Wild animals experience a series of stressful situations when going about their normal routine, from competing for food and mates to social status among their groups. To get a better idea of how an animal's genetic make-up influences the excretion of stress hormones and their subsequent behavioral response, researchers from the University of Vienna studied Japanese macaques living at Affenberg Landskron in Carinthia, Austria.

"Japanese macaques live in strict hierarchy which entails a high level of aggressive interaction. This makes them ideally suited for a study on stress behavior," Lena Pflüger, first author and an ethologist from the Department of Anthropology at the University of Vienna, said in a news release.

For their study, researchers observed 26 sexually mature males during the mating season, which is a particularly stressful time for males competing with others for a female's attraction. When analyzing the animals' feces, researchers found the amount of cortisol -- the adrenal hormone responsible for maintaining one's homeostasis, otherwise known as the "stress hormone" -- varied greatly among individuals.

"The macaques appear to handle stressful situations differently. Some are more courageous than others. We were interested to see whether there were genetic causes for this behavior and how genetics affects the hormonal stress reaction and social rank," Pflüger added in the release.

It follows then, increased levels of cortisol found in an individuals dung suggests they were experiencing higher levels of stress. To understand the role genetics has in all of this, researchers investigated how a gene known as COMT -- one of 18 genes that controls the dopamine system in the brain and gives humans the ability to plan, make decisions, and solve problems -- varied among these animals. Essentially, higher amounts of dopamine in the brain increases one's cognitive performance and stress reaction, meaning COMT is linked to the excretion of stress hormones. 

So what did they find? Macaques with high levels of stress hormones present in their feces also possess a certain COMT variant that presumably metabolizes dopamine in the brain more slowly, researchers say. However, further research is required to better understand this variant and what underlying mechanisms ultimately release higher levels of stress hormones. 

"Our results indicate that animals with stress-resilient COMT variants acquire higher rank positions in the group. But a direct correlation between COMT variant and social rank has to be investigated more closely in the future," Ralf Steinborn, co-author and Head of the Genomics Unit of the VetCore Facility for Research at the Vetmeduni Vienna, explained in the university's release. "The dopamine level in the brain controls various behaviors in people. On the one hand, there are the so-called warrior types. In warriors, the dopamine in the brain is metabolized more quickly. Warriors possess lower cognitive skills and are less easily stressed. The second type are worriers, who score higher in cognitive performance tests but are more easily stressed. However, the dopamine system functions like an orchestra and is not dependent on just one factor."

Researchers also plan to further study how this genetic variation affects the release of stress hormones in other groups of primate species with different social styles.

Their findings were recently published in the journal Hormones and Behavior.

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