Image caption The social movements of rhesus monkeys are well studied, but health effects remain to be understood

A study of rhesus macaque monkeys may have solved a long-standing puzzle on a link between social rank and health.

A study of 10 social groups of macaque females showed that the activity level of an individual's immune genes was an accurate predictor of her social rank.

In a paper in Proceedings of the National Academy of Sciences , the team also showed that the monkey's immunity changed when social rank was altered.

The work suggests that status drives immune health, rather than vice-versa.

A great many studies have shown associations in both humans and non-human primates between social environment and biological markers of health.

In previous studies of rhesus macaques, the so-called dominance rank has been correlated to levels of the stress-linked glucocorticoid hormones, sex hormones, the brain chemicals serotonin and dopamine, and white blood cell counts.

But one unanswered question concerns cause and effect: does a compromised immunity or imbalance of some chemical cause a particular social rank, or does taking on a particular social rank set the immune system and neural dials?

Jenny Tung, now at Duke University, and colleagues addressed this question by carefully assigning social rank to 10 groups of rhesus macaques, each containing five females.

This can be done by altering the order in which females are introduced into the group; the later she arrives, the lower her social rank.

Image caption The level of the blood's immunity seem to respond to changes in social rank

The team then measured the levels of a broad class of immune cells, peripheral blood mononuclear cells, in the bloodstream.

They found that on the basis of those levels of circulating immune cells alone, they could predict an individual female's social rank with 80% accuracy.

Further studies that investigated the degree to which hundreds of immunity-related genes were "switched on" also showed increased immune activity in higher-ranking females.

What is more, the team found that as rank shifted among seven of the females, the data corresponding to gene activity was again enough to guess an individual's new rank with an accuracy of 85%.

"The current results support the idea that changes in gene regulation help to explain links between the social environment and physiology, potentially supplying an important piece to the puzzle of how social effects 'get under the skin'," the team wrote.

Though the findings might seem to suggest that low social rank, or a decrease in social rank, can lead to reduced immune health, the team said it was "encouraging" that the effects can be counteracted by a change in the social environment.

"Our results motivate efforts to develop a nuanced understanding of social effects on gene regulation," they wrote, "with the aim of both exploring its evolutionary and ecological consequences and addressing its effects on human health."