Altruistic bet-hedging and the evolution of cooperation in a Kalahari bird

Analyses of the global biogeography of altruism suggest that unpredictable environments have favoured the evolution of altruistic helping behaviour (helping to rear the offspring of others). It has therefore been hypothesised that selection for altruism may frequently arise because helping reduces variance in the reproductive success of relatives in unpredictable environments (a scenario termed ‘altruistic bet-hedging’). Here we show that helping behaviour does reduce environmentally-induced variance in the reproductive success of relatives in a wild cooperative bird, the white-browed sparrow-weaver (Plocepasser mahali). Our decade-long study in the Kalahari desert reveals that non-breeding helpers have no overall effect on the mean reproductive success of related breeders, but instead reduce variance in the reproductive success of related breeders. Moreover, this variance reduction arises in part because helpers specifically reduce unpredictable rainfall-induced variance in reproductiv...