[Gymnorhinus cyanocephalus (Paz-Y-Miño et al., 2004)] and hyenas [Crocuta crocuta (Engh et al., 2005)], but also in less social territorial birds and fish [e.g., Oliveira et al. (1998), Peake et al. (2002), Grosenick et al. (2007)]. When joining a coalition, for example, hyenas and monkeys selectively recruit or support the higher-ranking of two combatants [e.g., Silk (1999), Schino et al. (2006); reviewed in Cheney and Seyfarth (2007)]. Capuchin monkeys (Cebus capucinus) selectively recruit allies who both rank higher than their opponents and have a closer bond with themselves than with their opponent, indicating that they are able to compare the bond between the ally and themselves with the bond between the ally and their opponent (Perry et al., 2004). In playback experiments involving wild baboons (Papio hamadryas ursinus), a sequence of calls that mimics a higher-ranking opponent threatening a lower-ranking animal elicits little response from listeners, but if the individuals’ roles are reversed, the response is significantly stronger—presumably because the rank-reversal sequence violates the listener’s expectations (Cheney et al., 1995; Bergman et al., 2003; Kitchen et al., 2005).
The ability to eavesdrop on the social interactions of others enables individuals to acquire knowledge about another’s competitive ability and probable allies without directly challenging him. In nonsocial birds and fish, males use information acquired through eavesdropping when deciding whether to challenge an intruder [e.g., Oliveira et al. (1998), Peake et al. (2002); see Paxton et al. (2010) for similar data on rhesus macaques, Macaca mulatta]. Similarly, female chickadees (Poecile atricapil-lus) assess their mate’s relative dominance status by attending to his singing contests with neighboring males. Females mated to males who are dominated in such contests are subsequently likely to solicit extra-pair copulations from apparently more dominant neighbors (Mennill et al., 2002).
Monkeys also recognize the close bonds that exist among others. In vervets (Chlorocebus aethiops) and macaques, an individual who has just been involved in an aggressive interaction will often redirect aggression by attacking a close relative of her opponent (Cheney and Seyfarth, 1990; Judge, 1991). Similarly, if a female baboon hears a call sequence that mimics a fight between one of her own close relatives and the close relative of a more dominant female, she will subsequently avoid that female (Cheney and Seyfarth, 1999). Playback experiments have also demonstrated that low-ranking male baboons monitor the status of other males’ sexual consortships to take advantage of opportunities to mate “sneakily” (Crockford et al., 2007).
If a baboon receives aggression from another and then, minutes later, hears a “reconciliatory” grunt from a previously uninvolved animal, the listener’s response to the grunt depends on the relationship between