Effects of between-group competition on sexual size dimorphism in primates

Sexual selection acting on males through intrasexual competition for mates is a well-established driver of sexual size dimorphism (SSD) in primates. However, studies typically focus on within-group competition, overlooking the potential significance of competition arising from interactions between neighbouring social groups, particularly when home ranges overlap. Here, we analysed the relationships between SSD, mating system and different proxies of range use across up to 143 species of anthropoid and strepsirrhine primates. Contrary to expectations, mating system-a commonly used proxy for male competition-did not significantly predict SSD. Instead, male-biased SSD increased with home range overlap and encounter rate between social groups, even after accounting for mating system and body size allometry. This suggests that spatial pressures, such as the latent threat of competition from rival groups, impose stronger selection on male compared with female size. Home range overlap may select for larger males to deter rivals, defend resources or monopolize females across shared territories, potentially without frequent physical contests. Our work calls for renewed attention to how spatial competition, including resource defence and mate guarding across overlapping territories, influences trait evolution in primates and other social vertebrates and to re-evaluate proxies of sexual selection.