Armed forces: Intrasexual selection for upper limb length in Homo sapiens

Numerous taxa have evolved physiological appendage-based weaponry to increase damage output in violent fights, but no research has empirically shown that Homo sapiens upper appendages have uniquely evolved to increase resource-provisioning potential in real-world combat. In Study 1, we used actual fight outcome data (N = 715 fighters) to examine multiple competing hypotheses—the striker, defender, grappler, and knockout hypotheses—for the evolution of Homo sapiens upper limb length, controlling for approximately a dozen confounding variables (e.g., biacromial width, lower limb length, age, weight, height). There was exclusive support for the knockout hypothesis: upper limb length increases fighting success through knockout power. There was also evidence for a real-world association between biacromial width and knockout power. Because sexual dimorphism often emerges from selection on morphological structures that improve male’s fighting success, we consequently expected sexual dimorphism in upper limb length. Studies 2a-2d provided powerful evidence for this new universal sexual dimorphism in upper limb length. Even after controlling for weight, height, and lower limb length, males exhibited longer upper limbs than females across the globe: from mixed-martial-artists (Study 2a) and Croatian adolescents (but not pre-pubertal children; Study 2b) to older Singaporean adults (Study 2c) and over 6,000 United States Army personnel (Study 2d) born across seven major world regions (Africa, Europe, Asia, Oceania, and North, Central, and South America). Combined, our results provide comprehensive support for the argument that intrasexual selection has uniquely shaped Homo sapiens upper limb length to enhance fighting performance in real-world combat.

Armed forces: Intrasexual selection for upper limb length in Homo sapiens

Over 150 years ago, Darwin (1871) argued that humans had evolved to inflict and resist damage in violent combat. Until now, however, no research in human fighting ability has examined the anatomical structures that specifically contribute to damage resistance and infliction. Numerous taxa, however, have evolved physiological outgrowths (i.e., appendages) -- biologically categorised as animal weaponry -- to increase their damage infliction capacity, and humans' upper limb length have long been speculated, but never empirically examined, to contribute to this capacity. There have been four leading explanations for human upper limb length's place as an indicator of resource-holding potential, such that upper limb length contributes to: (1) striking accuracy (the striker hypothesis); (2) grappling accuracy (the grappler hypothesis); (3) defensive capabilities (the defender hypothesis); and (4) knockout power (the knockout hypothesis). In 715 professional combatants, we find exclusive support for the knockout hypothesis: upper limb length increases fighting success as mediated by increased knockout wins, after comprehensively controlling for weight, height, sex, fighter's debut date, age, facial structure, lower limb length, biacromial (shoulder) width, wins by submission and decision, and losses by submission, decision, and knockout (Study 1). In Studies 2-5, we then find that upper limb length (controlling for weight, height, and lower limb length) is sexually dimorphic across the globe: in professional combatants (Study 2), Croatian adolescents (Study 3), older Singaporean adults (Study 4), and 6,000 male and female United States logistics personnel born from over 110 countries statistically grouped into 7 distinct world regions (Africa, Europe, Asia, Oceania, and North, Central, and South America). Our results paint a comprehensive and consistent picture that upper limb length in Homo sapiens was shaped by intrasexual selection to generate force output in agonistic exchanges.

Behaviour and beliefs related to male aggression: Evidence of intrasexual selection in humans?

Sexual selection favors traits that increase mating and, thus, reproductive success. Some scholars have suggested that intrasexual selection driven by contest competition has shaped human male aggression. If this is the case, one testable hypothesis is that beliefs and behavior related to male aggression should be more prevalent in societies where the intensity and strength of sexual selection is higher, as measured by factors such as the presence and scope of polygyny, the number of same-sex competitors relative to potential mates, and the amount of effort males have available to allocate to mating. Using mixed-effect linear regression models with data from 78 societies from the Standard Cross-Cultural Sample, we found strong support for this hypothesis. We were able to rule out some potential alternative explanations by controlling for confounding variables such as political complexity, warfare and geographic clustering.