Ecology, diet, competition, and ease of movement all affect the size of primate groups. The structure of primate groups is primarily affected by the mating system. There are several elements of primate mating systems. In most species, individuals of either one sex or the other disperse from their natal group --- the one they were born in --- when they reach adulthood. This dispersal affects the structure of the groups by breaking some kinds of relationships and preserving others.
For example, in primate species where maturing females transfer to a new group, males are often left within the group where they are born. This means that males can form relationships as juveniles that last their entire lives. The long-lasting male coalitions in chimpanzees are a side-effect of female dispersal, conditioned by large group sizes and other social factors. In contrast, male savanna baboons transfer to new groups when they reach adulthood. In baboon groups, the female associations are highly structured by kin relations, since mothers and daughters live in the same group as adults.
Mating is essential to reproduction, and is the contest through which individuals pass their genes into future generations. From an evolutionary perspective, individuals do whatever they can to promote their own chances to reproduce. Sometimes, individuals can promote their own reproduction by inhibiting the chances of others. In other instances, it may be in their best interest to cooperate with other individuals, or to bide their time waiting for higher-ranking individuals to die or lose status.
The most basic conflict of interest in mating is between males and females. Because of their biological role in carrying and providing nutrition for their offspring, both before and after birth, females must make a large investment in reproduction. Considering the large cost of reproduction, an adaptive mating strategy for a female is to mate with the male whose genes will contribute to the best possible offspring. For this reason, females are typically choosy about which males they will mate with. This element of female choice can give rise to sexual selection, in which males are advantaged by the possession of features that females value.
In contrast, males may have extreme levels of competition for mates. A single male may be able, through fighting, threat, or intimidation, to prevent other males from having mating access to females, or even to expel all other adult males from the group. If he is successful, the reproductive opportunities for such a male are tremendous. On the other hand, the opportunities of other males fall to zero. This places a tremendous genetic payoff on social competition for mating.
The intensity and form of mating competition vary from species to species. Some kinds of primates have a sparse diet that is simply unable to sustain the caloric requirements of huge male body sizes. Other primates or may modify the conditions of combat through coordination of activity with other individuals, emphasis on advertising the risks of combat rather than pursuing combat itself, or other means.
Sexual dimorphism is a difference in size or form between males and females.
Males and females within a species often differ in size or morphology. For example, male primates almost always have larger canine teeth than females. The canine teeth are important in male mating competition --- males can display their canines as a threat to other males, and at an extreme they can injure or kill other males with these teeth. One indication of the importance of the canines in displays is that they tend to be more dimorphic in species that are active during the day, as opposed to nocturnal species (Leutenegger and Cheverud 1982).
Sexual dimorphism in body size is very pronounced in many primate species. For example, orangutan males average around twice the body mass of females. The body mass dimorphism is even more extreme for gorillas than for orangutans. Many different factors influence the body size dimorphism in a primate species (Hedrick and Temeles 1989). One of these is mating competition between males --- intense male competition increases the value of male body size. Another factor can be food competition between the sexes --- when males are larger, they may be able to dominate a larger share of valued food resources. Additionally, males can take on important reproductive roles beyond mating, such as protecting young juveniles from predation or infanticide.
Territorial primate groups maintain their home ranges against incursions by other groups or individuals.
Mating competition in many primates involves territoriality, when males defend a home range against incursions from other males. Primate groups may be territorial as a result of a single male's action, or the coordinated activity of multiple males. For example, Males of some primate species dominate access to females by preventing other males from coming into their home range. These primate males are said to be territorial. Even small social groups, like the monogamous male-female pairs of gibbons, may be highly territorial. But chimpanzees provide one of the strongest instances of territoriality among primates. Groups of male chimpanzees walk the approximate perimeter of their territory, engaging in violent conflicts with any members of neighboring groups they might encounter (Wrangham 1999). As observed by Jane Goodall (1986) at Gombe in Tanzania, the males of one group of chimpanzees killed all of the adult males and several females and juveniles in a neighboring group over the course of several months.
