Abstract
In polygynous lizards, male–male competition is an important driver of morphologic and behavioral traits associated with intraspecific dominance. The extent to which females engage in aggressive behavior and thus contribute to competition-driven morphologic variation is not well studied. We used injury frequencies of brown anoles (Anolis sagrei) in 16 island populations to test the hypothesis that injury-inducing aggressive encounters increase with population density in both male and female lizards. We further asked whether intraspecific competition is a potential driver of phenotypic traits related to dominance by using population density as proxy for intraspecific competition. We found that the proportion of individuals with injuries was greater in populations with higher densities, suggesting that agonistic competitive interactions increase with population density. Size-adjusted head length of male and female lizards increased with population density, suggesting that larger heads might be advantageous when intraspecific competition is strong. We detected differences in morphology and injury frequency among islands for both males and females, which suggests that agonistic competitive interactions among females may be stronger than previously appreciated. Further research is needed to determine whether aggressive encounters involving females are restricted to intrasexual competition or whether they also involve males, and how morphologic traits of females are related to competitive dominance and reproductive success.
Highlights
In many vertebrate species, males compete for mating opportunities and engage regularly in aggressive male–male interactions (Kokko and Rankin, 2006; Knell, 2009)
Body size is correlated with head size and bite force and is the primary trait linked to reproductive success of males (Herrel et al, 2001, 2007, 2010; Verwaijen et al, 2002; Wittorski et al, 2016; but see Lappin and Husak, 2005)
We tested whether body size and head shape varied with population density in the direction predicted if stronger competition leads to selection on morphology
Summary
Males compete for mating opportunities and engage regularly in aggressive male–male interactions (Kokko and Rankin, 2006; Knell, 2009). The ability to obtain and defend territories disproportionally increases reproductive success for males in such species, and sexual selection is expected to shape the phenotypic traits associated with dominance and performance during aggressive male–male interactions. Body size is correlated with head size and bite force and is the primary trait linked to reproductive success of males (Herrel et al, 2001, 2007, 2010; Verwaijen et al, 2002; Wittorski et al, 2016; but see Lappin and Husak, 2005). The extent to which aggressive behavior among females contributes to phenotypic evolution is not well studied, primarily because past studies have mostly focused on males
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