Reserve. We are pleased to acknowledge Mokopi Mokopi for his eld assistance, as well as

Previous research has suggested that the quiet, tonal grunts given by female savanna baboons (Papio cynocephalusursinus) function to mollify lower-rankingfemales and thereby facilitate friendly social interaction with them (Cheney et al., 1995). In a two-year study of wild chacma baboons, we assessed whether or not grunts given by adult males function similarly to facilitate heterosexual interaction. Two patterns of male vocal behavior initially suggested this function. First, males grunted more often when approaching females with which social interaction was potentially highly bene cial and/or unlikely (due to female evasion), i.e. estrus females and lactating females (particularly friend females); males rarely grunted whe