Unlike many other animals, primates are highly social and many live in stable groups consisting of adult males and females, even outside the breeding season, when females are receptive and available for mating because they are not pregnant or nursing. Indeed, sociality, or the tendency to form social groups, is a key behavioral adaptation of the order primates (see Chapter 5). This has led primatologists to ask two questions: “Why do primates live in groups?” and “What types of groups do primates live in?”
Why Do Primates Live in Groups?
Primates live in groups when the benefits of doing so exceed the costs. Although there are many potential benefits to group living, enhanced feeding competition and predator avoidance are important benefits for many group living primates. When primates feed on high-quality, scarce food (like fruit), larger groups are more successful in competition with other groups. For example, in a long-term study of vervets in Kenya’s Amboseli National Park, larger vervet groups had larger and better home ranges, which is the area in which a group regularly moves around as it performs its daily activities, including searching for food and water. Females in larger groups had higher average infant and female survival rates than the smallest group. Because pregnancy and nursing are energetically expensive for females, female reproductive success, or genetic contribution to future generations (measured by the number of offspring produced), is limited by access to food. Although living in a group means females compete with members of their own group for food, the benefits of being a member of a larger vervet group outweigh the costs (Cheney and Seyfarth 1987).
However, because they contain more individuals, larger groups are more likely to attract the attention of predators compared to smaller groups. This is one of the reasons that primates who rely on crypsis, or the ability to avoid detection by others, including predators, are often solitary (the term used to describe individuals who do not live together with other members of their species) and nocturnal, or active at night. If an animal is already hard to see because it is active at night, then moving quietly in small groups is a good strategy to avoid detection by predators. The slow loris (Nycticebus coucang) of Southeast Asia is a good example of this strategy (Figure 6.9a). Nocturnal and solitary, the slow loris moves slowly and quietly as its primary strategy to avoid detection (Wiens and Zitzmann 2003). In contrast, primates who live in large groups and are diurnal, or active during the day (like gelada baboons [Theropithecus gelada]; Figure 6.9b) cannot avoid detection by predators. Instead, group-living primates rely on behaviors that alert others to the presence of danger and/or deter predators, including shared vigilance (watchful behavior to detect potential danger), mobbing (the act of cooperatively attacking or harassing a predator), and alarm calling (vocalizations emitted by social animals in response to danger). We will discuss alarm calls in the Communication section.
What Types of Groups Do Primates Live In?
Primates vary with regard to the types of groups in which they live. A social system describes a set of social interactions and behaviors that is typical for a species. The components that make up a species’ social system include:
- Group size, which refers to the number of individuals that typically live together. Primate group size can be highly variable, ranging from one or a few individuals, to a few dozen, upward to several hundred individuals.
- Group composition describes group membership in terms of age class (e.g., adult, juvenile, infant) and sex. In some primates, groups consist of a mother and her dependent offspring while in others, one adult male lives long-term with one adult female and their dependent offspring. In other species, one or more adult males live with multiple females and their offspring.
- A species’ mating systemrefers to which male(s) and female(s) mate. The terms that describe a mating system (e.g., polygyny, in which one male mates with multiple females) are sometimes used to describe a primate species’ social system, but a mating system is one component of the species’ social system. For example, two species might both have polygynous mating systems, but in one species, the group is composed of one male and multiple females, while members of the other species live as solitary individuals.
- Ranging behavior refers to the way in which animals move about their environment. Most primate species have a home range, where they perform their daily activities. Some primates defend a territory which is the part of the home range that the group actively guards in an attempt to keep out conspecifics.
- Dispersal patterns describe which sex moves to a new group to reproduce. In most primate species, males disperse because the benefits of dispersal, including increased access to mates and reduced competition from other males, outweigh the costs of migrating into a new group, which often comes with aggression from current group members. For many female primates, the opposite is true: females usually benefit from remaining philopatric , or in the group of their birth. This allows them to maintain strong alliances with female relatives, which helps them compete successfully against other groups for food. In solitary species, offspring of both sexes leave their mother’s home range and become solitary. If this did not happen, the species would not be solitary. Even though both sexes disperse in solitary species, males usually disperse farther than females.
