Mate competition encourages individuals to modulate characters involved in mating success. Adult Iberian red deer (Cervus elaphus hispanicus) males show a dark ventral patch (DVP) that plays a central role in mating rivalry, whose size and chemical compounds varied according to the level of male-male competition within the population. In the pigmentation of the DVP appears, after urinary excretion, a molecule called DOPEG originating from the metabolism of norepinephrine, leading us to investigate whether differential expression mechanisms of key genes dopamine B-hydroxylase (DBH) and tyrosine hydroxylase (TH), encoding enzymes catalyzing the process can be sensitive to different competitive population situations and responsible for the plastic development of the DVP in red deer. We found that social environment with higher intrasexual competition, where male invest more in sexual traits, was associated with increased levels of DBH and TH transcripts, while Dopamine showed reversed values. We found alternative splicing for the TH gene, although differences between social environments appeared just related to expression levels. Our results support the internal cause of trait modulation based on differential gene expression in relation to the conditions of intrasexual competition in social environment. We propose the quantification of DBH transcripts as a molecular biomarker of male red deer reproductive activity.