Seasonal variation in aggression and physiological stress in wild female and male redfronted lemurs (Eulemur rufifrons).

Intraspecific competition with fellow group members represents an unavoidable cost of group living. However, the causes of competition can vary among group members, and ecological and reproductive challenges faced by individuals throughout the year can trigger physical conflicts and or physiological responses. To date, few studies in mammals have described both physiological and behavioral responses to competition simultaneously across the year in both males and females. However, such an approach may shed light on ultimate drivers of sex-specific competitive strategies. In this six-year study on multiple groups of wild redfronted lemurs (Eulemur rufifrons), a primate species from Madagascar, we intended to identify the relative importance of feeding vs. reproductive competition for both sexes. We combined data on fecal glucocorticoid metabolite (FGCM) levels, a proxy for the physiological stress response, with behavioral observations on agonistic interactions during ecologically and socially challenging phases across the year. We found that while FGCM levels increased in both sexes with decreasing fruit consumption, this increase was not accompanied by concomitant changes in agonistic behavior. Female aggression and FGCM levels instead peaked during the birth season, while for males, aggression remained fairly constant across the year. Our results suggest that redfronted lemurs have mechanisms to avoid direct competition through aggression at times when individuals may need to conserve energy.