For monkeys, being on top of the social pyramid is likely good for the health -- and it could be true for humans, too.
Researchers led by a University of Chicago and Emory University team studying social hierarchies in rhesus macaques have found that a monkey's social status appears to influence how her genes are regulated, particularly those genes pertaining to the immune system.
The scientists studied 49 female macaques in 10 groups, experimentally manipulating their place in the pecking order and examining blood samples to look for signs that gene expression levels -- how often certain pieces of genetic code are transcribed into RNA by a cell -- are affected by a monkey's social standing.
Not all genes are turned on at the same time; some are switched on only at certain times, and others are expressed only in response to some sort of stimulus. By looking at levels of gene expression in monkeys of different social status, or in the same monkey before and after its status changes, the researchers were able to get a glimpse of how the animal's body is responding to changes in its environment.
The results were published in the journal Proceedings of the National Academy of Sciences on Monday.
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Of more than 6,000 genes examined, the researchers found 987 genes with expression levels linked to dominance rank, and genes related to the immune system tended to pop up almost twice as much as you'd expect, lead author Jenny Tung said in a phone interview.
This result suggests that a monkey's immune system is molded in part by its social relationships.
For instance, lower-ranking macaques tended to express higher levels of genes related to the body's inflammation response, even though they were perfectly healthy, according to Tung.
This could mean that lower-ranked monkeys are more prone to inflammatory responses, but further studies will be needed to establish a definitive link.
In any case, other studies have linked an increased inflammation response to premature aging caused by wear and tear on the body, Tung says.
Lower-ranking macaques also had a reduced proportion of killer T-cells -- a type of white blood cell that attacks threats to the body -- compared to their social superiors, according to the paper.
The relationship between gene expression and social status was strong enough for Tung and her colleagues to correctly predict where a female macaque was classed amongst her peers eight out of 10 times solely by examining a blood sample.
When seven of the macaques moved around in the social hierarchy, their gene expression patterns also changed.
Tung and her colleagues also examined the relationship between social status and the methylation of DNA. Methylation is a process in which bits of DNA are marked with a little chemical flag, and is thought to be a mechanism by which environmental factors influence gene expression.
In regulatory regions, having fewer methyl marks is associated with increased expression of a gene; genes with more methylation tend to be silenced.
For certain gene regions, there was clearly a difference in methylation between the low and the high-ranking females, the researchers said, indicating that the environmental stresses associated with social interactions may affect whether certain genes are turned off or on.
Both male and female macaques have dominance hierarchies, but females are better for modeling social stress because their rank hierarchies tend to remain stable for long periods, according to Tung.
In the same vein, studies of social hierarchies are easier to do in monkeys than in humans because researchers can experimentally manipulate a subject's standing, a feat which would be either impossible or unethical to do with people.
But some other studies have found similar links between human social pecking orders and immune system gene expression, and mounting evidence shows that children from lower socioeconomic backgrounds are more susceptible to chronic diseases in their old age than children raised in the upper classes.
In 2009, researchers led by a University of British Columbia team examined 103 healthy adults from various social strata, and found that those subjects who were poorer as children had increased expression of certain genes related to the immune system's inflammatory response and had higher circulating levels of the stress hormone cortisol in their daily life.
The Canadian-led team wrote in PNAS that their results show that repeated social adversity in early life can program a 'defensive' phenotype, characterized by exaggerated inflammatory and hormonal responses to various challenges.