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related to environmental variation. To understand the adaptive value of cooperative breeding behavior in variable environments, researchers must consider both mean and environmentally induced variance in fecundity. Determining how spatiotemporal environmental variation drives risk-averse strategies may provide insights into the evolution of complex social behavior.

Kin selection, or reproductive strategies that favor an organisms’ relatives, is often invoked to explain the evolution of cooperation and the formation of complex animal societies (Hamilton, 1964a; West-Eberhard, 1975). In cooperatively breeding systems in which some individuals delay independent breeding to help raise the offspring of others, the inclusive fitness benefits of helping genetic relatives may outweigh the potential costs of trying to breed independently (Brown, 1987). Recent theoretical (Boomsma, 2007, 2009) and comparative work in both invertebrates (Hughes et al., 2008) and vertebrates (Cornwallis et al., 2010) suggests that high relatedness among group members may be critical to the evolution of complex animal societies. However, despite renewed interest in determining how genetic relatedness among group members can influence social interactions and the evolution of family groups (Boomsma, 2007, 2009; Hughes et al., 2008; Hatchwell, 2009; Nam et al., 2010; Sharp et al., 2011), relatedness alone cannot explain why some individuals in a group breed whereas others do not, or why some species breed cooperatively whereas other closely related ones do not. In other words, relatedness may set the stage for cooperation in animal societies, but it is not sufficient to explain many individual differences in reproductive strategies or interspecific patterns of social diversity (Rubenstein and Lovette, 2007; Jetz and Rubenstein, 2011).

Environmental factors are known to influence complex vertebrate social behavior (Alexander, 1974; Jarman, 1974), as well as explain many of the individual differences in reproductive strategies (Emlen, 1982a; Komdeur, 1992; Covas et al., 2004; Rubenstein, 2007a) and interspecific patterns of sociality (Rubenstein and Lovette, 2007; Jetz and Rubenstein, 2011). The role of environmental factors in shaping animal societies is central to the ecological constraints hypothesis (Emlen, 1982a), which argues that when barriers to dispersal are high, offspring will be selected to delay dispersal and remain at home as part of a group because the probability of reproducing successfully outside the group is low. The ecological constraints hypothesis (Emlen, 1982a) and its other derivations (Koenig and Pitelka, 1981; Koenig et al., 1992) predict the environmental conditions under which delayed dispersal is likely to occur (Hatchwell and Komdeur, 2000). These conditions include a shortage of vacant breeding territories (i.e., habitat saturation), the costs

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