Macaque societies are female-bonded societies. The larger the maternal group, the more social interactions there are.
A macaque is the epitome of gregariousness. Remarkable social ties help them navigate their lives against predators and improve their ability to find food. But a new study suggests that some individuals of the rhesus macaque species are prone to social isolation.
There is no dearth of research that links social integration in human and non-human primates with better physical health and emotional well-being. The benefits of getting along with others help social animals lead longer and, at least in the case of humans, happier lives. Nevertheless, social isolation seems to have its virtues and is therefore being maintained in macaques.
Communal living comes with its costs, such as getting parasites. So how do social primates balance the benefits and costs of living in a group?
Social isolation is not the same as depression or anti-social behaviour. Social isolation is when one has few or no social relationships. Depression is when one is distressed over the quality or quantity of social relationships. Anti-social behaviour is when social relationships are broken by being disruptive to the society.
Researchers from the universities of Exeter, Pennsylvania and Puerto Rico studied rhesus macaques on the Cayo Santiago Island in Puerto Rico. They found that social isolation in the macaques was a combination of nature and nurture. Using behavioural data and social network analysis, they observed that sociality was a product of two distinct factors: their genetic makeup and external factors such as age, sex, social hierarchy, presence of relatives and familiarity with others in the group. These socially isolated individuals were very much part of the troop – but they had fewer social partners and spent far less time engaging in social interactions.
While it looks like the researchers have thrown every parameter into the analysis bucket, some independent scientists The Wire spoke to said that the study was well thought out.
Macaque societies are female-bonded societies. The larger the maternal group, the more social interactions there are. The males do not live with their families. So opportunities for social integration in males take a backseat. Nevertheless, it becomes easier over time for males in a troop to navigate their social world.
Older individuals tend to be more socially isolated than younger ones. This could be because of two reasons. Older macaques could be less energetic and so indulge in fewer social interactions. Second, they could also be cutting down on social interactions to avoid the risk of aggression.
However, the study found that though these older individuals were isolating themselves from others, the opposite was not true: though the older macaques groomed others less, the amount of grooming they received from others in the troop remained the same. This could have been because the younger macaques were less threatened by their elders and did not mind grooming them.
The study also found that some individuals were consistently more isolated compared to the rest. However, isolation was not learned from either the mother or the macaque’s natal group, suggesting the ‘condition’ could be inherited. Lauren Brent, from the University of Exeter and the lead author of the paper, found this surprising. “It was exciting to find … that if mothers are influencing the behaviour of their adult offspring, they are doing so through shared genes,” she said.
It was notable that social isolation varies within as well as across individual macaques over time. One size does not fit all in macaque societies. “This would mean that an individual has much more control over how much it wants to integrate within the macaque society,” said Anindya Sinha, a primatologist at the National Institute of Advanced Studies, Bengaluru. The hard and fast rules of who is socially isolated and who is not melt away even in apparently simple beings such as macaques. “It tells you how complex their lives are.”
What adds to the complexity is the that the factors influencing social isolation could change over time. For example, the Cayo Santiago island, where the study was conducted, was recently devastated by Hurricane Maria. What did this mean for the macaques’ sociality? Brent is currently collecting data but expects that the macaques might reduce the size of their social networks in the hurricane’s aftermath. “They might do so in order to focus their social efforts on their closest partners, those most willing to provide them with assistance or tolerate them at a food source,” she said.
When social networks shrink, some individual macaques could become more isolated as a result. Some older studies (such as this) have suggested that some people became more socially isolated after Hurricane Katrina. Could there have been a similar response in macaques?
Sinha thinks a macaque could become more isolated if another macaque key to its social life had died. Then, it would have to wait until it could forge new bonds with a different macaque.
Why is social isolation desirable at all from an evolutionary point of view? Avoiding aggression could be one reason. Sinha said he had observed this among the bonnet macaques he studies in Bandipur Tiger Reserve, Karnataka. Subordinate individuals tend to be more socially isolated than dominant ones. By being at the lower end of the hierarchy, these individuals have to forage more than the dominant individuals. They also have to travel greater distances away from the troop to look for food. “Social isolation is not just sitting quietly on my own,” Sinha said. “Social isolation is not interacting and I could not be interacting for a variety of reasons.”
On the other hand, social integration could be negated in some macaques to reduce the risk of contracting or transmitting diseases. The more macaques there are in a troop, the more interactions there are, and higher the chances of acquiring a diseases from one of the other members. However, how pathogens could jump from one macaque to another remains unknown.
Then again, there’s a flip side to being isolated. A May 2017 study found that female rhesus macaques that had fewer social relationships did not live as long as those with larger networks. Are these macaques also depressed or anti-social? Are there other costs to being less socially integrated? Studies of the future will have to answer these questions.
Macaques and humans share a common ancestry, and primate studies are important because they open a window into the origins of human behaviour. To understand social relationships among humans, Harry Harlow, an American psychologist, had performed controversial experiments on macaques in the 1950s. He had kept infant rhesus macaques completely isolated in steel chambers and examined their behaviour for up to 12 months at a time.
Harlow found that the macaques went into a state of emotional shock when removed from isolation. Those isolated for less than three months slowly developed normal social interactions. However, those isolated for longer were severely impaired. Based on these studies, he estimated that the corresponding period of time – being isolated for which could trigger irreversible changes – in humans to be six months.
So can we similarly come to any conclusions about human behaviour from the social isolation study? Not so fast, says Aniruddha Deb, a psychologist in Kolkata. “Humans behave differently from macaques; we have a larger variety of social activities than grooming. Hence the possibility of interaction is a lot more,” he said.
Sinha agreed: drawing lessons about our own lives from those of another species is complicated. While studying macaques, researchers are forced to think about behaviours in very functional terms whereas measuring social isolation in humans is more complex. “If a macaque is sitting on its own, it might actually be benefitting, though I would never know. If I knew what benefits it was getting, I might also want to sit on my own,” Sinha said.
The study was published in the journal Scientific Reports on December 19, 2017.
Rashmi Bhat is a wildlife researcher with an overarching interest in applied conservation biology. She is based in Bengaluru, India.