Humans aren’t the only ones whose social circles shrink in old age

Previous research has shown that some other species — not just people — see their social networks shrink as they move into old age. Exactly why this should be has not been clear; we have some understanding of why it happens in humans, and what can be done to mitigate any negative effects, but why exactly it happens beyond our species has so far been a mystery. New research, however, poses a novel suggestion.

As Erin R Siracusa at the University of Exeter's School of Psychology and colleagues write in their recent paper in Philosophical Transactions B, though the benefits of social living are well established, "sociality also comes with costs, including infectious disease risks." The team reasoned that with increasing age, and a less effective immune system, the risks of spending time with lots of people might start to outweigh the benefits — and this could drive changes in behaviour.

To explore this idea, they first analysed data gathered on a well-studied population of rhesus macaques on Cayo Santiago, nicknamed 'Monkey Island', off the southeast coast of Puerto Rico. These monkeys were first brought to Cayo Santiago in 1938 from India, and there are now about 1800 individuals living in 12 social groups. They are looked after by the Caribbean Primate Research Center, which keeps daily tabs on them, and collects data on births, deaths, and group memberships.

The team focused on data from 2010 and 2022 on adult females from six social groups. By looking at the total amount of time these animals spent grooming or being groomed by another macaque and their total number of grooming partners, they plotted these individuals' social connections over time.

Then, they modelled the spread of a hypothetical respiratory virus through these social networks. This model took into account that older animals are more at risk of developing an infection; that they are likely to stay sick for longer than younger monkeys; and that they are likely to fall more seriously ill.

The team found that older macaques were, in fact, likely to suffer less from infectious disease — and this was mostly because they had smaller social networks.

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

However, for some animals, at least, this may not be even an unconscious choice. In another recent paper, Julia Schroeder at Imperial College London and colleagues report the first evidence that with age, birds become less socially connected, too. This finding came from their analysis of six years' worth of social interaction data on a population of wild sparrows living on Lundy Island, in Devon, UK. Schroder and her colleagues found that as the birds got older, they interacted less overall with other birds, and they also interacted with fewer other individuals.

However, the team don't suggest that this might have reflected a drive to reduce infection risks. They think it could have happened because as older birds' 'friends' died off, their social network shrank. And, they add, while birds are capable of making new friends (or 'associates', as the researchers call them), with age, the costs of maintaining and making new connections might become greater, because they have to compete for associates with younger birds. More work is needed though, to explore this theory. "To fully understand the effects of demography on sociality, we need to better understand the changing costs and benefits across ages of maintaining and making new connections," Schroder and her colleagues write.

With more research, we should also get a better handle on all the reasons — and the potential pros, as well as the cons — for why older birds, monkeys, humans, as well as some other species, tend to have smaller social circles than they did when they were younger.

Read the first paper (preprint) here:
Siracusa, E. R., Pavez-Fox, M. A., Negron-Del Valle, J. E., Phillips, D., Platt, M. L., Snyder-Mackler, N., Higham, J. P., Brent, L. J. N., & Silk, M. J. (2024). Social ageing can protect against infectious disease in a group-living primate. BioRxiv : The Preprint Server for Biology, 2024.03.09.584237. https://doi.org/10.1101/2024.03.09.584237

Read the second paper here:
Schroeder, J., Dunning, J., Hoi, A., Janet, Y., & Burke, T. (2024). Not so social in old age: demography as one driver of decreasing sociality. Philosophical Transactions of the Royal Society B Biological Sciences, 379(1916). https://doi.org/10.1098/rstb.2022.0458

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