A change in the social position of a rhesus macaque can affect the expression of nearly 1,000 genes, perhaps partly explaining why poorer people tend to have worse health outcomes.
It's the first time that a link's been demonstrated between social status and genetic regulation in primates on a genome-wide scale.
Comparing high-ranking rhesus macaque females with others of low rank, researchers discovered significant differences in the expression of genes involved in the immune response and other functions. When a female's rank improved, her gene expression also changed - and within just a few weeks.
"We were able to use gene expression to classify individuals based on their rank," says Yoav Gilad of the University of Chicago.
The team studied rhesus macaques housed in groups of five, with dominance determined by the order of introduction into the group.
"In the wild, females would not ordinarily leave the social group they were born into," says former University of Chicago postdoctoral researcher Jenny Tung.
"They inherit their social rank from their mothers. But in this unnatural situation, order of introduction determines rank – the newcomer is generally lower status."
And comparing 49 different female monkeys of different rank revealed significant changes in the expression of 987 genes, including 112 associated with immune system function.
The result fits with previous studies, in which low rank and chronic stress in monkeys have been shown to lead to compromised immune function - and, more loosely, with human studies linking low socioeconomic status and high social stress to a greater risk of disease.
The link was so strong that researchers could predict an individual monkey's social rank very accurately from its gene expression profile alone - and even to predict changes in gene expression when a particular monkey changed rank.
"When a couple of animals were removed from cages for various reasons and new ones were introduced to the groups, it turned out to improve the rank of a few monkeys," says Gilad.
However, a very few monkeys managed to preserve their gene status, regardless of rank.
"There's a spooky side to this kind of research, in that an individual's social rank is partially determining health status," says Tung.
"But there's also a hopeful side. For the seven females that changed ranks, their gene status changed with them. They're not stuck in place, and I think that says something more broadly about the capacity for change."
As for the mechanism by which all this happens, the researchers believe it's because dominance rank affects signaling of the glucocorticoid stress hormone system and the cell composition of blood samples, both of which contribute to changes in gene expression.
"An encouraging message to humans is the fact that the effects are plastic, reversible and change on a really large scale when rank changes," says Gilad. "Whatever it is that causes stress through social environment, you might be able to fix."