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.