Cooperative breeding systems

Abstract

Cooperative breeding is a type of social system in which some group members (referred to as ‘helpers’) routinely provide care for offspring that are not their own, but retain the potential to reproduce themselves either currently or in the future. This broad definition (which derives from those suggested by Cockburn 1998; Crespi and Yanega 1995; Emlen 1991) includes a range of species, from primitively eusocial insects such as paper wasps, hover wasps, halictid bees, and ambrosia beetles; to avian, mammalian, and fish ‘helper-at-the-nest’ systems in which offspring delay dispersal and help dominant breeders with subsequent breeding attempts; and also larger animal societies with multiple male and female breeders and helpers per group (Fig. 12.1). From current information, 9% of birds (852 species; Cockburn 2006) around 2% of mammals (Lukas and Clutton-Brock in press; Riedman 1982), <0.5% of fishes (20–38 species; Taborsky 1994; Taborsky 2009), and hundreds of species of insect can be classed as cooperative breeders. There are also examples from arachnids (Salomon and Lubin 2007) and crustaceans (Duffy and Macdonald 2010). These societies, while very diverse in terms of social structure and basic biology, share some common features. Populations are usually subdivided into groups of kin (although non-kin individuals may also be present) with strong ecological constraints on dispersal or independent breeding (Hatchwell 2009). Within groups, there is usually (but not always) a reproductive division of labour in which high ranked or socially dominant individuals breed, and lower ranked individuals help (Field and Cant 2009b). Because helpers retain the ability to reproduce themselves, their behaviour reflects a trade-off between current and future fitness, and between direct and indirect components of their inclusive fitness. In this way cooperative breeders differ from eusocial species which have distinct reproductive and worker castes and helpers remain functionally or morphologically sterile throughout their lives (Bourke 2011). Cooperative breeders have been the focus of intense research in behavioural ecology for two main reasons. First, they embody a major puzzle of evolutionary theory: how can altruistic behaviour be favoured by natural selection? Helpers pay a fitness cost to boost the reproductive output of other group members. For example, subordinate foundresses of the paper wasp Polistes dominulus risk their lives foraging to feed larvae to which they are often unrelated (Leadbeater et al. 2010; Queller et al. 2000). Using the classification of social behaviours introduced by Hamilton (1964), helping is a form of altruism when it involves a lifetime direct fitness cost to the helper, and results in a lifetime direct fitness benefit to the recipient of help. In the case of paper wasps, foraging involves clear fitness costs because foundresses that do more foraging suffer higher mortality (Cant and Field 2001). Cooperative breeding systems provide concrete examples of altruism together with the possibility of measuring the fitness consequences of helping, and hence an opportunity to test evolutionary theories of cooperation. Second, cooperative breeders have proved to be excellent models for the study of evolutionary conflict and its consequences for behaviour and group dynamics. Evolutionary conflict arises whenever the optimum fitness outcomes for the participants in an interaction cannot all be achieved simultaneously. In the case of cooperative breeders, the role of breeder is usually more profitable (in terms of fitness) than the role of helper, which generates