Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability.

Sexual dimorphism often arises from selection on specific musculoskeletal traits that improve male fighting performance. In humans, one common form of fighting includes using the fists as weapons. Here, we tested the hypothesis that selection on male fighting performance has led to the evolution of sexual dimorphism in the musculoskeletal system that powers striking with a fist. We compared male and female arm cranking power output, using it as a proxy for the power production component of striking with a fist. Using backward arm cranking as an unselected control, our results indicate the presence of pronounced male-biased sexual dimorphism in muscle performance for protracting the arm to propel the fist forward. We also compared overhead pulling force between males and females, to test the alternative hypothesis that sexual dimorphism in the upper body of humans is a result of selection on male overhead throwing ability. We found weaker support for this hypothesis, with less pronounced sexual dimorphism in overhead arm pulling force. The results of this study add to a set of recently identified characters indicating that sexual selection on male aggressive performance has played a role in the evolution of the human musculoskeletal system and the evolution of sexual dimorphism in hominins.