Abstract
Sexual selection via male-male contest competition has shaped the evolution of agonistic displays, weaponry, and fighting styles, and is further argued to have shaped human psychological mechanisms to detect, process, and respond appropriately to cues of fighting ability. Drawing on the largest fight-specific dataset to date across the sports and biological sciences (Nā=ā2765 fights), we examined how different indicators of fighting ability in humans reflect unique paths to victory and indicate different forms of perceived and actual resource-holding power (RHP). Overall, we discovered that: (1) both striking skill and vigour, and grappling skill and vigour, individually and collectively predict RHP; (2) different RHP indicators are distinguished by a unique path to victory (e.g., striking skill is a knockout-typical strategy, whereas grappling vigour is a submission-typical strategy); and (3) third-party observers accurately track fighting skill and vigour along their unique paths to victory. Our argument that different measures of RHP are associated with unique paths to victory, and third-party observers accurately track fighting vigour and skill along their unique paths to victory, advance our understanding not only of human contest competition, but animal contest theory more broadly.
Similar content being viewed by others
Introduction
Sexual selection via male-male contest competition has shaped the evolution of agonistic displays, fighting styles, and weaponry1,2. During contests, horns, tusks, antlers and claws are employed to directly inflict harm on conspecifics2,3, while ornamental features broadcast social rank and dominance4,5. Both mechanisms are tied to male resource holding potential (RHP), wherein individuals compete for favourable territories and social rank to secure reproductive opportunities6,7. Fighting ability and social dominance may have evolved as honest signals of RHP, so that only individuals with greater underlying biological quality can bear the costs of outwardly displaying formidability to conspecifics8,9. Despite the fact that Darwin10 argued that humans evolved for āstriking or fighting with an enemyā a century and a half ago, researchers have only recently turned their attention to fighting skill and vigour in the context of human contest competition11.
Hand-to-hand combat is an ancient feature of human contest competition12,13, potentially driving adaptations that increase fighting ability and RHP. Striking an opponent with oneās fists during an agonistic exchange is universal among Europeans10,13, is a central component of Asian martial arts (e.g., Karate; Kung Fu; the Chinese martial art Xing Yi Quan means āform and intent fistā), appears in humanityās earliest written sources (e.g., Exodus 21:18; Ezekiel 6:11; Matthew 26:67), occurs among the Yanomamƶ of Brazil hunter-gatherers in Brazil and Venezuela14 and accounts for 46ā67% of fight-associated facial fractures in contemporary populations12. There is an emerging body of evidence that supports the argument that hand-to-hand combat has been a powerful selective pressure throughout our speciesā evolutionary history, shaping menās fist morphology13, arm power and force15, neck musculature16, and arm length and shoulder breadth17.
In addition to driving the evolution of morphological structures employed in contest competition, sexual selection is argued to have shaped human psychological mechanisms to detect, process, and respond appropriately to cues of fighting ability18,19. People show strong agreement across cultures which male faces look socially dominant and aggressive20 and using faces alone can accurately predict menās physical strength21 and the winners of fights22,23. Moreover, men displaying clear physical indicators of RHPāincluding overall body size, total upper body strength, shoulder breadth, bicep size, handgrip strength, and neck musculatureāare perceived as stronger16,24, more attractive16,25,26, more aggressive22,27, better fighters16,22, are more likely to be elected as leaders in politics and business28,29, and have higher mating and reproductive success30,31,32.
Sexual selection is also proposed to have favoured behavioral processes to deploy those fighting strategies and techniques that maximise the potential to inflict injury on an opponent33. These include fighting vigour, defined as the rate at which fighting manoeuvres are performed, and fighting skill, which refers to the capacity to effectively enact aggressive manoeuvres during agonistic exchanges33. A review of the contest competition literature in nonhuman animals indicates that a large swathe of research implies that fighting skill contributes to RHP (e.g., Mantis shrimp, Neogonodactylus bredini; Richardson's ground squirrels, Spermophilus richardsonii)34, with recent evidence directly showing that fighting skill contributes to RHP in hermit crabs35,36. The structural weapons that are used during fights may have been maintained by the emergence of specific fighting styles3,37, furthering the argument that sexual selection operates on behavioural processes that deploy fighting strategies and techniques during contest competition, such as fighting skill and vigour.
Although similar tests of contest competition in humans are sparse, Lane and Briffa11 reported that striking vigour contributed to RHP in mixed-martial-arts (MMA) fights (Nā=ā548) from the Ultimate Fighting ChampionshipĀ© (UFC). An interaction between striking vigour and striking skill was also reported, such that the effect of vigour on the likelihood of victory increased with skill, while there was no overall effect of striking skill on the likelihood of victory independent of vigour11.
Despite a long tradition of wrestling across cultures38 that is documented in ancient written sources (e.g., Genesis 32:22ā31; Hosea 12:3ā5) and observed in hunter-gatherer tribes38, whether grappling skill and vigour are indicators of RHP is still unknown11. Grappling involves gripping and grabbing an opponent, either leading to or during ground control, eventuating in the conspecific conceding defeat (termed submission). Attempts to submit an opponent are indeed defined as attempts to end the contest via grappling, locks, and chokes39, suggesting that grappling manoeuvres are an important contributor of submission victories. Indeed, submissions are sometimes specifically referred to as grappling submissions40, indicating the inextricable link between grappling abilities and submission victories. Recent evidence suggests that humans evolved morphological features that signal their grappling abilities, as all major facial width-to-height ratio measurementsāautomatic and manual measurements, using both eyelid and eyebrow landmarksāpredicted menās grappling behaviours in 1367 UFC fights, before and after controlling for 17 different control variables23.
Furthermore, whether fighting vigour and skill predict KO/TKO-typical, submission-typical strategies, or all-round indicators of RHP remains to be determined. For example, striking skill may be more strongly associated with KO/TKO victories whereas grappling vigour may be more strongly associated with submission victories. Evolutionarily, only one accurate but powerful strike would have been necessary to render an opponent unconscious41,42 whereas the process leading to a successful submission can be arduous, requiring continual efforts to grapple and lock the opponent40. This proposition that humans use alternative fighting tactics to their advantage is congruent with non-human animal research, in which animals (e.g., green anole lizard, Anolis carolinensis) adopt distinct fighting styles based on their morphology (e.g., greater jumping velocity and acceleration in lightweight males but greater biting force in heavyweight males)43.
Animal psychological systems have evolved to detect, process, and respond to agonistic behaviours deployed during contest competition1,10,19. This is the case for both non-human animals directly involved in the contest1,10,19 and third-party observers of these contests44,45,46. Given that human evolutionary history has been characterised by both group-based living and contest competition12,14,18,19, human third-party observers should possess psychological systems that allow them to gauge conspecificsā fighting skill and vigour11. More specifically, if human fighting abilities are associated with unique paths to victory, then we should expect that third party judgesā evaluations should accurately track the specific fighting manoeuvresā typical avenue to victory, or ābe analogous to the decision the loser would eventually make for themselves if the fight were to continueā11.
The present study
The present study uses the most comprehensive data set to date (Nā=ā2765 fights) to test how different indicators of human fighting ability reflect unique paths to victory and hence different forms of RHP. Moreover, we sought to test whether third-party observers accurately track these predictors of RHP along their unique paths to victory. First, we hypothesised that striking skill, vigour, as well as the interaction between striking vigour and skill would increase the likelihood of victory (Hypothesis 1). Next, we hypothesised that grappling skill, vigour, as well as the interaction between grappling vigour and skill would increase the likelihood of victory (Hypothesis 2).
Striking skill should share stronger associations with KO/TKO than submission victories, whereas striking vigour should be evenly associated with victories from KO/TKO and submission (Hypothesis 3). We postulated that striking vigour would be an all-round strategy to victory because conspecifics must repeatedly strike their opponent to emerge victorious by either knockout or submission. Fighters are required to repeatedly strike their opponent to render them incapacitated or unconscious, and similarly, submissions require vigorously striking an opponent to keep them in placeāto maintain the opponentās submissive position40.
