Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability Jeremy S. Morris1,*, Jenna Link2, James C. Martin2 and David R. Carrier3

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. KEY WORDS: Aggression, Human evolution, Fist, Hominidae, Male–male competition, Muscle performance, Primates INTRODUCTION In many species, male mating opportunity is determined by fighting ability and performance in male–male contests (Andersson, 1994; Darwin, 1871). This has led to the evolution of male-biased sexual dimorphism in traits that improve fighting performance, such as body mass, weapon size (e.g. canine teeth), and musculoskeletal traits that increase the ability to manipulate or injure opponents (e.g. Clutton- Brock, 1985; Crook, 1972; Morris and Brandt, 2014; Morris and Carrier, 2016; Morris et al., 2019; Plavcan, 2001; Plavcan and van Schaik, 1992, 1997). In many groups of mammals and primates, the degree of dimorphism in fighting-related traits is a general indicator of the intensity of sexual selection on male fighting performance (Clutton-Brock et al., 1977; Mitani et al., 1996; Plavcan, 1999, 2004; Plavcan and Van Schaik, 1997; Puts, 2010, 2016). The pervasiveness of aggression and violence among male primates has led to the suggestion that these behaviors are general 1Department of Biology, Wofford College, 429 N Church Street, Spartanburg, SC 29303, USA. 2Department of Nutrition and Integrative Physiology, University of Utah, 250 S 1850 E, Salt Lake City, UT 84112, USA. 3School of Biological Sciences, University of Utah, 257 S 1400 E, Salt Lake City, UT 84112, USA. *Author for correspondence (morrisjs@wofford.edu) J.S.M., 0000-0002-8647-4420; D.R.C., 0000-0002-9905-3131 Received 14 August 2019; Accepted 18 December 2019 characteristics of the primate order (Talebi et al., 2009; Wrangham and Peterson, 1996). Within this group, the great apes are also characterized by intense male–male competition (Carrier, 2007; Puts, 2016; Wrangham and Peterson, 1996) and pronounced male- biased sexual dimorphism in traits that improve fighting performance. Fighting among male chimpanzees, which can lead to severe injury and death, involves the use of the forelimbs to grapple, strike and slam an opponent to the ground (Goodall, 1986). Selection on male fighting performance in chimpanzees may be associated with the evolution of sexual dimorphism, with males being 27% larger (Smith and Jungers, 1997) and having broader forearm bones as compared with females (Morris et al., 2019). In gorillas, a more extreme example, fights between males occur in 50% of intergroup encounters (Harcourt, 1978). Male gorilla fighting involves using the forelimbs to strike with the hands and push and pin opponents to the ground (Rosenbaum et al., 2016). Selection on male fighting performance in gorillas may be associated with greater body mass (males are 100% larger than females, on average; Smith and Jungers, 1997), greater forelimb mass (Zihlman and McFarland, 2000) and a larger anatomical mechanical advantage associated with elbow extension, which increases force output https://cob.silverchair-cdn.com/cob/content_public/journal/jeb/223/2/10.1242_jeb.212365/4/jeb212365.pdf?Expires=1656947576&Signature=2IWb0mzVsXqpsW0wpPG5elo5B0Ouu2snoI~ukrm0GJkPuXNu0ynTk6YMXfMGx9PlZCTr7gNasJcvLttH3n9j9WXA-te8Px1RjTRv-4FQxV9xzQ2jv2lcrmsyekjYSRsnzBC7MeZY9yVH6yB2--H9UCU4bp~QUKNAEvrrZvXT32uSXp1-X8g~fSoriM9LSkeRWzlZBTWm54i6BBIgPoBa0bhWzazohrHi1cmOmXlaeeueeJ-2wUPZmpVb62qy8epjgAyW1jNSNqYR5svWTVhYACBa-rDblfgZxB4waYI7sAnYOtTqNdmsGm9tZ8UrXrFw6jmQs1b8qCrriM4-tTOKcw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA