Defeated chameleons darken dynamically during dyadic disputes to decrease danger from dominants
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Research on intraspecific aggression has typically focused on dominant individuals, but a better understanding of the consequences and mechanisms of agonistic encounters requires a balanced perspective that includes knowledge of subordinate animal behaviors. In contrast to signals of fighting ability, signals of submission are an understudied component of agonistic communication that could provide important insights into the dynamics, function, and evolution of intraspecific competition. Here, I use a series of staged agonistic trials between adult male veiled chameleons Chamaeleo calyptratus to test the hypothesis that rapid skin darkening serves as a submissive signal to resolve agonistic activity. Concordant with this hypothesis, I found that losing chameleons darkened over the course of aggressive trials while winners brightened, and the likelihood of darkening increased when individuals were attacked more aggressively. Additionally, I found that the degree of brightness change exhibited by individual chameleons was tied to both overall and net aggression experienced during a trial, with chameleons who received high levels of aggression relative to their own aggression levels darkening to a greater extent than individuals receiving relatively less aggression. Lastly, I found that aggression increased for losers and winners prior to the onset of darkening by the eventual loser but that both chameleons reduced aggression after the losing chameleon began to darken. Based on the theoretical prediction that signals of submission should be favored when retreat options are restricted, I suggest that limited escapability imposed by chameleon morphology, physiology, and ecology favored the evolution of a pigment-based signal of submission in this group.
KeywordsSubmissive signals Physiological color change Aggression Communication Reptiles Chamaeleo calyptratus Color signals
I thank Megan Best, Sarah Bruemmer, Brianna Bero-Buell, and Andrea Carpenter for their invaluable assistance with experimental procedures, data collection, and dedicated chameleon care. I thank the Animal Behavior Society, the American Society for Ichthyologists and Herpetologists, the ASU chapter of Sigma Xi, and the ASU Graduate and Professional Students Association for financial support of this research. I also thank my parents, David and Sandy Ligon, as well as Richard and Janet Steele and two anonymous donors for their generous financial support to this project. I thank my advisor Kevin McGraw, Dale DeNardo, and members of the McGraw lab group for input that greatly improved this manuscript. I also thank my wife, Veronica Ligon, for her support throughout this project.
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