Is arm length a sexually selected trait in humans? Evidence from mixed martial arts.

Abstract

Growing evidence suggests that human males have been sexually selected for violent contest competition. I propose the hypothesis that increased arm length is an intrasexually selected adaptation for fighting in males. Longer arms may have provided several advantages to our male ancestors during conflict. However previous research on the effects of arm span on fighting success have shown mixed results and may not have fully accounted for allometric scaling of arm span with size. In a sample of 1,660 modern mixed martial arts fighters, I find that arm span is sexually dimorphic and associated with fighting success, even when controlling for body size. However, effects of arm span on fighting success were very small, suggesting that selection may have been weak. I review evidence for alternative explanations for men’s longer arm span and propose future directions to further test this hypothesis.

Facial width‐to‐height ratio predicts fighting success: A direct replication and extension of Zilioli et al. (2014)

“…This association between fWHR and overall fighting success only held when we used Zilioli et al's (2014) original methodology (overall fighter data) and did not conceptually replicate when using fight‐specific data. This is consistent with previous critiques of using individual fight data, arguing that singular fights do not capture fighters' overall ability to succeed because singular fights can be suddenly and unexpectedly determined (Richardson, 2020). This supports the majority of research on human contest competition, which has elected to use overall fighter data (e.g., Aung et al, 2021; Richardson & Gilman, 2019; Richardson, 2020; Třebický et al, 2013, 2015, 2019).…”

“…This is consistent with previous critiques of using individual fight data, arguing that singular fights do not capture fighters' overall ability to succeed because singular fights can be suddenly and unexpectedly determined (Richardson, 2020). This supports the majority of research on human contest competition, which has elected to use overall fighter data (e.g., Aung et al, 2021; Richardson & Gilman, 2019; Richardson, 2020; Třebický et al, 2013, 2015, 2019). While individual fights would also be included in a fighter's win percentage, win percentage might: (1) better discriminate among fighters; and (2) more comprehensively capture fighters' overall RHP.…”

“…However, an additional limitation within Zilioli et al (2014) was the use of fighter rather than fight data, which could be argued to violate independence of observations. It should be noted that fight data has been criticised (e.g., Richardson, 2020), as the winner of a single fight can be suddenly and unexpectedly determined which might make dichotomous fight outcome measures (i.e., win/lose) less than preferable. Indeed, much of human contest competition research has used fighter data (Aung et al, 2021;Richardson & Gilman, 2019;Richardson, 2020;Třebický et al, 2013Třebický et al, , 2015Třebický et al, , 2019.…”

“…In the current study, then, we sought to reexamine the hypothesis that facial structure is associated with fighting success (Zilioli et al, 2014). Hereon, we use the terms fighting success, RHP, and win percentage (i.e., total wins divided by total fights) interchangeably because win percentage is the most often used measure of fighting success and RHP throughout the human contest competition literature (e.g., Richardson & Gilman, 2019;Richardson, 2020;Třebický et al, 2013Třebický et al, , 2019Zilioli et al, 2014).…”

“…It should be noted that fight data has been criticised (e.g., Richardson, 2020), as the winner of a single fight can be suddenly and unexpectedly determined which might make dichotomous fight outcome measures (i.e., win/lose) less than preferable. Indeed, much of human contest competition research has used fighter data (Aung et al, 2021; Richardson & Gilman, 2019; Richardson, 2020; Třebický et al, 2013, 2015, 2019). In Study 3, we sought to expand Zilioli et al (2014) by examining the associations between fWHR and contest data among individual fighters.…”