Hormones and honest signals: males with larger ornaments elevate testosterone more when challenged.

When male investment in mating varies with quality, reliable sexual signals may evolve. In many songbirds, testosterone mediates mating investment, suggesting that signals should be linked to testosterone production. However, because testosterone may change rapidly during behaviour such as territorial aggression and courtship, efforts to establish such a relationship have proved challenging. In a population of dark-eyed juncos, we measured individual variation in the production of short-term testosterone increases by injecting gonadotropin-releasing hormone (GnRH). We found a positive correlation between the magnitude of these increases and the size of a plumage ornament ('tail white') previously shown to be important for female choice and male-male competition. We then measured naturally elevated testosterone levels produced during male-male competition and found that they covaried with those induced by GnRH. We suggest that the association between tail white and testosterone increases may allow conspecifics to assess potential mates and competitors reliably using tail white.