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 reproductive success, just as hypothesised by global comparative analyses. Our novel analytical approach implicates effects of helping per se rather than correlated effects of group size and isolates within-mother effects of helping from potentially confounding among-mother variation in performance. Our findings lend new strength to the leading explanation for the global biogeography of altruism and highlight the wider importance of considering the impacts of altruism on both the mean and variance in performance of recipients.

"1 - Provisioning analysis - RAW DATA.csv": provisioning data, analysis presented in Fig. 1D, 1E

"2a - Experimental removal all female helpers - RAW DATA.csv": helper removal experiment data, analysis presented in Fig. 1F and Table S8

"2b - Experimental removal all male helpers - RAW DATA.csv": helper removal experiment data, analysis presented in Fig. 1F and Table S8

"3 - Bivariate models - RAW DATA.csv": data for the analysis of variance of reproductive success, analysis presented in Fig. 2

"4 - Nestling survival analysis - RAW DATA.csv": data for the analysis of nestling survival, analysis presented in Fig. 3