How do you define friendships in primatology?
Friendships are described as high rates of association between anestrus (non estrous ie no sexual activity) and lactating females with unrelated males of any rank (Strum, 1974; Altmann, 1980; Smuts, 1985).
Female and male friendship is a unique occurrence within mammals. In most mammal groups the two different sexes generally only interact in courtship or mating. Males will attempt to monopolize females while females attempt to find the fittest male (Greenwood, 1980). Males and females also will often associate more when females are in estrous. For examples female chimps when in estrous have been described as more sociable and more likely to join males in smaller bisexual groups. However, when they are no longer in estrous they are generally found in close proximity to their offspring instead of the males (Pepper et al. 1999). Thus female and male friendship outside of courtship and mating is highly unusual.
Female and male friendships were first documented in Savannah baboons (Ransom & Ransom, 1971). Savannah baboons are part of the Papio genus. Within Papio there are three species the exhibit female and male friendship: the olive baboon (Papio anubis), yellow baboon (Papio cynocephalus) and Chacma baboons (Papio ursinus). They are found in groups of multifemale and males and have a polygynandrous mating system (multiple partners for both the males and females); male rank however does affect access to females (Swedell, 2011). They are also characterized by female philopatry and male dispersal (females have very close bonds). Hamadryas baboons (Papio hamadryas) do not have female and male friendships. They also differ in their group structure compared to other savannah baboons. Females and males are within one male units in which several females only mate and affiliate with a specific male (Kummer, 1968), although both sexes disperse males tend to be philopatric (Swedell et al. 2011). Hamadryas baboons do not exhibit female and male friendship but the savanna baboons do suggesting a link to the differing group structure and the development of opposite sex friendships (Kummer, 1968; Swedell, 2006; Pines et al., 2011).
Male and female friendships also have differing benefits for the females.
- Yellow and savanna baboons
- Low infanticide rates thus friendships function as a protection against harassment of the female and her infant from other females and males (Lemasson et al. 2008; Nguyen et al. 2009)
- Chacma baboons
- Higher infanticide rates thus friendships may function as a counter against infanticidal immigrant males (Busse and Hamilton, 1981; Palombit et al., 1997)
Hypothesis for male and female friendship
- Anti infanticide
- Female benefit from protection by males friend from infanticidal males (Altmann et al., 1978)
- Chacma baboons only
- Female harassment hypothesis
- Males protect females from harassment by high ranking females (Altmann, 1980; Wasser, 1983)
- Yellow and Savanna baboons
- Male caretaker hypothesis
- Female develops friendship and males develop relationship with the infant (Ransom & Ransom, 1971). This would help protect the females investment in her offspring by providing added protection, access and socialization for the infant (Altmann & Altmann, 1970; Altman, 1980; Anderson, 1992)
But what do the males get out of it?
- Possible mating opportunities with the female (Wittenberger and Tilson, 1980)
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