Sexual Selection in Human Evolution - Sexual Dimorphism

Sexual Dimorphism

Men are generally hairier than women, and Darwin was of the opinion that hairlessness was related to sexual selection; however, several other explanations have been advanced to explain human hairlessness, a leading one is loss of body hair to facilitate sweating. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits.

Sexual selection is impossible to prove, as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selection, and some may be accidental and due to pleiotropy. For example monogamous primates are known to typically exhibit little sexual dimorphism such as particularly large males armed with huge canines; however, powerful big-toothed males can provide protection against predators and may be bigger for that reason rather than in order to win confrontations over females. Males and females differing in size can specialize in, and more fully exploit, different food resources while avoiding competing with each other; furthermore, body size can be useful in avoiding predators and may also be of assistance in securing a mate. This is further complicated by the consideration that with larger body size, the skeleton of mammals becomes much more robust and massive (relatively speaking). Bearing these caveats in mind, levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have shown the earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have became more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).

A study found that there is an evolutionary trend for men to have relatively shorter upper faces, and suggested that this trait may have been caused by sexual selection, possibly because women have preferred men who looked masculine but not aggressive.