Pick up almost any textbook account of why male gorillas, baboons, or proboscis monkeys are so much larger than females and you’ll read a version of the same explanation: bigger males win more fights, win more fights means more mates, more mates means more offspring, and over millions of years that arithmetic compounds into bodies that can be twice the mass of a female’s. It’s a tidy story. It’s also, according to a new comparative study, probably incomplete.
The research, published in Biology Letters1 by Cyril Grueter, Xiaoguang Qi, and Stefan Lüpold, examined sexual size dimorphism across up to 146 primate species and found that the factor most strongly associated with how different males and females are in body mass is not the mating system. It’s something more spatial: how much a group’s home range overlaps with the ranges of its neighbors.
That’s a meaningful distinction. Mating system, the typical proxy researchers use to infer the strength of sexual selection, describes the competitive dynamics happening inside a group. A polygynous species, where one or a few males monopolize multiple females, is presumed to have intense male-male competition and therefore strong selection for large male bodies. Monogamy, by contrast, implies less competition, less dimorphism. The logic is appealing. The data, once phylogenetic relationships are properly accounted for, don’t actually support it.
