The range of social behaviour is best understood by considering how sociality benefits the individuals involved. Because interacting with other individuals is inherently dangerous and potentially costly, both the costs and benefits of social behaviour and the costs and benefits of aggregating with others play a role in the evolution of aggregation.

On the positive side, aggregation may provide individuals with increased access to food through information sharing and cooperative defense against non-group members. Conversely, close contact with members of the same species increases the risk of cannibalism, parasitism, and disease. This is illustrated by studies of cliff swallows (Hirundo pyrrhonota), which suggest that the original benefit of nesting near other individuals and forming colonies was information sharing and increased ability to exploit a highly variable insect food resource. Once colonies were formed, other benefits arose including more efficient detection of predators. These benefits are countered by several costs of coloniality, including increased susceptibility to ectoparasites (that is, parasites such as fleas and ticks that live on the body surface of the host), increased incidence of food stealing (kleptoparasitism), and the need to travel greater distances to foraging areas.

Some costs and benefits overlap. For example, coloniality increases the opportunity for some males to mate with females other than their primary mate (extra-pair matings); it is a benefit for the males succeeding in obtaining extra-pair matings and a cost to the cuckolded males. Similarly, coloniality allows some females to lay eggs in the nests of other females in the colony (conspecific brood parasitism); it is beneficial to them but costly to the parasitized pair. The outcome of these multiple factors, which include the extent to which each individual involved is affected, is a delicate balance leading to wide variation in group sizes ranging from solitary nesting to nesting in colonies of several thousand pairs.

In groups, potential benefits of efficient predator detection and prey acquisition may be diluted by the costs of sharing food and reproductive opportunities. Furthermore, all individuals within groups are not equal; as dominant individuals monopolize a group’s resources more effectively, group living becomes less beneficial for subordinates. If social behaviour is to be maintained in a population, even subordinate individuals must gain more from being social than from leaving the group and trying to survive and reproduce on their own.

On the other hand, aggregation may be advantageous due to the energy saved by huddling during cold weather, increased survival through group defense, or increased ability to acquire, hold, and make efficient use of resources. Animals may aggregate by mutual attraction to each other, by mutual attraction to limited resources, or as a side effect of having hatched from eggs laid together in a clutch. In some cases more than one mechanism of attraction is involved. For example, bark beetles (family Scolytidae) form large aggregations by mutual attraction to the bark of a fallen log and also to the odours of other members of their species.

Regardless of the mechanism of attraction, once animals are grouped there is selection to evolve increasing degrees of communication, cooperation, individual recognition, and efficiency to better exploit the potential advantages of group living. For example, in some treehopper (family Membracidae) aggregations, nymphs communicate the threat of a predator by using vibrations, which humans can detect only with electronic instruments. A more sophisticated form of communication is found in eastern tent caterpillars (Malacosoma americanum), which rest in a communal tent that increases in size as they grow and add silk. Colony members leave the tent on brief forays to feed on foliage within their tree, at which point they lay chemical trails that other group members follow to locate high-quality feeding sites. A similar kind of central-place foraging is practiced by some colonial birds, such as cliff swallows, among which unsuccessful individuals often cue in on other birds returning to their nest with food and follow them to productive foraging sites.

In addition to feeding and defensive aggregations, some aggregations are based exclusively on mating. These include the explosive breeding assemblages of frogs and toads, the aggregations of male birds and mammals at leks (display sites used only for mating), and various insect aggregations including bees and wasps (order Hymenoptera), flies (order Diptera), and butterflies (superfamily Papilionoidea). Some species in each of these groups congregate at conspicuous landmarks visited by females. Frequently, the aggregation of one sex provides opportunities for the other. For example, when females aggregate due to the clumping of food or nest sites, males are likely to aggregate at these sites as well because they are the most efficient places to find females with which to mate.

Other groups include flocks or herds that form during migration and coalitions that form due to group advantages in holding or acquiring a reproductive vacancy. Coalitions of male African lions (Panthera leo) that compete for control of groups of females (called prides) are a classic example of the latter. Migration in herds is common and can involve tremendous numbers of individuals. For example, more than one million blue wildebeest (gnu; Connochaetes taurinus) typically migrate in a clockwise fashion over the plains of East Africa, covering a distance of over 2,500 km (about 1,550 miles) each year in search of rain-ripened grass. The record size for migratory aggregations is probably the African desert locust (Schistocerca gregaria), which forms huge swarms covering as much as 200 square km (about 80 square miles); each swarm contains upward of 10 billion individuals moving more or less cohesively in search of food. Among vertebrates, the largest known aggregations were probably those of the now-extinct passenger pigeon (Ectopistes migratorius) of North America. Vast groups of these birds migrating together in search of food, particularly large acorn crops, reportedly exceeded three to five billion individuals.

