Rhesus macaques, which are some of the best studied of all monkeys, establish hierarchies in their social groups. Whenever two macaques tussle over a piece of food, say, or the right to mate, the monkey with the higher rank usually wins. Primatologists have established that monkeys of a lower social status are generally more stressed out than their dominant peers—low-ranking monkeys have higher levels of stress hormones, for instance. But what about differences in gene activity? Does one's social stature change how one's genes are expressed. Yes, concludes a new study that used differences in gene expression to identify a monkey's social status with around 80 percent accuracy.

Jenny Tung of Duke University and her colleagues at the University of Chicago (where Tung worked at the time of the study)—as well as several collaborators at the Yerkes National Primate Research Center—studied 10 groups of adult female rhesus macaques made up of five females each. Researchers formed the groups one female at a time, which allowed them to carefully construct the social hierarchy: females introduced earlier generally assumed a higher rank. In this way, the scientists knew exactly which monkey held rank 1, 2, 3, 4 and 5 in each group.

Tung and her colleagues collected blood samples from the rhesus macaques, isolated the white blood cells and analyzed the DNA in those cells. They found 987 genes whose activity depended on social rank: 535 genes that were more highly expressed in high-ranking individuals and 452 genes with higher activity in low-ranking individuals. Many of these genes were involved with the immune system; in particular, genes involved in inflammation were more active in low-ranking individuals. Further testing revealed that low-ranking monkeys also had fewer cytotoxic T-cells, a kind of white blood cell that attacks infected and cancerous cells. Earlier research suggests that the stress of a low social rank compromises the immune system—which fits with the finding about T-cells—but may also trigger the immune system to respond when it does not need to, which fits with the finding about inflammation. Findings about the relationship between stress, social status and the immune system are not clear cut, however; for example, some studies have found that having a higher rank is more stressful than having a lower rank.

Tung chose 10 genetic profiles at random and tried to predict the corresponding monkey's social rank based solely on the gene activity—she was successful in eight cases. In another test, Tung showed that gene expression correctly identified the social status of six of seven monkeys after they changed rank. The new study appears in the April 9 issue of the Proceedings of the National Academy of Sciences.

Tung says this is the first time she has been able to predict social rank by looking at gene expression alone. "We have lots of biomarkers of stress," she says, "but they are not necessarily great predictors by themselves. With whole genome studies, we can look at thousands of biomarkers at once."