- Social interactions describe the ways in which individuals interact with members of their own and other groups of conspecifics. Affiliative (i.e., friendly or nonaggressive) behaviors include grooming(picking through the fur of another individual), playing, or coalitions (temporary alliances between individuals). Agonistic(i.e., aggressive) behaviors include fighting over food or fighting over access to mates. In groups that contain multiple adult individuals of the same sex, it is common to have a dominance hierarchy, or a group of individuals that can be ranked according to their relative amount of power over others in the hierarchy. Initially, dominance hierarchies are established through the outcome of conflicts. Individuals who lose conflicts with others are subordinate(or low rank) to those who win them. Those who win conflicts are dominant (or high rank). Dominant individuals gain access to resources, like food or mates, before subordinates. Once a hierarchy is established, agonism decreases because everyone “knows their place.”
The main types of primate social systems are as follows: solitary; single-male, single-female; single-male, multi-female; multi-male, multi-female; fission-fusion; and multi-male, single-female. These types are discussed below.
Recall that the term solitary is used to describe species in which individuals do not live or travel together with other members of the same species, except for mothers and unweaned offspring. Males typically occupy a large home range or territory that overlaps the home ranges of multiple females, with whom they mate (Figure 6.10a). Because one male mates with multiple females, the mating system of solitary primates is polygyny. Social interactions between adults are limited but because some males do not get to mate, competition between males is intense. When males compete physically, they benefit from large body size and weaponry. The result is sexual dimorphism, when males and females look different from one another. Both males and females disperse, although males move farther from their mother than females. The nocturnal West African potto (Perodicticus potto; Figure 6.10b) is solitary. Bornean orangutans, which are diurnal, are also solitary.
Primate species in which an adult male and adult female live together with their dependent offspring have a single-male, single-female social system, sometimes referred to as a “family,” with group sizes between two and five individuals. The adult male and adult female engage in behaviors that strengthen their social relationship, or pair bond, including mutual grooming and resting together. The pair defend a territory (Figure 6.11a) and keep same-sex individuals away from their mate. The adult male and adult female mate with each other, so the mating system is monogamy, although mating outside the pair bond may occur. Species with monogamous mating systems are usually sexually monomorphic (males and females look similar) because competition for mates is relaxed since most males are able to obtain a mate. Males are usually confident that they are the father of their mate’s infant, so they help with offspring care by carrying the infant when it is not nursing. Once offspring are sexually mature, both males and females disperse. As with solitary species, males disperse farther from their parents than females. Bolivian Gray titi monkeys (Plecturocebus donacophilus) are an example of a species that has a single-male, single-female social system. One of their signature behaviors is tail twining, when two individuals sit with their tails wrapped around each other (Figure 6.11b). This behavior reinforces the social bond among family members and is especially common between the adult male and female. Gibbons (Hylobates) and owl monkeys (Aotus) also live in single-male, single-female groups.
Single-male, multi-female groups consist of one adult male living with multiple adult females and their dependent offspring (Figure 6.12a ) . These groups can range from as few as five or ten individuals to as many as 50. Female social relationships are governed by the female dominance hierarchy. Females are usually philopatric and males disperse. Males who are unable to join a group of females may join a bachelor group with other males. Because a single male mates with multiple females, the mating system is polygyny. Species that form single-male, multi-female groups may or may not defend a territory, but the resident male, who lives with a group of females, is aggressive toward other males, who may try to take over the group and become the new resident male. Competition between males to be the resident male of a group is intense, and these species usually display sexual dimorphism, with males being larger than females and possessing large canines. Hanuman langurs (Semnopithecus entellus) of India form single-male, multi-female groups (Figure 6.12b). When a new male takes over a group of females and ousts the former resident male, he may commit infanticide, or kill the unweaned infants. This is especially likely if the new resident male has not yet mated with any of the females and thus cannot be the infants’ father. This causes the females, who were nursing, to become sexually receptive sooner, increasing the new resident male’s chances of producing offspring (Sharma, Ram, and Raipurohit 2010). Gorillas, patas monkeys, and golden snub-nosed monkeys (Rhinopithecus roxellana) also live in single-male, multi-female groups.
