Mountain gorillas – like humans – sometimes find crowds exhausting, it seems.
A new study shows that when living in preferred group sizes of 12 to 20 they have a diverse range of social relationships, but this changes if the group gets too big.
And that, the researchers suggest, may well be because they only have enough time and mental energy to maintain a certain number of relationships at a given strength.
“It is often assumed that animals living in larger groups will have more diverse and complex social lives,” says Robin Morrison from the Dian Fossey Gorilla Fund and the UK’s University of Exeter.
“However, our study suggests that social diversity is lower in very large groups where gorillas must maintain a larger number of relationships, with most relationships falling into the weakest category. Strong social relationships still exist in the big groups, but they seem to make up a smaller proportion of the total relationships.”
Morrison and colleagues examined 12 years of data on 13 gorilla groups in Rwanda’s Volcanoes National Park where – on a positive note – gorilla numbers have risen in recent years.
This may explain why unusually large groups have formed – sometimes as big as 65 individuals.
“Living in a social group requires mental effort,” says Morrison. “Indeed, one of the big ideas in social evolution is that humans developed large brains and language to deal with social complexity.”
In many primates, social interaction can be measured by how much time individuals spend grooming each other, but gorillas actually don’t spend much time on that at all. Instead, Morrison says, “a lot of gorilla society is about who individuals choose to sit next to, and who they move away from”.
The researchers therefore used proximity data for their study, and identified seven categories of relationship, ranging from close mother-offspring ties to “weak” associations.
However, they stress that the categories can’t be simply expressed in human terms such as best or close friends.
Both males and females experience a diverse range of relationships as youngsters, but this changes as they age.
While females maintain a relatively consistent diversity of relationships through adolescence and adulthood, this declines rapidly in males as they enter adolescence, reaching the lowest levels at around 14.
At this age, males show many characteristics of sexual maturity but are still several years from full sexual maturity.
This is also the period when males are most likely to decide whether to leave the group they were born into, the researchers say, so they may be socially distancing themselves in the lead up to this departure.
If they choose to remain, as roughly half do, they then gradually build up a diverse set of relationships through adulthood as they take on key social roles protecting the group and fathering and caring for offspring.
The research findings could be useful for gorilla conservation and – as with so much work in the current environment – efforts to limit the spread of disease.
“They catch many of the same diseases as humans, including Ebola, and it’s extremely likely they would also catch COVID-19 if they were exposed to it,” says Tara Stoinski, CEO of the Fossey Fund and a co-author of a paper in the journal Proceedings of the Royal Society B.
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.
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