Territorial conflict may explain male primates’ large size

It’s a game of monkey mean, monkey grew. Territorial tension may be behind the size of male primates.

In many primate species, males have evolved to be bigger than their female counterparts, a disparity typically attributed to competition among socially related males for access to females. But bigger bodies may be more about dissuading conflict with males from rival groups, researchers report May 13 in Biology Letters. 

“The traditional explanation is incomplete,” says Cyril Grueter, an evolutionary anthropologist at the University of Oxford. 

Many species in the primate order — which includes monkeys, apes and lemurs — have sexual size dimorphism, meaning an average size difference between the sexes. While some primates like gibbons show barely any size disparity, others such as baboons and gorillas can have males that are twice as massive as the females.

Most research on this pattern has centered on male-male competition for females within a social group, Grueter says. Bigger, stronger males can outfight or intimidate smaller males, giving them better access to mates. 

“But primate groups are rarely isolated,” Grueter says. Neighboring groups commonly interact. They overlap in territory and compete for resources such as food and mates.

During his Ph.D. research, Grueter found that African leaf-eating monkeys with more contact between groups had especially large males compared with females. He and his colleagues wanted to see if that relationship extended to other primates.

The researchers gathered data on 146 primate species from the scientific literature and compared female and male body mass against several measures of between-group contact — how much home ranges overlapped, how often groups met and how aggressive those encounters were — along with each species’ mating system. Not every measure was available for every species.

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

Being bigger may help males defend territory and any accompanying resources from rival groups. This may be through physical fights, but Grueter says the persistent threat of altercations may be enough to supercharge size evolution in male primates via a kind of chronic cold war.

“Larger males may discourage escalation before fights even happen,” Grueter says.

Surprisingly, the mating system — a proxy for competition between in-group males for mates — didn’t have much of an effect on the size split. But Grueter thinks male body size evolved due to multiple pressures, and competition between groups has just been underappreciated.

The findings may mean looking at the evolution of sexual body size differences through a new lens that accounts for broader social effects. And the same evolutionary forces may be at work in other social or territorial mammals, producing similar size patterns, Grueter says.

Evolutionary biologist Catherine Sheard says the high variation in primate social interactions and the relative glut of data makes the clever mammals “a great place to start” studying the evolutionary impact of social traits.

“I’m also wondering how these results would have changed if the researchers included solitary species in their analyses,” says Sheard, of the University of Aberdeen in Scotland. The researchers had left out solitary species because the competitive dynamics between and within groups the team investigated don’t easily apply to species that lack stable social groups.

Grueter says he and his colleagues want to examine other traits linked to sexual evolutionary pressures in primates and other mammals — such as large canine teeth or physical or vocal displays. “It would be exciting to see whether the same pattern shows up.”