New research suggests male primates grow larger not only for in-group mating fights, but also to deter rival groups across overlapping territories.
Territorial tension may be a key reason male primates look so much larger than females, reshaping a long-standing explanation for sexual size differences across the animal kingdom.
In many primate species, males evolve to be bigger than females, a pattern known as sexual size dimorphism.. For years, researchers have largely attributed this to competition among socially related males inside the same group.. The logic was straightforward: larger males can outfight rivals or intimidate them, gaining better access to mates.
But evolutionary anthropologist Cyril Grueter says that standard view does not tell the whole story.. Neighboring primate groups are often not isolated from one another.. They can overlap in territory. meet repeatedly. and compete for critical resources such as food and potential mates—creating conditions for conflict that extends beyond the boundaries of a single social group.
This idea traces back to Grueter’s earlier Ph.D.. work on African leaf-eating monkeys. where he observed that species with more interaction between groups tended to have especially large males compared with females.. The new report asked whether that relationship holds more broadly across primates rather than being limited to one lineage.
To test the hypothesis, the researchers assembled comparative data covering 146 primate species.. Using information compiled from the scientific literature. they compared female and male body mass while also examining several indicators of between-group contact.. These included how much home ranges overlapped, how often groups met, and how aggressive encounters were.. Because not every measurement existed for every species. the analyses relied on whatever between-group contact data were available for each case alongside the species’ mating system.
The pattern the team found was clear: the more territories overlapped and the more frequently groups encountered one another. the larger the male-to-female size difference tended to be.. In other words, primates living in a “crowded” social landscape—where individuals regularly face outsiders—show stronger male-biased size.
Larger bodies. the researchers suggest. may help males defend territory and any resources tied to it when rival groups come too close.. While conflict could involve direct physical fights, Grueter argues that something subtler may also matter.. A persistent risk of altercations—an ongoing. low-grade threat—could keep selection pressures on male size active over many generations. producing what he describes as a chronic cold war rather than only episodic combat.
One twist in the findings was that the mating system. used as a proxy for how strongly males compete for mates within the group. did not appear to have much effect on the size split.. That result does not negate in-group competition. but it does complicate the idea that it alone explains the magnitude of sexual size dimorphism.. Grueter thinks multiple pressures may shape male body size at once, and that competition between groups may have been underestimated.
The research points to a new way of looking at sexual body size differences: not simply as a consequence of who can win inside a group. but also as an outcome of how often groups collide in the wider landscape.. It also raises the possibility that similar social and territorial pressures could influence other species beyond primates.
Evolutionary biologist Catherine Sheard, who was not involved in the work, highlighted why primates are a useful test case.. She noted that the order shows wide variation in social interaction patterns and that there is relatively abundant data available. making it a strong starting point for studying how social traits can shape evolutionary outcomes.
Sheard also raised a question about the analysis itself: what might change if solitary species were included?. The researchers left out solitary primates because the kind of between- and within-group competitive dynamics they examined does not apply cleanly to species that do not form stable social groups.
Looking ahead, Grueter and his colleagues plan to investigate other traits that may respond to sexual selection and territorial pressures.. That could include features such as large canine teeth, as well as physical or vocal displays.. They also want to examine whether the same broad relationship between conflict-prone environments and male-biased traits appears in other social or territorial mammals.
If the results hold, the implications extend beyond primate curiosity.. They suggest that the evolution of sexual dimorphism may reflect a wider web of pressures. including encounters with outsiders. and that the “shape of society” in the wild—how groups overlap. meet. and clash—could matter as much as reproductive competition inside the group.
primate evolution sexual size dimorphism territorial conflict between-group interactions evolutionary anthropology mating systems