Allometry and sexual dimorphism in the mandible of South American coatis: contrasting sexual selection and niche divergence hypotheses

Abstract Sexual dimorphism is a common source of intraspecific variation among carnivorans. Although indirect sexual selection on a particular trait can lead to sexual dimorphism in others, the effects of cross-trait covariances on carnivoran mandibular morphology remain understudied, especially in more generalist species, such as the South American Coati Nasua nasua. Herein, we asked whether sexual shape dimorphism (SShD) in the mandible of N. nasua—which presents strong male-biased sexual selection—were biased to sexual size dimorphism (SSD). Based on analyses of 148 adult specimens, we found that male coatis exhibited mandibles 10% larger and typically more robust than females, with lengthier canines and proportionally shorter molar tooth rows. While allometry intensified this SShD, sex-related differences also persisted in its nonallometric shape. These differences were mostly concentrated to the anterior portion of the mandibular corpus, where canines are attached. Because male coatis use their atypically enlarged canines during combat, we hypothesized that these differences were primarily related to male-biased sexual selection, leading to trade-offs in the relative proportions of canines relative to molars. These results indicate that mandibular SShD in N. nasua is not merely a by-product of SSD, but it also reflects direct male-biased sexual selection on combat-related traits.