The structure of family groups gives animals an incentive to help or harm their social group as they age, research shows.
Researchers looked at how relatedness – the strength of genetic links – changes over a lifetime in seven mammal species.
This varies by animal depending on whether the offspring, male or female, or both, leave the group they are born into.
For example, male and female killer whales both stay in the same group as their mother, so females have a growing number of close relatives – children and grandchildren – around them as they age.
But other animals may live among fewer close relatives as time passes.
When living in a group of close genetic relatives, it might be in an animal’s interest to behave in a way that helps the whole group, the researchers suggest.
However, when living with fewer related animals, or unrelated ones, the best strategy could be selfish or even harmful behaviour.
According to the study, given that animals have evolved to ensure their genes survive, these long-term changes in relatedness to the family group give animals different incentives to engage in helping or harming behaviour throughout their lives.
Lead author Dr Sam Ellis, from the University of Exeter’s Centre for Research in Animal Behaviour, said: “We wanted to know how an individual’s relatedness to their group changes as they age, and what consequences this might have for behaviour.
“We made a model to predict these changes and then tested it using data on banded mongooses, chimpanzees, badgers, killer whales, spotted hyenas, rhesus macaques and yellow baboons.
“Our model fitted the real data.
“This is exciting because it allows us to make predictions about how and why social behaviours can change with age.”
Dr Ellis said: “Our findings suggest that incentives to help or harm the group change with age, depending on the social structure of each species.”
Professor Darren Croft said: “Across a wide range of species, we see age-related changes in helping and harming behaviour which can also differ between males and females.
“Our new work shows that understanding how relatedness to the family group changes with age is key in understanding how the incentives to help or harm the group changes across the lifespan, which can potentially explain these differences across species and between the sexes.
“This research opens the door for future studies by providing testable predictions for how patterns of helping and harming will change across the lifespan and we eagerly anticipate new work testing these predictions.”
Among the species included in the study, male spotted hyenas, rhesus macaques and yellow baboons usually leave their birth group once they reach maturity.
In chimpanzees, female offspring leave the group, while for killer whales and mongooses, both sexes usually stay in the group into which they were born.
The findings are published in the journal Nature Ecology and Evolution.
