No incentive for older birds to make new friends

In humans, it's often been assumed that older people have fewer friends because they're pickier about who they spend their time with. There's also the issue that there are fewer people of their own age around.

But it's hard to pick apart the various potential causes for humans, so researchers have turned to animals. The team behind the new research, led by Imperial College London, studied an isolated population of sparrows on the island of Lundy, in the Bristol Channel.

By mapping the ages and social networks of all the birds, they found that older sparrows do tend to have fewer friends, as with humans. The reason could be that there is no 'evolutionary pressure' to do so: while friendliness helps younger birds survive and breed more successfully, the same isn't true for older birds.

Lead researcher Dr Julia Schroeder, from the Department of Life Sciences (Silwood Park) at Imperial, said: "This evolutionary mechanism may also be at work in humans -- it could be that older people are less inclined to new friends as they age. Combined with fewer same-age potential friends available, this could be a factor in the loneliness crisis among older people."

Co-author Dr Jamie Dunning, now at the University of Leeds, explained: "Our study is one of the first to suggest that birds, like mammals, also reduce the size of their social network as they age. Specifically, the number of friendships, and how central a bird is to the wider social network, declined with age."

The study is published today in Philosophical Transactions of the Royal Society B, in a special issue of the journal on understanding age and society using natural populations.

Changing with age

Lundy Island hosts a 'closed' population of sparrows, meaning that no individuals leave or arrive on the island. This allows the team to collect lots of accurate data on the inhabitants, including their ages, breeding success, and social networks, which have been recorded for 25 years.

Previous research from the team showed that being friendly (especially with the opposite sex) helps sparrows on the island breed successfully, showing that when they're young, having a good social network is an advantage.

In the new study, the researchers looked at the other end of life, to see if friendliness is still a benefit. They found that, rather than having a benefit, the lack of friendliness simply seems to be of no cost.

Dr Schroeder explained: "'Friendliness', at least for sparrows, may change with age. When they're young, it helps them to make friends, giving them an evolutionary 'benefit'. But once they've reproduced, it seems like being unfriendly has no evolutionary 'cost' -- there are no downsides that mean those genes wouldn't be passed on."

There are parallels in humans for age-related conditions like Parkinson's -- while diseases like this clearly do have a cost for the individual, since they only occur after reproduction, they are passed on to offspring anyway, so there is no mechanism for evolution to lose those genes.

Dr Schroeder said: "Ultimately, the study of friendliness with age -- whether it be in sparrows or humans -- may help us understand how to help older people make new friends, and reduce the burden of loneliness."