In many primates, males are much larger than females, but the size gap varies widely. Western gorillas are a striking example, with males averaging about 170 kilograms and females just over 70. Cape baboons also show strong dimorphism, with males at nearly 30 kilograms and females under 15. By contrast, in species such as silvery gibbons, emperor tamarins and ring-tailed lemurs, males and females are about the same size.
For years, scientists explained this mainly through competition among males within the same group, since larger males are better able to win access to females. That idea fits gorillas, which usually live in one-male groups, and gibbons, which live in monogamous pairs with far less male rivalry. But baboons do not fit so neatly, because they live in mixed groups with roughly equal numbers of males and females, so in-group competition is not especially intense.
A new study suggests another evolutionary force matters too, competition between groups. In primates, males often defend the group and its territory, and they may fight neighboring males that try to intrude. A group with weak males can lose food, territory and sometimes females. The researchers examined 146 primate species and compared their social structure with the amount of overlap between neighboring territories, how often groups met, and how often those meetings involved threats or fights.
They found that the greater the overlap and the more frequent the encounters between groups, the larger males tended to be relative to females. Cyril Grueter, who led the study, told Science News that “life in a crowded social landscape, with many intergroup interactions, is probably linked to large males.” The team did not find a link between the share of intergroup encounters that included threats or battles and male size. Their view is that being visibly large may be enough to deter rivals before a fight starts, so size reduces escalation risk rather than reflecting more combat.
The same researchers previously showed that intergroup competition is also linked to male “ornaments” in some primates, such as the broad flat cheeks of orangutans, baboon hair tufts and the long nose of the proboscis monkey. They now plan to test other male traits that may help intimidate rivals, including large canines and loud calls.
