Chapter 13

Social Behavior and Sociality

1. Social behavior includes interactions that benefit one party (altruism, selfishness), both parties (mutual benefit, or mutualism), or neither party (spite). Identifying which individuals receive a benefit that overcomes the cost of a social interaction is essential for demonstrating that a behavior is truly cooperative. Cooperative behaviors are those in which the recipient of an interaction receives a fitness benefit (mutualism, altruism). In contrast, collective behaviors are the synchronized movements of individuals following a series of basic interaction rules.

2. Altruism is rare in nature, but reciprocal altruism (or reciprocity) between members of the same or even different species is more common and explains many of the cooperative interactions that behavioral biologists have observed, particularly among non-kin. In these cases, an individual (the donor) provides assistance to another animal from which it later receives repayment that on average increases the donor’s reproductive success enough to overcome the cost of its earlier helpfulness.

3. Individuals exhibit behavioral variation, including consistent differences over time (termed animal personalities) or across different contexts (termed behavioral syndromes). These behavioral differences can have positive effects on fitness.

4. Animals other than eusocial insects engage in various kinds of social interactions, including cooperation from which both parties derive immediate gains in direct fitness. In vertebrates, species that live in kin groups in which some individuals forego reproduction and help others rear young are called cooperative breeders.

5. Helpers in cooperatively breeding birds and mammals (and even in some insects, such as Polistes wasps) can gain direct and/or indirect benefits from aiding others in rearing their young. For example, in addition to gaining indirect fitness benefits, helpers may increase the likelihood of inheriting their territory.

6. In addition to these benefits, costs—often in the form of ecological constraints that limit dispersal and independent breeding—can influence the evolution of cooperative breeding behavior. Habitat saturation, which may be more likely to occur in long-lived species, can promote helping behavior, as can living in harsh and unpredictable environments where many individuals are needed to successfully raise young.

7. Reproductive conflict occurs in social vertebrates, just as it does in eusocial insects. In vertebrates, the fact that subordinate individuals are not sterile means that conflict over breeding can be intense and lead to reproductive suppression by dominant individuals. Suppression of subordinate reproduction increases reproductive skew within social groups, which can intensify social conflict and drive the evolution of traits used to gain access to reproductive roles, not just in males but in both sexes.