We argued that striking skill acts as a KO/TKO-typical path to victory because, evolutionarily, only one accurate but powerful strike was necessary to render an opponent unconscious41,42. Striking skill would not predominate as a submission-typical strategy because most strikes in a submission-oriented position (ground or clinch) would successfully land, as combatants are within each otherās immediate proximity23. There would have therefore been minimal variance in striking skill during submission attempts throughout our speciesā evolutionary history of contest competition, and thereby reduced selective pressures on striking skill as a submission-typical strategy.
With respect to grappling techniques, we postulated that grappling skill would be equally associated with both KO/TKO and submission victories and that grappling vigour would be more strongly associated with submission than with KO/TKO victories (Hypothesis 4). We postulated that grappling skill would act as an all-round strategy to victory because only one skilfully enacted takedown can lead to a successful ground and pound where, after the opponent has been taken to the ground, a series of strikes are repeatedly landed on the opponent23. This places the opponent in a highly susceptible position, in which they can quickly be rendered unconscious or submitted23. We postulated that grappling vigour would be a submission-typical indicator of RHP because, unlike when one skilfully enacted takedown can lead to a knockout, continual takedown attempts in a short period of time could serve as an indicator of the individualās intent to win by submission because submission victories require the continual and arduous enactment of grappling manoeuvres in ground combat to outmanoeuvre the opponent40.
If third-party observers accurately track striking skill as a KO/TKO-typical indicator of RHP, then striking skill should be more important for bystander evaluations than submissions for victory but equally important to bystander evaluations as KO/TKOs for victory. Likewise, if third-party observers accurately track striking vigour as an all-round indicator of RHP, then striking vigour should be equally important for bystander evaluations as it is to KO/TKO and submission victories (Hypothesis 5). If third-party observers accurately track grappling skill as an all-round indicator of RHP, then grappling skill should be equally important for bystander evaluations as it is to KO/TKO and submission victories. If third-party observers accurately track grappling vigour as a submission-typical indicator of RHP, then grappling vigour should be more important for bystander evaluations than KO/TKOs for victory but equally important to bystander evaluations as submissions for victory (Hypothesis 6).
Results
Hypothesis 1. Striking skill and vigour increase the likelihood of victory.
There was a significant association between striking skill and the likelihood of victory (Ī²ā=ā1.19āĀ±ā0.10, X2ā=ā12.43, pā<ā0.001), such that striking skill increased the likelihood of victory. There was a significant association between striking vigour and the likelihood of victory (Ī²ā=ā1.03āĀ±ā0.10, X2ā=ā10.30, pā<ā0.001), such that striking vigour increased the likelihood of victory. There was also a significant positive interaction between striking skill and striking vigour on the likelihood of victory (Ī²ā=ā0.35āĀ±ā0.07, X2ā=ā4.91, pā<ā0.001), which indicates that the probability of winning increased with striking skill and this effect of skill was enhanced by striking vigour. There were no significant effects for sex or its interactions.
Hypothesis 2. Grappling skill and vigour increase the likelihood of victory.
There was a significant association between grappling skill and the likelihood of victory (Ī²ā=ā0.72āĀ±ā0.07, X2ā=ā10.66, pā<ā0.001), such that grappling skill increased the likelihood of victory. There was a significant association between grappling vigour and the likelihood of victory (Ī²ā=ā0.24āĀ±ā0.08, X2ā=ā3.02, pā=ā0.003), such that grappling vigour increased the likelihood of victory. There was a significant positive interaction between grappling vigour and skill (Ī²ā=ā0.31āĀ±ā0.07, X2ā=ā4.65, pā<ā0.001), such that the probability of winning increased with grappling skill and was enhanced by grappling vigour. There were no significant effects for sex or its interactions.
Hypothesis 3. Striking skill is a KO/TKO rather than submission strategy, whereas striking vigour is evenly associated with KO/TKO and submission.
