In many primate species, males are much larger than their female counterparts, which is generally attributed to male competition for mates (sexual selection). But bigger bodies may not just be about alpha males defeating rivals. They could also come about because of competition between neighboring social groups, according to a new study published in the journal Biology Letters.
External threats
The authors felt that the standard scientific view that a primate male's size was mostly determined by rivalry within the same group was not the full story. They wanted to see if the threat from outsiders was an even more powerful evolutionary engine. One of the reasons for this was that primate groups are not isolated. They tend to live close to other groups, meaning they must share or fight for resources.
The team searched through scientific literature and collected data on 146 different primate species. They compared male and female size against several measures of between-group competition. These included the daily encounter rate, the proportion of encounters that were antagonistic, and the home range overlap (the percentage of a group's total range that is shared with a neighboring group).
Silent deterrent
It turns out that the mating system, which describes how males and females pair up for breeding, was not a strong predictor of size differences. Instead, it was pressure for space. The authors discovered that the more a group's territory overlaps with its neighbors, the larger the males are compared to the females. Frequent encounters with neighboring groups also appear to promote the evolution of larger male bodies.
Their hypothesis is that the evolutionary advantage and driver of size is that it acts as a silent deterrent. Being larger helps a male defend resources and guard mates by simply being intimidating. This reduces the need for risky physical fights. Over time, evolution favors these larger males because they can successfully protect their group and territory by their physical presence alone.
"Home range overlap may select for larger males to deter rivals, defend resources or monopolize females across shared territories, potentially without frequent physical contests," wrote the research team in their paper.
Perhaps surprisingly, groups that had more hostile encounters with their neighbors did not consistently have bigger males. This reinforces the idea that looking dangerous could be more important than actual combat. "Our work calls for renewed attention to how spatial competition, including resource defense and mate guarding across overlapping territories, influences trait evolution in primates and other social vertebrates."
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Publication details
Cyril C. Grueter et al, Effects of between-group competition on sexual size dimorphism in primates, Biology Letters (2026). DOI: 10.1098/rsbl.2025.0680
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Citation: Rivalry with neighboring groups may be a key driver of male size in primates (2026, May 13) retrieved 13 May 2026 from https://phys.org/news/2026-05-rivalry-neighboring-groups-key-driver.html
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