Male life history and the unusual adult sex ratios of redfronted lemur, Eulemur fulvus rufus, groups

Abstract

In group-living species, theoretical considerations indicate the existence of a fundamental conflict of interest between the sexes over the adult sex ratio within groups. Females may derive certain benefits from living with many males. Males, in contrast, should generally try to monopolize access to a group of females. Which sex ultimately controls adult group sex ratio is poorly known. We examined this conflict between the sexes in redfronted lemurs, Malagasy primates characterized by an unusual lack of female-biased adult sex ratios. Using various demographic and behavioural data from several groups collected over 6 years, we examined (1) the proximate determinants of this unusual sex ratio, (2) the temporal distribution of female fertile phases within groups as a determinant of male monopolization potential, (3) sources of between-group variation in the adult sex ratio, and (4) possible social benefits of the relatively high number of males for both sexes. Birth and mortality rates were not sex biased and males migrated considerably more frequently than females, providing no proximate explanation for the unusual sex ratio. However, certain life history traits (fast maturation, short interbirth intervals) may ultimately play a role because they act to facilitate joint group transfers of male coalitions. Despite a relatively small female group size and an associated high monopolization potential, female oestrous synchrony may prevent the formation of single-male groups. Reduced male group size seems to be the main predictor of take-over rate, and, thus, infanticide risk, suggesting that both sexes may benefit from the high number of coresident males, thereby considerably defusing the conflict of interest between the sexes.