Cooperative breeding beyond kinship: why else do helpers help?

One of the most widely studied forms of cooperative behaviour is the phenomenon of cooperative breeding, where 'helpers' care for young that are not their own (Cockburn 1998; Pacheco 'et al'. 2008). Cooperative systems are present in taxa as diverse as slime mould, eusocial insects and a broad-range of vertebrates, including mammals, fish, reptiles and birds (Queller and Strassmann 1998; Clutton-Brock 2002; Mehdiabadi 'et al'. 2006). Fittingly, a large body of research has been devoted to understanding the causes and consequences of cooperative interactions (e.g. Cockburn 1998; Lehmann and Keller 2006), stemming back to Darwin himself who pondered if eusociality might be a fatal flaw in his theory of natural selection (Darwin 1859). Interest in this research question has not been lost over time; a recent article (Nowak 'et al'. 2010) prompted a joint reply from no less than 137 authors (Abbot 'et al'. 2011)!