What conditions foster close friendships between female duos? Certainly, they often flourish where adult females are familiar with each other. Sisters are often especially close (or might sibling rivalry sometimes carry on into adulthood, as it does in some women? Female infant spotted hyena siblings will fight until the death of the weaker if food is limited [Wahaj et al., 2007]). The relationships of mothers and their adult daughters, who are often best buddies, will be considered in the chapter: Mothers and daughters.

Theoretically, social species develop bonds of friendship most strongly between same-sex partners in the sex that does not leave the group when it reaches puberty, thus avoiding the possibility of inbreeding. This makes sense. One would expect that individuals who have known each other for a lifetime will have the strongest potential for friendship. It applies to the much-researched olive baboons where males emigrate at puberty and the females stay in their natal troop, continuing friendships that have been in place since their infancy. It is also generally true for chimpanzees where males are more likely to be affiliated with other males than females with females; in this species as in gorillas, most (but not all) females transfer from their natal group into a new one during adolescence. At Gombe, Tanzania, once in their new troop as adults, female chimpanzees spent relatively little time together with the exception of the few special friends that interest us here (Goodall, 1986). But this theory is decidedly not true for bonobos. In this species, females emigrate at puberty, but when they join another group they form a remarkably strong gregarious sisterhood with their new female friends.