Infants as costly social tools in male Barbary macaque networks

In mammals, females are the main caretakers of offspring. In a number of species, however, males also care for young, and may spend considerable time with infants. In various nonhuman primates, male interactions with infants appear to benefit mainly the males, who use them to regulate their relationships, particularly with other male group members. We studied whether interactions with infants were related to glucocorticoid output in male Barbary macaques, Macaca sylvanus, at the enclosure ‘La Forêt des Singes’, France. We used a combination of behavioural observations, social network analysis and measures of faecal glucocorticoid metabolites as an indicator of physiological stress, and compared the results for the birth season in spring, during a time of intense interactions with infants, with a period in autumn, when males interacted less frequently with infants. Social network analyses revealed that infant carriers had stronger ties with other males than noncarriers. Overall, male relationships were stronger in spring than in autumn. Infant care was unevenly distributed among males and not related to rank, suggesting that the motivation to invest in infant carrying depends on the position in the social network and the current constellation of relationships. Despite the correlation of infant carrying with male interactions, the time spent with infants alone explained most of the variation in glucocorticoid levels, suggesting that access to the male–male network is paid for by an increase in stress hormone levels. Taken together, these results show that Barbary macaque infants can be viewed as costly social tools that facilitate networking among males.