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of dispersal, difficulties in finding a mate, and a low chance of successful reproduction once a territory is established [reviewed in Hatchwell and Komdeur (2000)]. Although each of these conditions represents an external constraint or cost associated with dispersal and independent breeding, such conditions represent only one side of the cost–benefit equation of social living. An alternative theory, the benefits of philopatry hypothesis, instead argues that delayed dispersal is the result of intrinsic benefits gained by remaining on the natal territory (Stacey and Ligon, 1987, 1991). These benefits include enhanced survival, indirect fitness gains from helping relatives, opportunities for obtaining a nearby breeding vacancy in the future, and the chance to inherit the natal breeding territory itself [reviewed in Cockburn (1998)]. Despite much initial debate over the relative importance of these two hypotheses in shaping cooperative groups (Emlen, 1994), it is now widely accepted that they are more similar than they are dissimilar, because they place different emphasis on the costs of dispersing vs. the benefits of not dispersing as a result of environmental constraints (Emlen, 1994, 1997a; Hatchwell, 2009).

Nearly all of the early environmental hypotheses for cooperative breeding behavior have focused primarily on the costs and benefits associated with breeding on territories of varying quality, or the fitness consequences of living in a spatially heterogeneous landscape where suitable territories are limiting (Emlen, 1982a; Stacey and Ligon, 1991; Koenig et al., 1992). However, spatial constraints on dispersal (i.e., habitat heterogeneity) are not the only form of environmental variation that can influence cooperative breeding behavior. Environmental variation in time can also influence social behavior, including dispersal decisions and the adoption of different breeding roles. Although the ecological constraints hypothesis is generally used to emphasize the role of habitat heterogeneity in influencing dispersal decisions, its original description also recognized the importance of environmental unpredictability in driving cooperative breeding (Emlen, 1982a), although this idea went largely untested for decades [but see Curry (1989) and Curry and Grant (1990)]. Recent work in cooperatively breeding birds living in unpredictable environments suggests that erratic and variable climatic patterns can also influence social complexity. Temporal environmental variation resulting from unpredictable patterns of rainfall (i.e., climatic uncertainty) has been shown to influence not only individual behavioral decisions and reproductive roles (MacColl and Hatchwell, 2002; Canario et al., 2004; Rubenstein, 2007a; Covas et al., 2008) but also interspecific patterns of sociality on continental and global scales (Rubenstein and Lovette, 2007; Jetz and Rubenstein, 2011). Together, spatial and temporal patterns of environmental variation explain many of the individual-level coopera-



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