Male animals are far less likely to be bonded in male–male friendships than are female dyads because, at heart, male priority is to mate with females. Males are therefore usually rivals. However, most females only mate when they are in estrus, which never lasts very long, so male friendships may blow hot and cold. For example, adult male blue monkeys (Cercopithecus mitis) sometimes groom each other and hang out with particular friends, but this easy companionship exists only when there are no females around. The same is true of patas monkeys (Erythrocebus patas). In species in which males and estrus females are present together in a group, males are typically antagonistic or, at best, grudgingly tolerant of each other (Kappeler, 2000). In such groups the biggest, most aggressive males tend to have the most success with females, so males have evolved to be larger than females (Neuhaus and Ruckstuhl, 2002). Male primates may form coalitions to attain power for some end result (for example, combat against another group), but these are ephemeral in nature and do not usually last. Human brothers and male relatives are often close friends but so, too, can they become enemies. About 1800, ruling sultans had a tradition of fratricide, where brothers, nephews, and any other threats to ascendancy were generally imprisoned and killed (Nagel, 2004).

This chapter considers friendships among primates (chimpanzees, gorillas, Tibetan macaques, and woolly spider monkeys), horses, lions, cheetahs, wolves, bears, dolphins, and birds (geese, rheas, black swans, Humboldt penguins, and long-tailed manakins).