Social environment with high intrasexual competition enhances the positive relationship between faecal testosterone and cortisol metabolite levels in red deer

The relationship between reproductive and physiological stress hormones in vertebrates is poorly understood. In many species of mammals and especially in humans, the most widespread idea is that there is a negative relationship between them, i.e. higher stress levels are associated with lower testosterone levels. Likewise, the subordination stress paradigm supports that subordinates suffer greater stress in a competitive situation, while the dominant ones have higher levels of testosterone. However, this predominant idea of a negative relationship between testosterone and cortisol concentrations may be influenced by unnatural circumstances, such as chronic stress in humans or forced interactions between subordinates and dominants in laboratory or captivity. Some studies have reported that dominant males under natural conditions may show higher physiological stress and also higher testosterone levels than subordinates. But for this positive relationship, the question is whether there is a causal link or whether both hormones only coincide due to other factors. We hypothesized that testosterone should be related to physiological stress only as a result of individual males being subjected to stressful situations, such as intrasexual competition. We studied this topic in Iberian red deer (Cervus elaphus hispanicus) males in two types of populations with high and low levels of intrasexual competition. We found a positive relationship between faecal testosterone and cortisol metabolite levels, but also a significant interaction showing that this relationship occurs more intensely in populations with high competition level for mating. These results reinforce the positive relationship between both hormones under natural conditions and support the hypothesis that it is mediated by male-male competition for mates.