Pflüger, first author of the study and scientific director of the Affenberg facility, studied 26 sexually mature males during the mating season, a particularly stressful time for male animals as they must compete for females. She discovered that the amount of a metabolic product of the hormone cortisol in the animals' faeces differed across 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 behaviour and how genetics affects the hormonal stress reaction and social rank," says Pflüger.