Imma Perfetto
Cosmos science journalist
Like long-term friendships between humans, African superb starlings become besties with individuals they are not related to and help each other repeatedly over time.
These are the findings of a study which analysed more than 20 years of behavioural observations of starlings in east African savannahs. The starlings demonstrated “reciprocity” – helping each other with the expectation that the favour will eventually be returned.
This kind of behaviour is integral to human society, but proving it occurs in the wider animal kingdom remains difficult.
“Starling societies are not just simple families, they’re much more complex, containing a mixture of related and unrelated individuals that live together, much in the way that humans do,” says Professor Dustin Rubenstein of Columbia University in the US.
“Many of these birds are essentially forming friendships over time.”
Rubenstein is senior author of a study describing the research in the journal Nature.
Like humans, superb starlings (Lamprotornis superbus) are “cooperative breeders” in which helpers forego reproduction to contribute to raising the young of others, typically relatives.
It has long been known that animals indirectly ensure that their genes are passed on by supporting the survival and reproduction of their relatives. This is known as “indirect fitness”.
“Breeding pairs of this obligate cooperative breeder are aided by up to 16 non-breeding helpers … in both offspring provisioning and nest defence from predators,” the authors write in their study.
“Helpers provide fitness benefits to breeders by increasing the breeder’s reproductive success and decreasing the costs of reproduction (that is, load lightening), especially for mothers.”
But whether these birds also help non-relatives to raise their babies wasn’t clear.
To find out, between 2002-2021, Rubenstein and collaborators studied interactions between thousands of starlings, and collected DNA to examine genetic relationships within the population.
The team combined 40 breeding seasons’ worth of behavioural and genetic data and found that, while helpers preferred to aid their closest relatives, they also frequently and consistently helped non-related birds.
“Past work has shown that individuals in larger superb starling social groups have greater adult survival and reproductive success,” the authors write.
“These findings suggest that individuals could augment the size of their social group by helping unrelated breeders (group augmentation), thereby increasing both the helper’s own survival and the survival and reproductive success of their related group mates.”
But the researchers also found something unexpected – some specific pairs of starlings maintained long-term reciprocal helping relationships by swapping social roles across their lifetime.
These bird besties chose to help each other even when a direct relative was also in need of help.
The researchers suggest that forming reciprocal helping relationships with non-kin in the harsh and variable east African savannah environment in which these starlings live may “ensure that the necessary help is available in frequent but unpredictable times of need”.
In other words, reciprocal helping could also contribute to the individual starlings’ own survival and reproduction, known as “direct fitness”.
“Alongside group augmentation, we suggest that within-group reciprocity may be a cryptic but crucial source of direct fitness that promotes the stability of complex cooperative societies,” the authors conclude.
“I think this kind of reciprocal helping behaviour is likely going on in a lot of animal societies, and people just haven’t studied them long enough to be able to detect it,” Rubenstein says.
The next step, he says, is to explore how these relationships form, how long they last, and why some relationships stay robust, while others fall apart.
