Multi-male groups, also known as multi-male/multi-female, are a type of social organization where the group contains more than one adult male, more than one adult female, and offspring of all ages. Within Order Primates, it is the most common social group type, with group sizes ranging from 10 to 100 individuals. Large groups of primates are called "troops." Examples of species that can be categorized under this type of social organization include many diurnal lemurs, langurs, and most members of the family Cebidae.[1]
Definition Multi-male groups, also known as multi-male/multi-female, are a type of social organization in which a group consists of more than one adult male, more than one adult female, and offspring of all ages (Sussman, 2003). Within Order Primates, it is the most common social group type, with group sizes ranging from 10 to 100 individuals composed of several males, and numerous females and offspring (Sussman, 2003). Large groups of primates are called "troops” which are characterized by complex intratroop politics and competition (O’Neil, 2011; Sussman, 2003). Within troops, there are no stable heterosexual bonds—both males and females have a number of different mates (O’Neil, 2011).
Species Multi-male groups are most common among semi-terrestrial primates such as savanna baboons, macaques, langurs, and other new world monkey species as well as chimpanzees, gorillas, and other old world primates (cercopithecine) (O’Neil, 2011). Some species such as the leptodactylid frog and many rodent species such as the prairie vole have also been known to have multi-male/multi-female spawning (Haddad, 2003; Hodges, 2002;).
Social structure Multi-male/multi-female groups commonly have a dominance hierarchy among both males and females (O’Neil, 2011). Each individual is ranked relative to all other community members of the same gender (O’Neil, 2011). This tends to reduce serious violence within the community since all individuals know in advance who they must defer to and who must be submissive to them. For example, among rhesus macaques, one's position in the dominance hierarchy is determined by the rank of his or her mother (O’Neil, 2011). The top ranking individuals are referred to as the alpha male and the alpha female by primatologists (O’Neil, 2011). All other community members defer to them (O’Neil, 2011). A female's rank in the hierarchy stays with her throughout her life (O’Neil, 2011). However, most young adult male rhesus macaques leave their natal community and ultimately join others to find mates (O’Neil, 2011). When they do so, they begin again at the bottom of the male dominance hierarchy (O’Neil, 2011). Alpha males usually mate more often than others (O’Neil, 2011). This makes the social organization superficially seem as if it is a one-male-several-females group. However, younger females often sneak off to mate with males lower down in the dominance hierarchy (O’Neil, 2011). The stable core of rhesus macaque communities is the group of female relatives (O’Neil, 2011). They stay within their natal community throughout life and work as a team to defend it against other females (O’Neil, 2011).
Female Reproductive Behavior
In multi-male groups, there is typically no single male that has full control over the reproductive share of females (Bradley, 2005). Instead, the top ranking males in the groups have an approximately equal share of siring all offspring born to females within the group (Bradley, 2005). Females in multi-male groups will mate multiple times with different top ranking males as well as lower ranking males and occasionally bachelor “lone wolf” males outside of the group (Bradley, 2005). Most female primates do not have a particular breeding season but can be receptive all year around, however they will most likely not mate if they are already caring for an infant (Bradley, 2005).
Parental Care
Although male parental care is rare among mammals, adult males of many old world primate species provide care for infants and juveniles (Alberts, 2003). This care is often in the form of grooming, carrying, support in agonistic interactions, and protection against infanticide (Alberts, 2003). A traditional view of multi-male primate groups is held that males provide relatively little direct care to infants, possibly as a result of the low confidence of paternity that is associated with this relatively promiscuous breeding system (Johnstone, 2010). In the last five years this view has changed however, as a result of a careful documentation of intimate male-infant affiliations in certain species, especially the savanna-dwelling baboons (Papio spp.) (Johnstone, 2010). Recent field studies of baboons suggest that male-infant relationships are mediated through affiliations between the males and the infants' mothers, but the degree to which these male-female affiliations are based on prior mating experience (hence, paternity) has not been established (Johnstone, 2010). Comparative studies of male-infant relations in primates have given little attention to the variation in the intensity and form of male care patterns within the set of species that have a multi-male social organization (Borries, 1997). Among multi-male species, male care of young is reported most often in baboons and barbary macaques (Macaca sylvanus) and least often in other macaques, chimpanzees (Pan troglodytes), and vervet monkeys (Cercopithecus) (Borries, 1997). This interspecific variation may result both from differences in the importance of male care to infant survival and from differences in male confidence of paternity, which in turn may relate to seasonal breeding patterns and, in particular, to the presence or absence of conspicuous signs of ovulation in females (Borries, 1997).
Infanticide
In some multi-male groups, the costs for infanticidal males seem likely to be high, since other resident males might defend the victim, and the benefits seem likely to be low because of the generally lower paternal probability in multi-male groups (Fruteau, 2010). However, in other primate species living in multi-male groups, males have been observed to kill infants (Borries, 1997). The aggressors are typically extra-group males, recent immigrants, have recently been introduced or are conceived outside the group, mainly due to the fact that they could not possibly be related to the infants they kill (Borries, 1997). A male’s age and rank influence the occurrence of infanticide (Borries, 1997). The youngest and highest-ranking immigrant males are more likely to commit infanticide than their older and lower-ranking counterparts if putative fathers fail to protect infants (Borries, 1997).
Costs/Benefits (Trade-Offs)
A benefit of living in multi-male groups is the collective protection that the group as a whole receives from outsiders as well as sufficient protection of infants against infanticide from other outside roaming males (Cheney, 1988). Another benefit is the low costs of finding a mate and reproducing since the males and females are always together (Cheney, 1988). Also, participants in multi-male groups have better access to resources such as food and living quarters (Cheney, 1988). However, a major cost to living in a multi-male group is the constant competition for mates, mainly among males but also among females (Cheney, 1988).
Interactions between Overlapping Multi-Male Groups Most non-human primate communities are more or less closed to contact with members of other communities (Alfred, 1995). Most often, they are tied to a particular locale and rarely migrate outside of their home range (Alfred, 1995). This aloofness from other troops prevents high concentrations of individuals, which could result in rapid depletion of local resources (Alfred, 1995). Communities usually avoid each other and are aggressive towards outsiders (Alfred, 1995). As a result, social interactions between members of different troops are usually very rare, especially for females (Alfred, 1995). Chimpanzees are a notable exception. When chimpanzees from different troops come together, there is often an exciting, friendly encounter lasting several hours, following which, some of the adult females switch groups (Alfred, 1995). Apparently, they are seeking new mates (Alfred, 1995). Occasionally, however, contact between communities of the comparatively unpredictable chimpanzees will develop into genocidal violence (Alfred, 1995). Interactions within non-human primate communities are usually unlimited. Subgroups are rarely closed from group interaction (Alfred, 1995). All members of a community have daily face-to-face, casual communication. The most common type of subgroup consists of a mother and her young offspring (Alfred, 1995). In some forest living primates, contact between groups of the same species is in the form of a specialized territorial defense behavior (Alfred, 1995). Instead of avoiding each other, groups actively converge near their common territorial border and make hostile displays (Alfred, 1995). Howler monkeys, indris, siamangs, and gibbons all produce exceptionally loud vocalizations for this purpose (Alfred, 1995). This is a ritualized, essentially harmless form of aggression that is intended to intimidate members of the neighboring community (Alfred, 1995). All four of these species live in home ranges that are usually so small that the food resources of neighboring territories can be seen and become attractive (Alfred, 1995).
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