unknown. More than that, systematic data on the expression of female dominance are missing in many lemurs, especially in nocturnal species. Therefore, within this thesis female dominance was investigated in two captive, closely related mouse lemur species with different socio-ecology, the gray and the Goodman´s mouse lemur (Microcebus murinus and M. lehilahytsara). Three studies were designed to answer specific questions about the ontogeny, expression and determinants of female dominance in combination with dominance-related behaviors in mouse lemurs, which are generally assumed to represent a suitable model for ancestral primate conditions. With the help of the first part of this thesis the ontogeny of intersexual dominance in parallel to age-related changes in dominance-related behaviors in young gray mouse lemurs was investigated within their first year of life. Male-female dyads were observed in encounter experiments in three different age categories: juvenile, adolescent and young adult. All occurrences of play and marking behavior, social tolerance and agonistic behavior were recorded and analyzed systematically. Age-dependent behavioral changes were found in play behavior, social tolerance, agonistic behavior and in the marking behavior of females, but not of males. First decided dominance relationships were established around puberty, when animals were adolescent and the number of dominant females further increased with age. Within the second part of the thesis physical and social characteristics between young adult and fully grown gray mouse lemurs were compared. The results showed that young adult gray mouse lemurs of about one year of age were sexually mature since they had comparable testicular volumes, regular estrus cycles and did not differ in breeding success in comparison to fully grown adults. Behavioral differences between both test groups indicated, however, that young individuals had not yet achieved the full appropriate behavioral repertoire of adults and were lacking complete social maturation. Therefore, this study presented first results that sexual maturation is preceding social maturation in this primate, which might be an adaption to the very high predation pressure. The third part of the thesis dealt with the plasticity and different determinants of female dominance in gray and Goodman´s mouse lemurs. Females of both species won the majority but not all conflicts throughout the year resulting in a moderate form of female dominance. In Goodman´s mouse lemurs the expression of female dominance was described for the first time and the behavior was moderately stable when the agonistic behavior of two successive reproductive seasons was compared. Overall, the tested determinants species, season and age (age difference between dyad partners) were identified to influence the agonistic behavior – and therefore the expression of female dominance – in both mouse lemur species, although with varying intensities. The dominance status of both sexes, however, was not identified to influence the marking behavior of males and females, whereas species and season were identified as influencing factors of marking. In general, both sexes of gray mouse lemurs marked more frequently than Goodman´s mouse lemurs, and in both species the marking behavior of females, but not of males, was clearly influenced by season with higher female marking rates during the reproductive season. In conclusion, the different studies provided new insights into female dominance in mouse lemurs and into the complexity of this behavioral phenomenon that seems to interact with other dominance-related behaviors. Social and ecological differences between both target species were evaluated to identify possible key variables causing sex-specific cost asymmetries that may explain the superiority of females. Furthermore, the relevance of current hypotheses on the evolution of the variable pattern of female dominance in lemurs and in particular in mouse lemurs was discussed.