Is dominance in your blood?

Humans are social creatures. We form close family structures, we surround ourselves with friends, and we learn from each other. In fact, a supportive social environment can have very positive effects on our individual behavior and health. When surrounded by close family members and friends, our bodies fight infections more effectively, we are able to overcome addictions more easily, and we are even faster learners.

On the other hand, when resources are scarce we compete for them, leading to uneven distribution of resources (think of socioeconomic class). Or we may take out our frustrations on others (think of physical or psychological bullying). These negative social interactions can stress us severely and ultimately negatively impact our health.  They may even encourage us to start abusing alcohol or drugs and even alter the way our brains function. But how does our social environment affect our health and behavior?

Top: Acts of aggression can result in lethal injuries in the socially living rhesus macaque monkeys. Unnecessary violence can be avoided by establishing strict dominance hierarchies in which individuals show their dominant or submissive status. Photo by Kevin Rosenfield.

Bottom: Rhesus macaque monkey being displaced by another individual. The monkey on the right is displaying a fear grimace, a signal of submission from which researchers infer the dominance of the monkey on the left. Photo by Dr. Alexander Georgiev.

Dr. Jenny Tung and a team of dedicated researchers at Duke University decided to address this question on how our social environment influences our biology by studying rhesus macaques, a species of monkey originating from India. Much like humans, rhesus monkeys have a social status system (i.e. a dominance rank) which dictates everything from who is the most popular monkey, to who gets first access to the best foods, to who gets beat up. The higher the rank, the better, more stress-free lifestyle you lead. By taking blood samples of monkeys from different social statuses (i.e. ranks), the investigators were able to quantify the production levels of a set of about 6,000 different genes in immune cells,  and compare the levels of each gene across ranks. They found a subset of ~1,000 genes that are over-produced only by the high-ranking monkeys or only by the low-ranking monkeys. This relationship was so strong that they were able to obtain samples from other monkeys and correctly guess the new monkey’s rank 8 out of 10 times solely based on this gene expression data! This would be the equivalent of being able to determine approximately how much money a person makes just based on a single blood sample!

Compelling as this relationship between gene production and rank is, it does not establish causality. One possibility is that having a different rank changes gene levels. Alternatively, the different gene production levels could be predetermined and instead produce differences in monkey behavior (such as acting more aggressively) and thus influence the rank of the individual. To address the direction of this relationship, the investigators waited until 7 of the monkeys switched dominance rank—which happens occasionally—and took additional blood samples after this fall or rise in rank. The gene production information still let them predict the new rank of the monkeys 6 out of 7 times, supporting the idea that rank is what influences the pattern of gene production.

Finally, the researchers decided to take a peek at the genes that differed in production between different ranks and try to find any patterns in their identity or function. Not surprisingly, a portion of the 1,000 affected genes were involved in immune responses (proteins that are necessary for inflammation, for the detection and effective fighting of invading microorganisms, etc.). This finding fits very well with our previous understanding that stress can influence our immune responses. However, the functions of most of these genes that are changed by the rank of the monkey are still left to be uncovered.

The work that the investigators present leaves many important questions to be answered. What are those other non-immune system genes that are affected by rank? Can the patterns of gene levels give us any useful information on gene levels in the brain? How is it that the social environment is able to influence gene production? (Hint: the authors also begin exploring how epigenetic changes relate to rank). This is an exciting area of research that may one day help us find therapies to diminish the health and behavioral consequences of negative social environments such as poverty, abuse and neglect.