Abstract
Social defeat induces stress-responses in a wide array of vertebrates and can generate winner-loser effects. Dynamic condition-dependent signaling systems that reflect preparation for subsequent agonistic interactions, and thereby mediate winner-loser effects, should be more sensitive to competitive history than to non-social sources of stress. Bill color of female American goldfinches (Spinus tristus) is a dynamic condition-dependent ornament that functions as a signal of competitive status and mediates intrasexual agonistic social interactions. We tested the “social experience signaling hypothesis” in female goldfinches by (1) manipulating a non-social energetic stressor by experimentally elevating flight costs via wing-clipping in free-ranging birds, and (2) manipulating social experience by staging competitive interactions among captive birds. Bill color change did not differ between wing clipped and non-clipped females, even though stress-response, as measured by the heterophil to lymphocyte (H:L) ratio, increased significantly in clipped females relative to unclipped females. In contrast, winners and losers in the social experience experiment differed significantly in the degree and direction of bill color change following social contests, with bill color increasing in winners and decreasing in losers. These results suggest that dynamic bill color of female American goldfinches signals recent social history, but is less sensitive to some stressors stemming from non-social sources, and thereby suggest that signals can evolve sensitivity to specific types of processes relevant to the context in which they are used.
Highlights
Ornaments that reliably signal condition are important mediators of social interactions, including mate choice, offspring provisioning, and social competition (Lyon and Montgomerie, 2012)
Our study provides support for the hypothesis that dynamic condition-dependent bill color change in female American goldfinches signals the outcome of prior social interactions
Our results indicate that bill color of winners and losers of competitive social interactions changes in opposite directions, with winners becoming brighter and losers becoming more drab
Summary
Ornaments that reliably signal condition are important mediators of social interactions, including mate choice, offspring provisioning, and social competition (Lyon and Montgomerie, 2012). Dynamic condition-dependent ornaments have the capacity to signal recent changes in condition (i.e., the “relative capacity to maintain optimal functionality of vital systems within the body;” Hill, 2011), and may provide information about current conditional state with very short time lags (Sullivan, 1994a,b; Torres and Velando, 2003; Velando et al, 2006; Bamford et al, 2010; Rosenthal et al, 2012). Because dynamic condition-dependent signals can be informative regarding short-term changes in condition, they may be especially important during competitive social interactions because competitive ability or motivation may covary with current physiological state. It is possible that stress-responses resulting from social interactions may manifest differently than stress-responses derived from other sources, in which case certain dynamic signals could be influenced more by social experience than by other physiological perturbations
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