Social Organization refers to the types of groups in which primates live. Social groups/types of social organization serve several important purposes for primates. They give individual primates:

• • access to resources within a particular territory. This can include defending that territory from predators or conspecifics (members of the same species)

  • opportunity to mate. This is particularly important for males, since most females reproduce at some point in their lives, but not all males do, depending on the social organization. The social organization makes sure that only the correct individuals mate, that is, individuals who are not too closely related, to avoid incest. In many primate societies dispersion is important. This is the movement of some individuals (usually one sex or the other, if not both sexes) at the age of sexual maturity, to prevent incest.

  • opportunity to raise offspring in relative safety. Social organization is a type of adaptation, and therefore it is critical for reproductive fitness. Reproductive fitness is increased when more offspring survive and thrive into adulthood. Social groups help to protect infants (from predators or conspecifics), to provide resources, and to provide care, depending on the structure of the group.

  • social interactions. Primates are social animals, in general. We need a certain amount of interaction with others.

Types of Social Organization

Solitary (also called noyau)

Solitary animals do not live in a group. Instead, males live alone and females live either alone or with their dependent offspring. Many nocturnal, arboreal prosimians live like this, but so do orangutans. Usually, one male's range overlaps multiple female's ranges. They don't spend much time together, except when the female is in estrous (fertile and ready to mate).

This type of social organization allows each primate (or mother-child group) to eat the resources within their own territory, without competing with other conspecifics (although they may need to fight to maintain the borders of the territory). It also allows mating opportunities for the males who can hold overlapping territories with females. Note, however, that if one male's territory overlaps with several female's, then some males are left out of this system entirely. They can only hang around at the edges of another male's territory, hoping to take over in the future, or to sneak in while the territorial male's back is turned. Solitary females raise their own offspring with no help or hindrance from other animals.

One-male/Multi-female (also called polygyny)

In this type of social organization, a single adult male lives with several females and their immature offspring. This is one of the most common types of social organization among primates. It is found among a number of monkey species and also the gorilla. Frequently (although not always), the females are related and remain as the permanent nucleus of the group. The male joins the group of females by pushing out the previous male. Males may be part of a group for relatively short periods of time (one or two years). They defend their territory against other males, until they themselves are pushed out. Animals with this type of social organization often have significant sexual dimorphism (sexual differences in size). Males are bigger than females, both so they can dominate females in the group, but also because they are under significant selection for size so they can fight off other males.

This type of social organization gives the small group of primates access to the resources within their jointly-held territory. The territory tends to not be very large (depending on how plentiful resources are), and is defended against conspecifics. Mating opportunities are only found within the group. Since only one male is in the group with multiple females, this means that lower-ranking males do not have mating opportunities. Usually, female offspring stay with their mothers after maturity, while male offspring leave (or are driven out by the dominant male) at sexual maturity. These younger males may form bachelor herds, groups of males without mates, who live together away from the territories of females and offspring, until they are old or strong enough to take over a group of their own. Offspring are raised by their mothers, but often mothers are helped by the other female members of the group, who are their sisters, aunts, and daughters. The biggest threat to the offspring comes when a new male takes over the group. He will often kill the youngest (unweaned) offspring so the females will become fertile more quickly and mate with him. It's not in his best interest to protect the offspring of another male while waiting for that infant to grow up enough that its mother becomes fertile.

Multimale/Multifemale (also called promiscuous hoard*)

In multimale/multifemale groups, as the name implies, a number of adult males and females live together, with offspring. Usually, most or all of the males will reproduce with most or all of the females. Groups of this sort often have dominance hierarchies to determine access to mates, food, sleeping areas, and other key resources. This is a very common type of social organization among primates and is found in some prosimian, monkey, and ape species, including the chimpanzee.

These groups have access to the resources within the territory they hold. Often, these are relatively large territories, and they may be aggressively defended. Chimpanzees, for example, send out patrols to walk the borders of their territory, and they will kill trespassers. In this type of social organization, all individuals have access to mates, although dominant individuals have the first choice. Since many males will mate with each fertile female, primates with this type of social organization often have very large testes, reflecting the importance of sperm competition. That is, if several males mate with a female in estrous, then the individual who produces the most sperm is more likely to be the father of any subsequent offspring. Males may also be somewhat bigger than females, because males are at an advantage if they can dominate females for mating opportunities, and size helps in male-male fighting for dominance.