Multi-male, multi-female groups consist of multiple adult males living with multiple adult females and their dependent offspring. Although there is more than one adult male, there are more adult females than adult males in the group (Figure 6.13a). Multi-male, multi-female groups can range in size from about ten to as many as 500 individuals. They occupy a home range but may or may not defend a territory. In groups that contain multiple males and multiple females, it is not possible for one male to monopolize all the matings, so the mating system is polygamy, in which multiple males mate with multiple females. However, this does not mean that all males have an equal opportunity to mate with all females. In multi-male, multi-female groups, both males and females form a dominance hierarchy. The male dominance hierarchy determines their access to females for mating in much the same way that a female dominance hierarchy determines a female’s access to food. Because their place in the hierarchy can affect their reproductive success, males compete with each other, but because it is rare for males to be excluded from mating altogether, the level of competition and degree of sexual dimorphism are less extreme than what we see in polygynous species. Usually, females are philopatric and males disperse. Vervet monkeys (Figure 6.13b), ring-tailed lemurs (Lemur catta), white-faced capuchins (Cebus capucinus), and black-capped squirrel monkeys (Saimiri boliviensis) live in multi-male, multi-female groups.
Fission-fusion is a fluid social system in which the size and composition of the social group changes, with groups splitting (fission) or merging (fusion) depending on food availability (Pinacho-Guendulain and Ramos-Fernández 2017). When key resources are scarce, individuals spread out (fission) and move and feed individually or in small subgroups (Figure 6.14a). When key food resources are plentiful, individuals come together (fusion) and individuals travel and feed as a more cohesive group (Figure 6.14a). Fission-fusion social structure is believed to reduce feeding competition when resources are scarce. Because group composition changes over time, species with fission-fusion social systems are referred to as a community. Communities consist of multiple adult males, multiple adult females, and offspring, and group size varies but typically ranges from ten to a few dozen individuals. Females typically disperse and males are philopatric. Thus, community males are related and display unusual forms of cooperation. The mating system associated with fission-fusion is polygamy. Because males are not excluded from mating, competition for mates is relaxed and sexual dimorphism is moderate (males are slightly larger than females). Geoffroy’s spider monkeys (Ateles geoffroyi) (Figure 6.14b) and chimpanzees both have fission-fusion social system.
In multi-male, single-female groups, two or more males live with one breeding female, her dependent offspring, and non-breeding females (Figure 6.15a). This type of social system is found in the callitrichids, the primate family that includes marmosets (Callithrix; Figure 6.15b) and tamarins (Saguinus) of Central and South America. Their groups rarely exceed 15 individuals, and each group actively defends their territory from conspecifics. Although more than one adult female may live in the group, the mating system is polyandry because there is only one breeding female who mates with all of the adult males. This is achieved through reproductive suppression, which involves the breeding female preventing other females from reproducing through physiological and/or behavioral means (Digby, Ferrari, and Salzman 2011). This limits the opportunities for other females in the group to become pregnant. Instead, these females, and the males in the group, help raise the breeding female’s offspring. This is referred to as cooperative breeding and usually takes the form of carrying infants, grooming them, and protecting them from danger (de Oliveira Terceiro and Burkart 2019). Because reproductive opportunities for female tamarins and marmosets are limited, they are very competitive, and females are slightly larger than males, which helps them compete for the breeding spot in a group.