Variation in individual investment strategies among social animals

Cooperation and conflict are ubiquitous features of life in the vast majority of animals and can occur over a wide range of functional contents and at various levels. In this review I describe known and less well-known proximate aspects of cooperation and conflict over reproductive behaviour in social animals, where individuals other than the genetic parents contribute to the provisioning of care ('alloparental care'). Traditionally the evolution of alloparental care is viewed as a two-step process: the decision to delay dispersal and independent breeding, usually as a consequence of the existence of constraints on independent breeding, and the decision to behave as alloparents by which individuals that have delayed dispersal gain a net fitness benefit. Behavioural ecological theory generally assumes that all individuals are similar in 'make-up' and that life history and behavioural decisions are facultative. However, there is probably more individual variation in the amount and type of social behaviour than originally anticipated. Here, I demonstrate that some of these differences in behaviours are because of environmental factors, which may be associated with 'reaction norms' or the genotype's quantitative phenotypic variation, or which may yield polyethisms. Most evolutionary models of animal cooperation are based on optimality approaches that do not consider individual genetic variation and maternal effects on the variation in the expression of social behaviour. Further research on the genetic basis of cooperations and subordinate-breeder interactions may be crucial for understanding the evolution of social behaviour. If we take individual differences into account our conclusions and explanations of social behaviour may change. Given the conceptual similarities between the various research disciplines addressing different types of cooperation and conflict over investment, the issues described here should lead to more mutual attraction between the different disciplines and stimulate further development in our understanding of cooperation strategies in general.