Keeping fewer friends protects aging monkeys from diseases

Many animals, including humans, are known to experience "social ageing" -- reductions in their number of social connections as they get older. But why this happens remains a mystery.

The new study, led by the universities of Exeter and Edinburgh, used long-term data on rhesus macaques on Cayo Santiago -- known as Monkey Island.

They found that older macaques are likely to suffer less from infectious disease, mostly because of their smaller social networks.

The study is published as part of a special issue of Philosophical Transactions of the Royal Society B, guest edited by Professor Lauren Brent from the University of Exeter.

"Social ties bring huge benefits to a vast range of species -- but sociality also comes with costs, including infectious disease risk," said Dr Erin Siracusa, from Exeter's Centre for Research in Animal Behaviour.

"This cost-benefit ratio can change across individuals' lifespans, which may drive changes in social behaviour.

"Older individuals may be more susceptible to diseases -- but once we accounted for that in our data, we found that older macaques suffered lower infection costs than their younger counterparts.

"Our findings suggest a powerful reason why many animals, including humans, might reduce their social connections as they age."

The benefits macaques gained from social ageing depended on the diseases in question.

Unsurprisingly, the benefit was strongest when diseases were highly infectious and were more severe for older macaques.

Dr Matthew Silk, from the University of Edinburgh, said: "Our results point to illness potentially helping to explain why 'social ageing' evolves -- something we are keen to test in future research."

The research was partly funded by the National Institutes of Health and Dr Silk's Royal Society University Research Fellowship.

The study is entitled: "Social ageing can protect against infectious disease in a group-living primate."

The special issue of the journal is called: "Understanding age and society using natural populations."