Unpredictable environments can help explain the evolution of cooperative behavior, according to a decade-long study of birds in the Kalahari Desert. The research showed that white-browed sparrow-weavers (Plocepasser mahali) work to raise the offspring of other birds as a strategy to cope with spells of dry weather. The findings are among the first to support an evolutionary theory called altruistic bet-hedging, which states that traits can emerge to reduce variability in reproduction, even if they do not increase the number of offspring produced overall.
“A large number of studies have highlighted the role that relatedness and kinship within families can play in the evolution of cooperation,” says Andrew Young, an evolutionary biologist at the University of Exeter, UK, who led the research, published in Science Advances. “But one thing that we didn’t have a good handle on was the ecological drivers.”
The weavers live in family groups, with a single breeding pair and up to 10 nonbreeding helpers that forage and bring food for chicks. Rainfall is unpredictable in the Kalahari, and food is scarce during dry spells.
The results showed that groups with the most helpers significantly improved the number of chicks raised in dry conditions. The reverse was also true: the groups with the most helpers raised fewer chicks in wet weather. That effect was robust enough for helpers to reduce breeding success overall, which suggests the cooperative behavior evolved because it reduces weather-driven variation in reproduction.
The hedging strategy works because the most important consideration is relative fitness—how many offspring each group produces when compared to others. Although the groups with lots of helpers take a hit in wet weather, those groups’ helper-assisted offspring success during dry periods more than compensates. “One offspring is effectively worth a lot more in poor conditions than in really good conditions,” Young says. “Because you could be contributing one to a thousand [offspring] in good conditions, but one of just ten offspring that survive in your competitor pool in dry conditions.”
The paper offers some of the first empirical evidence for altruistic bet-hedging, according to Patrick Kennedy, an evolutionary biologist at the University of Bristol, UK. “It’s a compelling demonstration of this effect in the wild, and is impressively complete,” he says.
Kennedy notes that if researchers only recorded average reproductive success, they might assume that helpers were pointless. “This paper is really neat,” he says, “because it disentangles how the effect of the helpers is different in different environmental states.”
The evolutionary mechanism is a form of kin selection, which explicates how and why animals assist related animals even when such behaviors do not directly help the individual to pass on their own genes. Because the helper weavers are older siblings of the chicks they help to feed, they indirectly contribute to the spread of shared genes.
The researchers tested the effects of helpers by observing 400 broods born to 68 mothers across 36 social groups in the Tswalu Kalahari Reserve in the Northern Cape province of South Africa. “They’re fantastic birds,” Young says. “The name mahali means fierce and they do have quite a bit of attitude.”
The results showed that only female birds help raise the chicks. Removing male helper birds from groups had no impact on the number of offspring produced. This is important, Young says, as it shows the increased number of chicks produced by helpers in dry conditions is not simply a result of larger group size.
The researchers are not sure why groups with more helpers actually produced fewer offspring in wet conditions. One possibility is increased visits from predators, perhaps due to extra birds at the nest attracting more attention.
The study highlights the value of long-term studies and datasets, says Dustin Rubenstein, an evolutionary ecologist at Columbia University in New York, who first proposed the concept of cooperative breeding as a bet-hedging strategy a decade ago. And the weavers made for the perfect subjects. “They don’t live that long,” Rubenstein explains, “so you can look at reproductive success over lifetimes for more individuals.”
About 5% of mammals show cooperative breeding behavior, Rubenstein adds, including lions, elephants, mongooses, meerkats, and wild dogs. But those animals’ longer life spans make long-term studies less practical. “There is huge variation among social insects,” he adds, “so it would be interesting to look across a wider swathe of those.”
Young and his group would next like to examine other ways that cooperation could benefit the birds, beyond breeding success. Mothers who do not have to forage for food might be able to dedicate more effort to improving egg quality, for example. “There is some evidence that she can front-load her investment into the egg stage,” he says.