Evolution of Age‐Dependent Social Behaviour

Abstract

Abstract Age‐dependent social behaviour is widespread in the animal world, and it represents an important source of intra‐ and interpersonal phenotypic variation within populations. Age‐dependent social behaviour occurs across all major taxa, ranging from microbes and insects to birds and mammals, including humans. Inclusive fitness provides the key theoretical framework for explaining age‐dependent social behaviour. It posits that the process of natural selection leads to organisms that behave as if maximising their inclusive fitness. When the inclusive fitness of an organism changes with its age, the theory predicts the evolution of age‐dependent behaviours. Inclusive fitness can explain highly diverse forms of behaviour across a wide range of species. It explains the self‐sacrificing defensive behaviour of juvenile aphids and older termites; age discrimination in insects, birds and mammals or the prolonged postreproductive lifespans observed in humans and cetaceans. Key Concepts The concept of inclusive fitness is seen as an intrinsic attribute of organisms and the quantity that is maximised by the action of natural selection. The condition for the evolution of a social behaviour is given by Hamilton's rule, − c + br > 0, where c is the fitness cost of the behaviour, b is the fitness benefit and r is the relatedness between the actor and the recipient. Inclusive fitness partitions fitness into a direct fitness component, − c , corresponding to Darwinian fitness, and an indirect fitness component, br . The signs of costs and benefits define four types of social behaviours: altruism (+/+); cooperation (−/+); spite (+/−); and selfishness (−/−). When the inclusive fitness of organisms varies with their age, and organisms are able to express conditional behaviours, inclusive fitness theory predicts the evolution of age‐dependent social behaviour. Age may affect any of the dimensions of life of an organism, which include personal condition, and social and natural environment, and all dimensions can contribute to the evolution of age‐dependent social behaviour. Relatedness, reproductive value and marginal fitness effects are three key measures of value that determine the inclusive fitness of an individual, all of which can change with the age of the interacting individuals. Because residual reproductive value often varies with the age of organisms, reproductive value can mediate the evolution of age‐dependent social behaviour. Residual social (helping) value is a cumulative measure of the future benefits an actor provides to its social partners, a quantity that may change with age and therefore mediate the evolution of age‐dependent social behaviour. Group‐age composition is the age profile of a group, a factor that may affect group productivity and the inclusive fitness of its members. Age‐discrimination mechanisms may evolve when the indirect fitness benefits are sensitive to the age of the recipients of a behaviour.