What was the primary driver of sexual dimorphism in body size among single-male groups?

The primary driver of sexual dimorphism in body size among single-male groups is male competition for females. In species where a single male mates with multiple females, there is often intense competition among males to gain access to mates. This competition can manifest physically, where larger males tend to have an advantage in battles for dominance or territory. As a result, these larger males are more likely to reproduce successfully, thereby passing on their genes to future generations. Over time, this evolutionary pressure leads to a pronounced difference in size between males and females, as males evolve to become larger and more competitive.

In environments where multiple males could try to mate with the same females, sexual selection intensifies. Thus, size becomes a crucial factor, and the benefits of being larger—such as intimidation of rivals and the ability to physically overpower them—promote a greater degree of dimorphism. This phenomenon is particularly evident in many primate species and other mammals where social structures and mating systems are largely influenced by male-female interactions centered around mating opportunities.