Territorial conflict with rival groups over food and mates may explain male primates’ large size, some male primates are up to twice the size of females

Amazing stuff!

"... 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. ...

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

"Sexual selection acting on males through intrasexual competition for mates is a well-established driver of sexual size dimorphism (SSD) in primates. However, studies typically focus on within-group competition, overlooking the potential significance of competition arising from interactions between neighbouring social groups, particularly when home ranges overlap.

Here, we analysed the relationships between SSD, mating system and different proxies of range use across up to 143 species of anthropoid and strepsirrhine primates.

Contrary to expectations, mating system—a commonly used proxy for male competition—did not significantly predict SSD.

Instead, male-biased SSD increased with home range overlap and encounter rate between social groups, even after accounting for mating system and body size allometry.

This suggests that spatial pressures, such as the latent threat of competition from rival groups, impose stronger selection on male compared with female size.

Home range overlap may select for larger males to deter rivals, defend resources or monopolize females across shared territories, potentially without frequent physical contests.

Our work calls for renewed attention to how spatial competition, including resource defence and mate guarding across overlapping territories, influences trait evolution in primates and other social vertebrates and to re-evaluate proxies of sexual selection."

Territorial conflict may explain male primates’ large size

Effects of between-group competition on sexual size dimorphism in primates (open access)

Fig. 1 In contrast, there was mixed evidence for evolutionary covariation between SSD and the proportion of encounters reported to be agonistic.