Primates form different kinds of groups. While there is variation within every species, each species has its own typical range of group sizes. Primate groups also vary in their structures. The structure of a group involves details about what kinds of individuals live in a group together. The group structure includes the number of adult males compared to females, called the sex ratio. It also includes the pattern of interactions among individuals, such as whether the entire group spends all its time together, or whether different individuals break into smaller parties for substantial amounts of time. The size and structure of groups have an adaptive role --- some kinds of groups are better suited to certain ecological or social contexts than others.
The size and structure of groups in a species is a product of selection on individuals. Individuals always have choices: they can stay in a group, or they can leave; they can permit others to join their group or they can confront newcomers aggressively; they can join together to form a new group, or they can go it alone. Some choices will be more adaptive than others, and individuals who make the right choices about what kind of group to live in will tend to survive or reproduce more. This is how group sizes and structures evolve.
In an evolutionary context, the kinds of groups found in different primate species are products of their ecology and social interactions. Typical group sizes can vary greatly among even closely related species or primates. The typical social group in orangutans, for example, consists of a single mother and her offspring. In contrast, chimpanzees --- who are relatively close in body size and diet to orangutans --- live in complex communities averaging over fifty individuals. The differences between these species' social groups must be explained by differences in their habitat, movement, mating system, and history. By studying primate groups, primatologists attempt to understand how these factors result in certain kinds of groups but not others.
Most primates are small to medium-sized mammals, and they are vulnerable to many kinds of predators. Carnivores, including leopards and hyenas in the Old World, and jaguars in the Americas pose a serious threat, even to large primates. Other natural predators include hawks and eagles, snakes, humans and other primates. Indeed, other primates are the most significant predators of some species. For example, chimpanzees are the most common predator of red colobus monkeys, as described below.
Most monkeys suffer high rates of predation by carnivores and other predators, but these rates are highest for those species that live in relatively open, exposed habitats. In vervet monkeys, which live in relatively open savanna, predators account for up to 70 percent of deaths (Cheney and Wrangham 1986). Baboons are much larger primates than vervet monkeys, but the rate of predation on savanna baboons is very high as well. At these levels, predation exerts a strong selective force on these populations.
For these primates, large groups tend to minimize predation risk. A large group of primates has more eyes, and is therefore more likely to notice the approach of a predator. Another strategy to resist predators is for individuals to raise a cry or alarm when they see a predator approach. Alarm calls can be a benefit to such groups, but an individual raising an alarm also raises its own risk --- predators will notice its noisy alarm. But in larger groups, a sudden scatter of alarmed primates competes for attention with the alarm call itself. In large groups, individuals can spend less time scanning for predators, meaning that they can devote more time to foraging. Likewise, the collective action of some individuals may deter predators from attacking. The mere presence of a collection of large male baboons may deter a not-so-hungry jackal, when a lone female and infant would be more likely to suffer attack.
Movement and food patches
Some kinds of habitat are much harder to move through than others. Terrestrial primates can move very quickly on the ground. But if they are on the ground far from a tree, they can be highly exposed to terrestrial and aerial predators. Small primates may be highly mobile through dense forest, because they can move quickly from tree to tree. Large primates must move more deliberately through trees, since not every branch can easily support their weight.
Size and difficulty of movement may partially underlie the differences in group size between chimpanzees and orangutans. Chimpanzees can move effectively across the forest floor, and this terrestrial movement is their major way of traveling from place to place. In this way, they can maintain relatively large groups, as sets of individuals can forage or patrol over relatively long distances and still return regularly to other group members. In contrast, much of the forest habitat of orangutans has a swampy or closed forest floor through which it is very difficult to move. In some parts of the orangutan range, tigers add to the danger of the forest floor. As a result, orangutans spend the overwhelming majority of their time in the forest canopy, where movement is slow but safer (Knott 2001).
Another element of ecology interacts with movement costs to affect group size: the distribution of food. Foraging for food is one of the major activities that primates conduct every day. Whether a group of primates is large or small, every individual must eat. For a group of primates that finds a huge tree with ripe fruit, life is relatively easy --- they can stay there and eat as long as they are not driven off by competitors or predators. If such large patches of food are common enough, the primates can stay in a large group and move from one patch to another. But nature does not only consist of large patches of ripe fruit and other primate foods. Sometimes fruits ripen a few at a time, so any tree can only feed a few primates. The availability of food can also vary from season to season, so that sometimes there may be plenty of food to eat, but at other times there is little.
The availability of food limits group size. If patches of food are small, then individuals within a large group may compete intensely for each bite. Being in a large group is a serious cost when resources are slim. This kind of limit to group size also affects orangutans. Orangutans eat a large amount of ripe fruit, but once they find a patch the amount of ripe fruit is usually relatively small for such a large-bodied primate (Terborgh and van Schaik 1987). Larger groups are generally impossible to maintain --- the orangutans would just go hungry unless they could find their own food sources, and it is too costly to move quickly between small patches of fruit. Indeed, in some forests where food is more plentiful, orangutans do tend to spend more time in groups of multiple individuals (Delgado and van Schaik 2000).