Male birds help their parents less than females because they're too busy scouting for new places to live and breed, a remarkable new study shows.
The study, led by researchers at the Centre for Ecology and Conservation at the University of Exeter, examined the cooperative behaviour and movement patterns of social birds called white-browed sparrow weavers, which live in the Kalahari desert.
These birds live in family groups in which only a dominant pair breeds - and their grown-up offspring, particularly females, help to feed nestlings.
The new study aimed to understand why in many animal societies one sex tends to invest more in helping within the family than the other.
"Female sparrow-weaver helpers contribute more to cooperative nestling care than males and also stay for longer in their family groups than males," said Dr Pablo Capilla-Lasheras, who led the study during his PhD at Exeter and now works at the Swiss Ornithological Institute.
"We wanted to understand why such sex differences in cooperation arise across the animal kingdom.
"The leading hypothesis is that the sex that lives for longer in its family group cooperates more because it stands to gain the downstream benefits of cooperation for longer.
"For example, the sex that stays in their family group for longer could receive more help in return from those family members they've helped in the past than the sex that leaves earlier."
Through over a decade of field research monitoring the cooperative behaviour of these birds and a ground-breaking tracking study of their movements, the team's work suggests that this is not the case after all.
"Our findings point instead to an alternative explanation that has attracted much less attention," said Dr Andrew Young, who leads the Kalahari sparrow-weaver project.
"Instead, males appear to help less because they spend more time prospecting for opportunities to live and breed elsewhere, and these efforts trade-off against their investments in cooperation at home."
Based on their findings, the team suggest that this 'dispersal trade-off hypothesis' may provide a more general explanation for the evolution of sex differences in cooperation across animal societies than the more widely held view that "the longer you stay, the more you'll benefit from having helped out".
This trade-off is an example of a universal challenge that all organisms face, including ourselves; there is never enough time or energy available to do everything well at once.
The study was funded by the Natural Environment Research Council and the Biotechnology and Biological Sciences Research Council.
