This historic discovery explains the long-standing mystery of why cooperation between birds is associated with unpredictable environments around the world.

White-browed weavers live in the Kalahari Desert, where rainfall varies widely and food is scarce during times of drought.

Birds live in family groups, with a single breeding pair and up to ten non-breeding “helpers” who help feed the chicks.

Explaining the evolution of such seemingly selfless behavior has been the focus of evolutionary research for many years.

The new study, conducted by a research team at the University of Exeter, shows that families with more helpers successfully raise more chicks in dry conditions.

Interestingly, these families do less well in humid conditions than those with less helpers, meaning that cooperative help does not improve overall reproductive success, but rather reduces variation in breeding success. reproduction due to weather conditions, which may be beneficial in other respects.

“We were really surprised – our results suggest that birds are cooperating to cope with the unpredictability of their environment,” said Dr Andrew Young, from the Center for Ecology and Conservation at the Penryn Campus in Exeter in Cornwall.

“This is an exciting discovery, as other studies have shown that cooperative behavior in birds is associated with unpredictable environments around the world, especially those with varying precipitation.

“Our findings help explain this enigmatic global pattern; cooperation can be most beneficial in unpredictable environments, as it helps families cope with such delicate conditions.

“For evolutionary biologists, our study is also exciting because it provides the strongest evidence to date for a new strategy called ‘altruistic betting hedging’, in which cooperation evolves as it allows organisms to do in the face of unpredictability. “

Most animal species have evolved to pass on their genes through reproduction, but a process called “parentage selection” causes some individuals who cannot reproduce on their own to help close relatives reproduce, thus ensuring the transmission of the same. genes of their family.

In families of white-browed weavers, helpers are usually the offspring of the breeding pair. Helping could therefore improve the chances of survival of their younger siblings.

“In this case, things are more complicated,” said Dr Pablo Capilla-Lasheras, now at the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow.

“We show that in this species, cooperation is probably not explained by classical evolutionary theory, because the presence of helpers does not increase the overall reproductive success of the parents.

“Instead, cooperation reduces reproductive variation, helping birds cope with their unpredictable environment.

“Bet hedging strategies like this all aim to reduce the risk of total failure when you can’t predict what’s going to happen.

“Our results suggest that the cooperation can serve this purpose – it allows families of weaver sparrows to enjoy some reproductive success regardless of the precipitation conditions they experience.”

The findings, based on 12 years of fieldwork in the Kalahari, also give clues as to how different bird species might adapt in the future.

Dr Young explained: “Species that reproduce cooperatively – those that live in societies with helpers – may be better placed to cope with unpredictable environments, which are expected to become increasingly common with climate change. “

The study found that female assistants were more cooperative (feed chicks more) than male assistants.

“It is specifically the number of female assistants that affects a family’s ability to cope with unpredictable precipitation conditions; the number of male assistants is less,” said Dr. Young.

“This observation allows us to be more confident that having more helpers – rather than just living in larger groups – is the cause of the effects we have seen.”

The study was carried out in the Tswalu Kalahari reserve in South Africa and was funded by the Biotechnology and Biological Sciences Research Council.

The article, published in the journal Scientists progress, is titled: “Altruistic Betting Coverage and Cooperation Evolution in a Kalahari Bird.”