Multimale/multifemale groups may have a core of related males, or a core of related females, depending on which sex disperses at sexual maturity. It can be either, but in the best-studied groups, the chimpanzee, it is the female that disperses. In groups where males are related, females tend to suffer more violence. The opposite is true in groups where the females are related. They tend to band together to drive off aggressive males. (Although, interestingly, among the bonobos unrelated females frequently band together to punish male aggression and prevent violence.) Offspring are raised by their mothers. In groups where the females are not related to each other, females may need to protect their offspring from other females in the group, as well as from males. Because no one male is particularly likely to be the father of any one offspring, males seldom help raise offspring and may be violent toward them. Infanticide has been well documented among chimpanzees, and is particularly likely if the infant is not the offspring of any male in the group. For example, if a female chimpanzee enters the group already pregnant, or if she disappears for a while and returns pregnant, then the males of the group may kill her infant. Both common chimpanzees and bonobos have developed a system of false estrous (presenting signs of fertility even when not fertile) which makes it very hard for males to know when a female conceived. In common chimpanzees, females will show signs of fertility (sexual swellings) and continue to mate each month, even though they are already pregnant. Among bonobos, females maintain their sexual swellings and willingness to mate at all times. It is thought that these traits are an attempt at paternity confusion, basically, making it hard for male chimps to know when the females conceived so they fool themselves into thinking they could be the father, even if it was impossible. Note, the third chimpanzee, humans, has a similar adaptation. We don't have false estrous, we have hidden estrous (it's hard to know when human females are fertile), but it serves a similar function.

*OK, it really hasn't been called that since the 19th century, but it's too good a name to leave off

Pair-bonding (also called monogamy or monogamous pair)

Monogamy is when a mated pair and their immature offspring live together in a small territory. Adults usually do not tolerate other same-sex adults, and will defend their mates and territories from them. These animals usually have little sexual dimorphism, since size is not needed to obtain mates (everyone has a partner), nor are males more territorial than females. This is a relatively uncommon breeding structure in primates, although it is found in a number of prosimian, monkey, and ape species, including the gibbon. Note that a number of primate species - including humans - are facultatively monogamous, meaning that many or most individuals live with one partner and their shared offspring, but they are perfectly capable of living in other types of mating structures if possible, and in fact would often prefer to do so under the right conditions. The term "facultative" will occur frequently in this class. In the evolutionary or behavioral sciences, "facultative" means "optionally" or "usually" as opposed to "obligate" or "required".

In monogamous primates, the bonded pair usually share a relatively small territory, and they control all the resources found there. In general, all individuals will find a mate, and at sexual maturity, offspring of both sexes leave their home to find mates of their own. In most species, both mother and father will care for the offspring. Because of the monogamous social organization, males can be relatively certain of paternity, so it is worthwhile for them to invest in the offspring.

One-female/Multi-male (also called polyandry)

In one-female/multi-male groups, as the name implies, one adult female has two or more male mates living with her and their offspring. This is a rare form of social organization that is best known in New World monkeys. It was described in detail in the reading about marmosets and tamarins.

The small group shares the resources from a (generally) small territory. Even in cases where many individuals are in one-female/multi-male groups, others of the same species are in facultatively monogamous groups, so mate competition is relatively rare. Instead, some females stay with their mothers, helping to raise their siblings. While living with their mothers, these daughters have their reproductive cycles suppressed, to prevent pregnancy. By helping to raise their siblings, they are passing down their own genes just as surely as they would through their own offspring. This is a form of cooperative breeding and inclusive fitness, both of which are commonly seen in humans. Male marmosets and tamarins who are in polyandrous groups are often brothers (a trend that is also found in polyandrous human societies). Again, this is a form of inclusive fitness, since the males are helping raise their nieces and nephews, even if they are not the father of the offspring themselves. This type of social organization is most common in species or environments where a female would not be able to raise her offspring without significant help from her mates and/or other offspring. The males help protect, carry, and feed the infants.