Dominance rank, facial morphology, and testes size in male white-faced capuchins: evidence for pre- and post-mating competition

Male reproductive success is determined by the interplay of female mate choice and male-male competition, often linked to dominance rank in social animals. Across taxa, elaborate ornaments, such as bright coloration or large antlers, often function as badges of status, signaling male competitive ability to rivals. In species where females mate with multiple males, post-mating sperm competition also plays an important role in male reproductive success and is associated with larger relative testes size. We investigate the relationship between morphological features and dominance rank in wild male white-faced capuchins. Using parallel-laser photogrammetry, we measured aspects of facial morphology, including facial width-to-height ratio, and testes size. We found that alpha males had significantly larger facial width-to-height ratios, wider faces, and wider scrota than subordinate males. These results suggest that facial traits potentially function as badges of status in male white-faced capuchins and may play a role in pre-mating competition and/or mate choice, while differences in scrotal size reflect adaptations for post-mating competition. This study highlights the underrecognized role of facial trait evolution in sexual selection among relatively gracile yet highly visually oriented mammals and highlights the potential variability of sexual traits in species characterized by strong reproductive skew among males.

Methods
We collected data on all males (6+ years old) residing in five habituated groups of white-faced capuchins (Cebus imitator) between January 2021 and May 2024 in the Santa Rosa Sector of the Área de Conservación Guanacaste, Costa Rica (SSR). We used parallel-laser photogrammetry (108,109) to noninvasively measure facial features, scrotal width, and body length, of male white-faced capuchins during periods of group stability. For each male, we collected multiple photographs (3 – 5) of their face, body length, and scrotum at least twice per month every three months over both dry and wet seasons. Selected photographs were uploaded in jpeg format to the GIMP GNU image manipulation program (version 2.10.34), a free and open-source image editing program (https://www.gimp.org). We applied GIMP’s standard high contrast filter to each photograph, which enhanced the visibility of the morphological features of interest due to the black and white pelage of the study species. Following Richardson et al. (2022), we used the calibrated inter-laser distance (30.00 or 45.00 mm depending on the laser used for the photograph) divided by the inter-laser distance from the image in pixels (calculated using the ‘measure tool’ in GIMP), to create a millimeter per pixel scale (112). We measured four facial features (Figure 1): a) brow-width – i.e., the lateral-medial width of the supraorbital torus, b) facial height – i.e., the height of the face from between the eyes to the middle of the upper lip, c) muzzle width – i.e., the lateral width of the upper snout, measured across the maxilla just above the upper lip and d) facial width – i.e., the width from the most lateral points of each side of the mandible, which is the widest visible portion of a capuchin male face. From these measurements, we calculated the fWHR by dividing facial width by facial height.