Testosterone and the dark ventral patch of male red deer: the role of the social environment.

The expression of male sexual traits, which is stimulated by testosterone, entails significant costs for individuals. Consequently, natural selection is expected to favour the modulation of sexual trait development according to the balance between its costs and benefits. The proportion of rivals in a population may affect this balance by increasing or decreasing the reproductive benefits associated with the development of sex traits. Here, we explore the relationship between testosterone level and sex trait size under two populational conditions of mate competition: fenced (i.e. high male-male competition; all male age groups are present) and unfenced (i.e. low competition; most males present are juveniles). Our model species is the Iberian red deer (Cervus elaphus hispanicus), and the sex trait is the dark ventral patch that males exhibit during the rutting season. Our results showed that the positive relationship between testosterone levels and the size of the dark ventral patch depends on the environmental level of male-male competition. Only in populations where the operational sex ratio was high (i.e. high proportion of rival males), individuals with high levels of testosterone developed the sex trait. Conversely, when mate competition was low, there was no significant relationship between testosterone level and trait size. This result reinforces the idea that the effect of testosterone in promoting the development of sex traits may be mediated by the intensity of mate competition in the population, as well as the role of sexual selection in the evolution of the dark ventral patch in red deer.