Study Finds Between-Group Competition Correlates With Greater Male-Female Size Differences in Primates

A study on sexual size dimorphism in primates was published in Biology Letters on May 13, 2026. The research indicates that pressure from rival groups, rather than solely competition among males inside social groups, helps explain why males in many primate species have evolved to be larger than females.

Cyril Grueter, an evolutionary anthropologist at the University of Oxford, stated that the traditional explanation for male primates being larger is incomplete.

Researchers gathered data on 146 primate species from the scientific literature. The study compared female and male body mass against measures of between-group contact including home range overlap, frequency of group encounters, and aggressiveness of encounters. The study also incorporated each species’ mating system in the analysis.

The more territories overlapped, the bigger males were compared with females, the team found. The more often groups encountered each other, the bigger males were compared with females. “Living in a crowded social landscape with lots of interaction between groups seems to be linked to bigger males,” Cyril Grueter stated.

Being bigger may help males defend territory and resources from rival groups. Larger males may discourage escalation before fights even happen, he added. The mating system, used as a proxy for in-group male competition, didn’t have much of an effect on the size split.

Solitary primate species were excluded from the analysis because competitive dynamics between and within groups do not easily apply to species lacking stable social groups. Science News reported that most prior research on this pattern has centered on male-male competition for females within a social group.