Evolutionary battle: How sexual selection may have shaped men’s upper limb length

A series of studies published in the American Journal of Human Biology suggests that human upper limb length may be a result of sexual selection, as it enhances effectiveness in fighting contests between males. This idea is supported by findings that professional combatants in the Ultimate Fighting Championship (UFC) with longer upper limbs tend to be more successful. Additionally, significant relative differences in upper limb lengths between males and females were observed in samples of Croatian adolescents, Singaporean adults, and United States Army personnel.

Sexual selection is a form of natural selection in which traits evolve because they increase an individual’s chances of attracting mates or outcompeting rivals. It occurs through mate choice (intersexual selection), where individuals prefer certain traits in partners, and competition (intrasexual selection), where individuals of the same sex compete for access to mates. This process explains the evolution of features such as peacock tails, antlers in deer, and courtship behaviors, which may not directly aid survival but enhance reproductive success.

Over time, sexual selection can lead to elaborate traits, pronounced differences between sexes, and complex mating strategies. In human males, characteristics such as a strong jawline, a prominent brow ridge, body strength, deep voices, ambition, and a drive to acquire wealth are likely results of sexual selection. For example, a strong jawline and a pronounced brow ridge are associated with higher testosterone levels and perceived genetic fitness. Deep voices signal maturity and dominance, while ambition and wealth have historically increased reproductive success by attracting mates who value stability and protection.

Study authors Neil R. Caton and David M. G. Lewis sought to test the hypothesis that human upper limbs have been shaped by selection to enhance male fighting effectiveness. They note that upper limb length exhibits pronounced differences between males and females in anthropoid primates. In humans, upper limb length appears to correlate with fighting ability, particularly in hand-to-hand combat.

However, because humans typically use weapons and teamwork in conflicts with animals and other groups, differences in limb length might not be crucial in such fights. Instead, the type of contests where limb length is most relevant are male-against-male competitions, which often follow rules and exclude weapons. These fights, commonly involving grappling and wrestling, usually result in submission rather than death. Some studies suggest that human upper limbs evolved specifically to deliver strikes to the head and facial skeleton—common targets in male fights. The study authors hypothesized that human upper limbs may have evolved to maximize the chances of knocking out opponents (the knockout hypothesis) and subduing them.

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The researchers conducted four studies. In the first study, they estimated the shoulder breadth of 715 UFC professional combatants (from photographs) and compared this to their fighting performance over their careers, analyzing factors such as striking accuracy, strike defense, grappling defense, and grappling accuracy. Among these fighters, 113 were female.

In the subsequent studies, they examined differences in upper limb length relative to other body proportions across various populations. These included the UFC fighters from the first study, 5,155 Croatian adolescents aged 3–18 (divided into 16 age groups), 100 older Singaporean adults, and 6,068 U.S. Army personnel from the Anthropometric Study of U.S. Army Personnel. All data were obtained from previously published studies.

The results showed that UFC fighters with longer upper limbs were generally more successful in fights. The study authors tested a statistical model proposing that greater upper limb length increases the likelihood of knocking out opponents, which, in turn, enhances a fighter’s success. Analyses supported this relationship, providing evidence for the knockout hypothesis. Another analysis indicated that fighters with longer upper limbs also tended to achieve more submission victories, further contributing to their success.

Results from the other studies revealed that even when controlling for factors such as age, height, lower limb strength, and shoulder breadth, significant differences in upper limb length between males and females persisted. Males had relatively longer upper limbs than females across all studied groups—UFC fighters, Croatian adolescents, older Singaporean adults, and U.S. Army personnel.

“Upper limb length may have been shaped by intrasexual selection, with implications across the biological, anthropological, and psychological sciences,” the study authors concluded.

The study sheds light on the differences between sexes in upper limb length and also on the links between upper limb length and fighting ability. However, it should be noted that this study does not provide definitive answer on how male upper limbs evolved. It is entirely possible that evolutionary pressure that led to longer upper limbs in men was the need for throwing power, and therefore hunting success. This is, however, not mutually exclusive with the argument that longer upper limbs make men more successful in fighting contests with other men.

The paper, “Intrasexual Selection for Upper Limb Length in Homo sapiens,” was authored by Neil R. Caton and David M. G. Lewis.