Male grunts as mediators of social interaction with females in wild chacmababoons (Papio cynocephalus ursinus)

Previous research has suggested that the quiet, tonal grunts given by femal e savanna baboons (Papio cynocephalus ursinus) function to mollify lower-ra nking females and thereby facilitate friendly social interaction with them (Cheney et nl., 1995). In a two-year study of wild chacma baboons, we asses sed whether or not grunts given by adult males function similarly to facili tate heterosexual interaction. Two patterns of male vocal behavior initiall y suggested this function. First, males grunted more often when approaching females with which social interaction was potentially highly beneficial an d/or unlikely (due to female evasion), i.e. estrus females and lactating fe males (particularly friend females); males rarely grunted when approaching pregnant females. Second, higher-ranking males grunted significantly more o ften than subordinates when approaching females in most reproductive states . In spire of these two patterns, however, male grunts had contrasting effect s on the probability of supplanting a female and interacting affinitively w ith her. Supplanting of females was just as common when the approaching mal e grunted as when he did not. Instead, variance in supplanting was better e xplained by female avoidance of high-ranking and nonfriend males than by th e male's vocal behavior. Results suggest that male grunt:, themselves do no t generally determine whether a supplant of the female occurs. Rather, the female's reproductive stare and social relationship with the male (i.e. Iri s 'friendship' status and/or lank) affect hertz the male's tendency to call to her and the female's tendency to move away from him. In contrast to supplanting, affinitive interaction occurred significantly m ore often when males grunted than when they silently approached females. Ta ken together, results suggest that a female chacma baboon's 'spatial' respo nse to a male's approach ('stay or leave') depends upon her assessment of n on-vocal factors, but her 'social' response ('interact or not') is influenc ed by the grunts given by the male.