There was a stronger association between striking skill and KO/TKO victories than with submission victories (Ī²ā=āāā0.48āĀ±ā0.14, X2ā=āāā3.54, pā<ā0.001) indicating that striking skill was a KO/TKO-typical strategy (Fig.Ā 1). Striking vigour was equally associated with both KO/TKO and submission victories (Ī²ā=āāĀ 0.31āĀ±ā0.18, X2ā=āāā1.74, pā=ā0.08) indicating that striking vigour was an all-round strategy to victory (Fig.Ā 2).
Hypothesis 4. Grappling skill is equally associated with KO/TKO and submission victories, while grappling vigour is associated with submission and not KO/TKO victories.
In line with Hypothesis 4, grappling skill was not more strongly associated with either KO/TKO or submission victories (Ī²ā=ā0.13āĀ±ā0.13, X2ā=ā1.01, pā=ā0.31) indicating that grappling skill was an all-round strategy to victory (Fig.Ā 3). Grappling vigour was more strongly associated with submission than KO/TKO victories (Ī²ā=ā0.32āĀ±ā0.13, X2ā=ā2.38, pā=ā0.02) indicating that grappling vigour was a submission-typical strategy (Fig.Ā 4).
Hypothesis 5. Third-party observers accurately track striking skill as a KO/TKO-typical indicator of RHP; striking vigour is tracked as an all-round indicator of RHP.
Striking skill was more important for bystander evaluations than for submissions for victory (Ī²ā=āāā0.61āĀ±ā0.13, X2ā=āāā4.66, pā<ā0.001) but was equally important (i.e., analogous) for bystander evaluations as KO/TKOs for victory (Ī²ā=āāā0.11āĀ±ā0.13, X2ā=āāĀ 0.82, pā=ā0.41). Collectively, our results indicate that: (1) striking skill is, most broadly, an indicator of RHP; (2) striking skill is, more specifically, a KO/TKO-typical indicator of RHP; and (3) third-party observers (judgesā decisions) accurately tracked striking skill as a KO/TKO-typical, but not submission-typical, strategy (Fig.Ā 1).
Striking vigour was also equally important to KO/TKO (Ī²ā=ā0.08āĀ±ā0.15, X2ā=ā0.56, pā=ā0.58) and submission victories (Ī²ā=āāā0.23āĀ±ā0.17, X2ā=āāā1.38, pā=ā0.17) as it was to judgesā decisions. Collectively, our results indicate that: (1) striking vigour is an indicator of RHP; (2) striking vigour is all-round indicator of RHP; and (3) third-party observers (judgesā decisions) accurately tracked striking vigour as an all-round indicator of RHP (Fig.Ā 2).
Hypothesis 6. Bystanders evaluate grappling skill equally for KO/TKO and submission victories, while grappling vigour is more important for submission than KO/TKOs.
Grappling skill was more important for bystander evaluations than for KO/TKO for victory (Ī²ā=āāā0.29āĀ±ā0.11, X2ā=āāā2.67, pā=ā0.01) but was equally important (i.e., analogous) for bystander evaluations as submissions for victory (Ī²ā=āāā0.13āĀ±ā0.12, X2ā=āāā1.11, pā=ā0.27) indicating that third-party observers inaccurately perceived grappling skill as a submission-typical strategy. These results indicate that grappling skill is an all-round indicator of RHP that is not distinguished by a unique path to victory, and that third-party observers inaccurately tracked grappling skill as a submission-typical indicator of RHP (Fig.Ā 3).
Grappling vigour was more important for bystander evaluations than for KO/TKO for victory (Ī²ā=āāā0.53āĀ±ā0.12, X2ā=āāā4.39, pā<ā0.001) but was equally important for bystander evaluations as submissions for victory (Ī²ā=āāā0.14āĀ±ā0.13, X2ā=āāā1.15, pā=ā0.25). Combined, these results indicate that: (1) grappling vigour is an indicator of RHP; (2) grappling vigour is a submission-typical indicator of RHP; and (3) third-party observers (judgesā decisions) accurately tracked grappling vigour as a submission-typical indicator of RHP (Fig.Ā 4).
