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 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.