Cooperative breeding beyond kinship: why else do helpers help?

Abstract

One of themost 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 andKeller 2006), stemming back toDarwin 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 fromno less than137authors (Abbot et al. 2011)! These issues are relevant to Emu: Austral Ornithology as this region is ideally placed to contribute to the field, because of a disproportionately high prevalence of cooperative avifauna, particularly in South Africa and Australia (Cockburn 1998; Jetz and Rubenstein 2011). Famous Australian examples include the diverse mating systems of Maluridae fairy-wrens (Margraf and Cockburn 2013), distinctiveCorcoracidae societies built on longterm associations (Beck et al. 2008; Griesser et al. 2009), and the extraordinarily complex societies of Manorina honeyeaters (Dow and Whitmore 1990; Wright et al. 2010). In Africa, the Turdoides babblers have attracted considerable interest (Zahavi 1977; Ridley 2007) whereas Merops bee-eaters have long been a model system (Emlen and Wrege 1988). This extensive research effort has elucidated spectacular life histories, such as the identification of some of the world’s least faithful birds (Double et al. 1997; Durrant and Hughes 2005) and feats of extraordinary cooperative behaviour, such as broods of two to four Noisy Miner (Manorina melanocephala) nestlings being fed by as many as 20 helpers (Poldmaa et al. 1995)! Despite this long history of research, few broad-scale factors that favour cooperative breeding have been uncovered, other than helpers tend to favour relatives. By contrast, there is comparatively little consensus on why cooperative breeding involving non-relatives has evolved (e.g. Clutton-Brock 2009). Although it is possible that there are no common drivers favouring cooperative breeding across taxa beyond kinship, I do not believe that present data allow us to definitively reach this conclusion. I therefore first investigate the evidence for any emerging consensus on cooperative breeding between non-relatives, before discussing suggestions as to how researchers might maximise the likelihood of uncovering, or at least refuting, non-kin based hypotheses for the evolution and maintenance of cooperative breeding.