When group members are genetically related, interactions will potentially involve nepotism, the tendency for individuals to favour kin. Examples of this sort of favouritism include parents favouring their own offspring, siblings forming alliances, and a tendency for individuals to favour their closest relatives. In Siberian jays (Perisoreus infaustus), parents tolerate their offspring on their territories for up to three years, allowing them preferential access to food. Like the African lions mentioned previously, acorn woodpeckers (Melanerpes formicivorus) in southwestern North America and Central America form same-sex sibling coalitions to better compete for reproductive vacancies. European long-tailed tits (Aegithalos caudatus) return home to help feed young still residing in their parents’ nests when their own breeding attempts fail. All these behaviours are facilitated by the genetic relatedness between the individuals involved.

The pinnacle of social behaviour is found in eusocial species. Eusocial species live in multigenerational family groups in which the vast majority of individuals cooperate to aid relatively few reproductive group members (or even a single member). Eusocial behaviour is found in ants, bees, some wasps in the family Vespidae, termites (order Isoptera; sometimes placed in the cockroach order, Blattodea), some thrips (order Thysanoptera), aphids (family Aphididae), and possibly some species of beetles (order Coleoptera). Blesmols, such as the naked mole rat (Heterocephalus glaber) and the Damaraland mole rat (Cryptomys damarensis), engage in truly eusocial behaviour; they are the only vertebrate species known to do so. Eusocial species often exhibit extreme task specialization, which makes colonies potentially very efficient in gathering resources. Workers in eusocial colonies are thought to forgo reproduction due to constraints on independent breeding. Such constraints include shortages of food, territories, protection, skill, nest sites, appropriate weather for breeding, and available mates. Workers may never reproduce during their entire lives, but they nonetheless gain inclusive fitness benefits by aiding the reproduction of a queen, who is typically their mother. Such assistance often takes the form of foraging for food, caring for the young, and maintaining and protecting the nest.

The cost-benefit approach can also be applied to reproductive interactions. For example, the reproductive behaviour of many species is designed to achieve multiple mating, but this behaviour may be associated with certain costs, such as the increased risk of injury or of contracting a sexually transmitted disease. Among many species, mating is essentially promiscuous, with individuals either shedding their gametes into the environment without mating or with individuals pairing briefly, just long enough to effect fertilization. The other extreme includes long-lived animals such as Bewick’s swans (Cygnus columbianus), which may live for up to 25 years and mate for life. In addition, it has been shown that within many species some individuals engage in same-sex mating; the individual and group costs and benefits of these behaviours likewise vary among species. Mating behaviour in animals includes the signaling of intent to mate, the attraction of mates, courtship, copulation, postcopulatory behaviours that protect a male’s paternity, and parental behaviour. Parental behaviour ranges from none to vigilant care by both parents and even by additional group members. Biologists refer to the investment in interactions that influence the likelihood of parenting offspring as “mating effort” and the investment that increases the survival or condition of offspring as “parental effort.”

Social behaviour is also involved in social dominance and the maintenance of territories, regardless of whether dominance status or territories are held by individuals or by groups of individuals. Territorial species tend to be distributed over the landscape in a more regular fashion than would be predicted if they used the landscape randomly. An important concept in understanding territorial behaviour is the notion of economic defendability. Economic defendability postulates that, in order to be territorial, the benefits of maintaining exclusive access to a space must outweigh the individual’s or group’s costs of defending the space from other members of its own kind. In the territorial systems of many species, overt defense in the form of direct aggressive behaviour against intruders has given way to indirect defense in the form of vocalization and scent marking.

Social behaviour is a complex combination of the costs and benefits of living in groups, dominance interactions, conflict between the sexes, nepotism when groups are composed of relatives, and cooperation. The diversity of social behaviour has provided significant material for evolutionary biologists interested in understanding natural selection and the process of evolution.

A historical perspective on the study of social behaviour

Various aspects of animal social behaviour intrigued humans for hundreds, if not thousands, of years. Social behaviour has been documented by writers starting with Aristotle (c. 330 bce); however, it was Charles Darwin’s On the Origin of Species in 1859 that initiated the modern approach with its assertion that behaviour, like morphology and physiology, evolves through natural selection. Darwin is also remembered for being the first to discuss sexual selection, the special form of natural selection that acts via competition for mates and female choice of mating partners, accounting for such elaborate traits as the antlers of red deer (Cervus elaphus) and the tails of peacocks (Pavo cristatus).

