The Princeton Guide to Ecology

Social life is a mix of cooperation, altruism, and selfishness. In species as diverse as slime molds, army ants, and great apes, individuals coordinate actions to achieve common goals. Yet competition and conflict are common within social groups and may lead to lethal altercations. Consider, for example, a well-integrated, long-lived society such as a colony of the honeybee Apis mellifera. Cooperative foraging is organized by communication among the worker bees, allowing the colony to allocate effort flexibly over a large region surrounding the hive. By acting in concert, nestmates build combs, raise young, and maintain a comfortable nest temperature even through snowy winters. When the hive is threatened by a vertebrate predator, workers sacrifice their lives to protect the queen and her offspring, perhaps the most celebrated example of altruism among the insects. Yet the benefits of this collective activity are not evenly shared. Although there may be well over 15,000 females in the society, one of them—the queen—lays the vast majority of eggs while eggs laid by other females are quickly eaten. Other conflicts are evident. As winter approaches, males, which do no work, are dragged out of the hive and left to die. When a new queen is reared, she may sting to death other prospective queens still developing in their royal cells, bringing reproductive competition to a deadly conclusion. Like other social species, the honeybee prompts two central questions. How do organisms benefit from group living? What prevents conflicts within the group from undermining cooperative aspects of social life?

Allee effect. An inverse relationship between population density and per capita population growth rate. Allee effects can accelerate the decline of a shrinking population.

altruism. Behavior that is costly to the individual performing it and is beneficial to one or more other individuals; costs and benefits are measured in terms of effects on fitness, which can be quantified by lifetime reproductive success.

coefficient of relatedness. The probability that one animal shares an allele carried by another as a result of descent from a common ancestor.

cooperation. Behavior that benefits two or more interacting individuals.

kin selection. Selection resulting from the effects of an organism on the fitness of relatives, as well as through the organism’s own reproduction.

policing. Actions by group members that suppress or punish selfish behavior by other group members.

selfishness. Behavior that benefits the individual performing it at a cost to one or more other individuals.

self-organization. In social species, this refers to phenomena in which group organization arises spontaneously, without central control, because of the actions and interactions of multiple individuals.

Why do so many organisms live in groups? To behavioral ecologists, the abundance and diversity of social species suggest that in many environments the benefits of group living outweigh its costs. The forces driving sociality vary, but field studies have revealed a few principal advantages, which recur in diverse taxa. One set of advantages emerges in the context of foraging. By acting in groups, animals can improve foraging success through enhanced search, ability to overcome prey defenses, or ability to outcompete other groups or individuals. On the other side of the hunt, potential prey often seek to escape capture by clustering and moving together. Group activity makes it more difficult or more dangerous for a predator to attack, and potential prey can maneuver for positions in which they are less vulnerable than other group members. Shared vigilance

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