Testosterone, signal coloration, and signal color perception in male zebra finch contests

Many animals use assessment signals to resolve contests over limited resources while minimizing the costs of those contests. The carotenoid‐based orange to red bills of male zebra finches (Taeniopygia guttata) are thought to function as assessment signals in male–male contests, but behavioral analyses relating contest behaviors and outcomes to bill coloration have yielded mixed results. We examined the relationship between bill color and contests while incorporating measurements of color perception and testosterone (T) production, for an integrative view of aggressive signal behavior, production, and perception. We assayed the T production capabilities of 12 males in response to a gonadotropin‐releasing hormone (GnRH) challenge. We then quantified the initiation, escalation, and outcome of over 400 contests in the group, and measured bill color using calibrated photography. Finally, because signal perception can influence signal function, we tested how males perceive variation in bill coloration, asking if males exhibit categorical perception of bill color, as has been shown recently in female zebra finches. The data suggest that males with greater T production capabilities than their rivals were more likely to initiate contests against those rivals, while males with redder bills than their rivals were more likely to win contests. Males exhibited categorical color perception, but individual variation in the effect of categorical perception on color discrimination abilities did not predict any aspects of contest behavior or outcomes. Our results are consistent with the hypotheses that T plays a role in zebra finch contests and that bill coloration functions as an aggressive signal. We suggest future approaches, based on animal contest theory, for how links among signals, perception, and assessment can be tested. Green et al. show that male zebra finches perceive the continuum of orange‐red colors that mirror bill color in a categorical, not continuous fashion, matching prior findings in female zebra finches. In male–male contests, testosterone expression and bill color, not color perception, were related to contest behaviors. These results are consistent with the hypotheses that testosterone functions in avian aggression and that bill color is an aggressive signal in zebra finch contests. (a) Photo credit: Jim Bendon, Wikimedia Commons.