The study of social behaviour during the remainder of the 19th century focused largely on description and gradual acceptance of Darwinian evolution. Starting in the early part of the 20th century, however, several workers embarked on the study of animal social behaviour from an evolutionary standpoint—for example, British naturalist H. Eliot Howard (Territory in Bird Life, 1920), American entomologist William Morton Wheeler (Social Life Among the Insects, 1923; and The Social Insects, Their Origin and Evolution, 1928), British statistician and scientist R.A. Fisher (The Genetical Theory of Natural Selection, 1930), American ecologist W.C. Allee (Animal Aggregations, 1931; and The Social Life of Animals, 1938), English ecologist Fraser Darling (A Herd of Red Deer, 1937; and Bird Flocks and the Breeding Cycle, 1938), and English ornithologist David Lambert Lack (The Life of the Robin, 1943). In addition, English biologist Sir Julian Huxley’s Evolution, The Modern Synthesis (1942) merged Darwin’s thinking with new knowledge of genetics to transform evolutionary biology into a comprehensive paradigm for understanding evolutionary change.

Lack became particularly influential in the second half of the 20th century, championing the view that virtually all aspects of behaviour could be understood in an evolutionary context by focusing on benefits for individuals. His career peaked with the publication of Ecological Adaptations for Breeding in Birds (1968). This work was one of the first major studies of social behaviour to make extensive use of the comparative method, which attempts to understand how natural selection favours particular traits by comparing the ecology of related species.

Other particularly influential workers in that era included Austrian zoologists Konrad Lorenz, who first described the social phenomenon of imprinting, and Karl von Frisch, who made extensive observations of the social communication and dance-language of honeybees, and Dutch-born British zoologist and ethologist Nikolaas Tinbergen, who was one of the first to perform field experiments to test hypotheses of social behaviour. These three are often considered the founders of ethology, and they shared the Nobel Prize in Physiology or Medicine in 1973 for their “discoveries concerning organization and elicitation of individual and social behaviour patterns.” That was the only time workers in this field have been so honoured.

Several watershed events in the study of social behaviour took place in the 1960s and ’70s. First was the challenge to Lack by English zoologist V.C. Wynne-Edwards, whose controversial Animal Dispersion in Relation to Social Behaviour (1962) proposed a pervasive role for group selection, allowing sacrificial behaviour for the good of the group or species. Although largely discounted by the majority of workers, who believed that such altruism should rarely evolve, Wynne-Edwards’s advocacy of this view prompted a careful reappraisal of the evolutionary basis of social behaviour that continues to this day.

Second was British evolutionary biologist W.D. Hamilton’s proposal in 1964 that kin selection plays a role in the evolution of altruism, cooperation, and sociality. Kin selection is based on the concept of inclusive fitness, which is made up of individual survival and reproduction (direct fitness) and any impact that an individual has on the survival and reproduction of relatives (indirect fitness). The elements of kin selection lead directly to the concept now known as Hamilton’s rule, which states that aid-giving behaviour can evolve when the indirect fitness benefits of helping relatives compensate the aid giver for any losses in personal reproduction incurred by helping. Hamilton’s theory of kin selection is now considered one of the foundations of the modern study of social behaviour.

The third major advance in social behaviour during this era was the sweeping summary and prospectus of the field provided by American biologist E.O. Wilson with Sociobiology: the New Synthesis (1975), which laid the cornerstone for the modern interdisciplinary study of animal behaviour. Although the bulk of Wilson’s book is not controversial, a final chapter attempting to understand the evolution of human social behaviour using adaptationist principles ignited such an intense debate that the very word sociobiology, until that time used synonymously with animal social behaviour, is now usually restricted to the application of such principles to human behaviour. Although some people remain disturbed by the idea of applying sociobiological principles to human behaviour, the approach has flourished and provided insights into human behaviour that could not have come to light with alternative, nonevolutionary worldviews.

The study of social behaviour remains active, involving the investigation of proximate mechanisms (that is, behaviour triggered by immediate stimuli coming from the outside world or inside the body), the survival and reproductive consequences of sociality, and the evolution of human behaviour and cultural traditions. Social behaviorists today study a wide range of species from ants to whales and an equally wide range of topics that span from the genetic basis of particular social characters to the evolutionary origins of group living.