Kinds of groups in primates
Group size, dispersal, and mating competition all contribute to the proportion of males and females found in any given group. Sometimes a single male and female form a group; sometimes a single male and multiple females; sometimes multiple males and females; and occasionally a single female and multiple males.
Monogamous, or pair-bonded, species have long-lasting mating relationships between a single male and a single female.
Gibbons tend to form long-lasting associations between a single adult male and a single adult female. These pair-bonded primates occupy territories that they defend against incursions from other individuals. Both males and females make long vocalizations, called songs, to establish their shared territory. They often vocalize together in duets, although in different contexts based on whether threats come from males or female intruders (Geissmann 2000, Mitani 1987). These groups of a single adult male and female and their offspring are called monogamous groups.
Polygynous groups have a single dominant male and multiple females.
In many kinds of primates, a single male may dominate a group with multiple females. This is a polygynous group --- a multifemale, single-male group. Polygyny results from strong mating competition among males. Only if a single adult male can repel other males from the group can he reap the powerful reward of mating with many females.
A well-known species with polygynous groups is the gorilla. Gorilla groups generally have one adult male, up to eight or more females, and their dependent offspring. Solitary males live outside of these polygynous groups and sometimes manage extragroup matings with females. These groups are maintained by strong mating competition --- a dominant male in a group repels other males by threats, intimidation, or violence.
Even so, there is variability in gorilla societies. In mountain gorillas, multimale groups are common (Robbins 1999). In such groups, the dominant males have a majority of matings, and often harass subordinate males that attempt to mate. But subordinates do have many mating opportunities, demonstrating a social flexibility among gorillas.
Polyandrous groups have a single dominant female and multiple males.
Marmosets and tamarins, called callitrichids, are the smallest of the New World monkeys. These monkeys are among the few primates for which twin births --- and sometimes triplets --- are common. This means that females tend to have high energetic requirements in pregnancy, lactation, and caring for young. Callitrichids therefore face unique challenges compared to other primates.
One way that these monkeys adapt to caring for more young is that older offspring of a female may stay with her for a longer time instead of quickly going off on their own. These older offspring help to watch and sometimes provide food for their younger siblings (Bales et al. 2000).
Another behavioral adaptation is for a single female to mate with and coexist with multiple males. This kind of mating system is called polyandry. Mating with multiple males reduces the paternity certainty of the males --- a male cannot know if a female conceived her offspring with him or another male. As long as mating opportunities are limited, males may cooperate in a group with a single female on the chance of having offspring with her. These males help to provide food and defense for the young juveniles in polyandrous groups. Polyandry is not universal among callitrichids; it is adopted more when resources and mating opportunities are rare (Goldizen 1988).
Fission-fusion societies are large multimale, multifemale communities that spend much of their time divided into smaller units that combine in different combinations.
Some primates coexist in large groups numbering 50 individuals or more. A group this large is always a multimale group --- there is no way for a single male to deter other males from twenty or more adult females. But multiple males sometimes coordinate their behavior to deter neighboring groups. A large multimale group may occupy and defend a large territory, especially where movement costs are relatively low.
Large primates who live in large groups have a problem: there are very few food patches large enough to feed them all. So even though a large multimale, multifemale group may occupy a substantial territory, it may not be possible for them to feed together much of the time. Chimpanzees live in such large multimale, multifemale groups. Even though members of the group share a dominance hierarchy, social interactions, and relationships, they spend much of their time apart. Smaller groups of individuals --- sometimes a single female and her young, sometimes male-female pairs, and sometimes small groups of either sex --- split apart in order to forage for food. These small groups recombine and split in different combinations, and sometimes all of them come together, especially when food is plentiful. This kind of social organization is called a fission-fusion society. Individuals divide into small foraging groups and come back together into the full community for social interactions.