Facial Width-To-Height Ratio Relates to Alpha Status and Assertive Personality in Capuchin Monkeys

Abstract

Social dominance hierarchies play a pivotal role in shaping the behaviour of many species, and sex differences within these hierarchies often exist. To date, however, few physical markers of dominance have been identified. Such markers would be valuable in terms of understanding the etiology of dominant behaviour and changes in social hierarchies over time. Animals may also use such traits to evaluate the potential dominance of others relative to themselves (i.e. a physical “cue”). Facial width-to-height ratio (fWHR), for example, has been suggested as a cue to dominance in humans, with links to both dominant behaviour and the perception of dominance in other individuals. Whether this association is present in non-human animals is currently not known. Therefore, here we examine within-species links between fWHR and dominant behaviour in 64 brown capuchin monkeys (Sapajus spp.) aged between 2 and 40 years. fWHR was positively associated with alpha status and with a dimensional rating of assertive personality in both males and females. Moreover, fWHR showed significant sexual dimorphism in adults but not juveniles, suggesting a developmental change may occur during puberty. In a sub-sample, sex differences were mediated by weight, suggesting fWHR dimorphism does not exceed what would be expected by differences in body weight. This is the first report of an association between face shape and behaviour in a non-human species. Results are discussed in terms of the role that face-behaviour associations might play within capuchin societies, and the possible selective forces that might have led to the evolution of fWHR-dominance associations in humans.

Response: Commentary: Facial Width-to-Height Ratio (fWHR) Is Not Associated with Adolescent Testosterone Levels

“…The research on fWHR thus far has been intriguing—in particular, findings on the associations between low fWHR and rated or actual aggression, dominance, strength (e.g., Carré and McCormick, 2008; Ozener, 2012; Lefevre et al, 2014; Geniole et al, 2015). Taken as a whole, the surfeit of converging evidence casts doubt on the hypothesis that fWHR is a sexually selected signal of T and T-derived traits.…”

“…Given continuing interest in the relationships between facial width-to-height ratio (fWHR) and behaviors such as aggression and dominance (e.g., Carré and McCormick, 2008; Ozener, 2012; Lefevre et al, 2014; Geniole et al, 2015), it is important to address the potential role of pubertal testosterone (T) in facial development.…”

Does the facial width-to-height ratio map onto variability in men's testosterone concentrations?

“…Studies in both humans (e.g., Weston et al, 2007) and non-human species (e.g., capuchin monkeys: Lefevre et al, 2014) also find that the fWHR becomes sexually-dimorphic around puberty, suggesting that pubertal T shapes, at least to some degree, variation in the fWHR. Other research shows that accurate identification of male and female faces does not happen until late adolescence, and importantly, accurate identification is strongly predicted by the target's T levels, even after controlling for age (Marečkova et al, 2011).…”

“…For instance, the facial widthto-height ratio (fWHR)-the distance of the bizogymatic width divided by the distance between the brow and upper lip-is positively correlated with measures of aggressive behavior (Carré & McCormick, 2008;Welker et al, 2014;Lefevre et al, 2014;Goetz et al, 2013, but see Ozener, 2012and Gomez-Valdes et al, 2013, psychopathic traits (Geniole et al, 2014a;Anderl et al, 2016), achievement drive (Lewis et al, 2012), competitive success (baseball study of homeruns:…”

Social status, facial structure, and assertiveness in brown capuchin monkeys

“…Lefevre et al (2014) did not probe this interaction, presumably because it did not reach the conventional level of statistical significance (i.e., p < 0.05). However, the lack of statistical significance is almost certainly due to a lack of statistical power given the small sample size ( n = 43).…”