Scientists from the UK monitored white-browed sparrow weavers at the 111,000ha Oppenheimer-owned Tswalu in the southern Kalahari, where rainfall varies dramatically and food is scarce during dry spells.
The birds live in family groups, with a single breeding pair and up to ten non-breeding “helpers” that assist with feeding chicks.
Explaining such seemingly altruistic behaviour has been a focus of evolutionary research for many years and the new study in ScienceAdvances, by a team from the University of Exeter, shows families with more helpers successfully raise more chicks in dry conditions.
In wet conditions, though, the families are less successful than those with fewer helpers. This means co-operative helping does not improve overall breeding success but reduces weather-driven variation, which can be beneficial in other ways.
“We were really surprised — our findings suggest that the birds co-operate to cope with the unpredictability of their environment,” said Andrew Young, of the university’s centre for ecology and conservation.
“This is an exciting discovery, because other studies have found that co-operative behaviour among birds is associated with unpredictable environments worldwide, especially those with variable rainfall.
“Our findings help to explain this enigmatic global pattern; co-operation may be at its most beneficial in unpredictable environments because it helps families to cope with such tricky conditions.
“To evolutionary biologists, our study is also exciting as it provides the strongest evidence to date for a novel strategy called ‘altruistic bet-hedging’, in which co-operation evolves because it allows organisms to cope with unpredictability.”
Most animal species have evolved to pass on their genes by reproducing, but the process of “kin selection” leads some individuals that cannot breed themselves to help close relatives breed instead, ensuring their family’s genes are passed on.
In white-browed sparrow weaver families, helpers are usually the offspring of the breeding pair, so helping could improve the survival chances of their younger siblings.
“In this case, things are more complicated,” said lead researcher Pablo Capilla-Lasheras. “In this species co-operation is probably not explained by classical evolutionary theory because the presence of helpers does not increase the overall reproductive success of relatives.
“Instead, co-operation reduces reproductive variation, helping the birds cope with their unpredictable environment.
“Bet-hedging strategies like this are all about reducing the risk of total failure when you can’t predict what is going to happen.
“Our findings suggest that co-operation may serve that purpose — it allows sparrow weaver families to enjoy some breeding success whatever rainfall conditions they experience.”
The findings also offer clues about how bird species might adapt in the future.
Young said: “Co-operatively breeding species — those that live in societies with helpers — may be better placed to cope with unpredictable environments, which we expect to become increasingly common under climate change.”
The study found that female helpers were more co-operative – feeding chicks more – than male helpers.
“It is specifically the number of female helpers that affects a family’s ability to cope with unpredictable rainfall conditions; the number of male helpers is less important,” Young said.
“This observation allows us to be more confident that having more helpers — rather than simply living in bigger groups — is the cause of the effects we witnessed.”
Tswalu has the largest population of black rhino and is also home to threatened species such as the pangolin and canid African wild dog.Internet Explorer Channel Network