Territorial Conflict Drives Male Primate Size Evolution

New research published in Biology Letters challenges the traditional view that male primates evolved larger bodies primarily to compete for mates within their own social groups. Instead, a study led by evolutionary anthropologist Cyril Grueter from the University of Oxford suggests that pressure from rival groups plays a significant role. By analyzing data from 146 primate species, researchers found a strong correlation between the degree of territorial overlap and inter-group contact with increased sexual size dimorphism. Males in species with frequent interactions between neighboring groups tend to be significantly larger than females. This size advantage likely helps deter conflict and defend resources through intimidation rather than just physical combat, acting as a chronic deterrent. Surprisingly, the mating system, which reflects internal competition, had little effect on size differences. The findings indicate that evolutionary pressures from broader social landscapes and inter-group dynamics have been underappreciated in understanding primate evolution. Experts suggest these insights could apply to other territorial mammals, offering a new lens for studying how social traits influence physical evolution. The study excluded solitary species, focusing only on those with stable social structures.