Costs and benefits of heterosexual interactions on female mandrills

In primates, most evidence linking sociality to fitness comes from studies of female-female bonds, whereas the consequences of heterosexual relationships remain understudied. This is particularly true in species such as mandrills (Mandrillus sphinx), where male group tenure is short, and interactions with females can range from affiliative to coercive. Using 13 years of behavioural and life-history data on 76 adult females, we show that grooming exchanged with all males has contrasting fitness consequences depending on its direction. Grooming given to males tended to improve female survival and was associated with higher juvenile offspring survival, whereas grooming received from males reduced female survival. These opposing effects suggest that heterosexual grooming interactions may entail both benefits and costs for females, depending on whether females provide or receive affiliation. Finally, agonistic interactions between females and males showed modest evidence of negative associations with juvenile survival, whereas male-female spatial proximity had no detectable effect overall. Our results show that even transient male-female associations can have lasting consequences for fitness and that these effects depend critically on the direction and timing of specific interactions rather than on global measures of sociality.