Discussion
Drawing on data from 2765 fightsāthe largest fight-specific sample to date in contest competition researchāthe present study supports that: (1) fighting skill in humans is an indicator of RHP that exists independently of vigour; (2) different RHP indicators are distinguished by a unique path to victory; and (3) third-party observers accurately track fighting vigour and skill along their unique paths to victory.
Striking skill, vigour, as well as their interaction, increased the likelihood of victory. This finding lends support to prior research, which showed that striking vigour and the interaction between striking vigour and skill increase the likelihood of victory in non-human33,34,35,36 and human11 contests. In the present work, we provided the first evidence that striking skill contributes to RHP independently of striking vigour. We also found that grappling skill and vigour act individually, and in concert, to predict individual differences in the pathway to victory. Performing distinct striking and grappling manoeuvres requires complex neurocognitive capacities, which suggests that fighters differ in their ability to adjust their fighting skill and efforts against their opponent. Indeed, striking skill was a KO/TKO-typical strategy whereas striking vigour was an all-round strategy to victory. Further, grappling skill was an all-round strategy to victory whereas grappling vigour was a submission-typical strategy. Our findings provide the first evidence that fighters pursue specific techniques and expend greater effort to follow specific fighting strategies along their unique paths to victory33. This is consistent with non-human animal research, in which animals (e.g., green anole lizard, Anolis carolinensis) adopt distinct fighting styles based on their individual characteristics (e.g., greater jumping velocity and acceleration in lightweight males but greater biting force in heavyweight males) 43.
Given that non-human animal research indicates that bystanders possess cognitive capacities to detect cues of RHP during agonistic exchanges44,45,46, we also expected that human third party spectators (i.e., UFC judges) would accurately assess individual performances within contests11. We report that judgesā decisions accurately tracked the association between striking skill and victories from knockouts, but not victories from submission. Judgesā decisions also accurately tracked the association between striking vigour and victories via KO/TKO and submission. Judgesā decisions further accurately tracked the positive association between grappling vigour and submission but not knockout victories, again supporting the overarching claim that judgesā rulings accurately track the eventual decision the loser would reap upon themselves if the fight were to continue11. These findings lend further support to theoretical claims that human psychological systems can forecast the avenue to victory held by specific agonistic behaviours11.
There were two methodological advantages to using judgesā decisions as a proxy for perceived RHP11. Firstly, to measure perceived RHP, previous research has only used online participants that rate the static stimuli of professional fighters on perceived fighting ability22,47. These data are limited to people from contemporary societies wherein exposure to hand-on-hand combat as a method of conflict resolution is likely lower than ancestral populations14,48. Using UFC judgesā rulings allowed us to examine whether humans who are exposed to many contests accurately track RHP11. Secondly, this approach allowed us to examine a very large sample of 2765 fights, the largest to date in contest competition research (see11,49). Given that human contests can end suddenly and unexpectedly, large samples of individual contests are necessary for the statistical power required to detect significant differences in fighting styles on fight outcomes, should they exist.
Despite the extensive experience required to officiate professional mixed martial arts competitions in the UFC, human psychological systems are naturally susceptible to errors and inaccuracies. We found that judgesā decisions did not accurately track grappling skill as an all-round but rather as a submission-typical strategy, potentially because there is a such a strong perceptionānot reflected in realityāthat all grappling behaviours are inextricably tied to submission victories40. Given the extent to which grappling vigour is positively associated with submission victories, but negatively associated with knockout victories, human psychological systems might perceive grappling skill as submission-typical strategy as a form of error-management. Lane and Briffa11 indeed found that judges overestimate the importance of vigour in contests, especially when compared to skill, suggesting that displays of grappling vigour might overpower perceptions of grappling skill. It should be noted that judgesā rulings for grappling skill as a submission-typical strategy were a very slight inaccuracy given the p-value (0.01) was small for a sample size of this magnitude. Nevertheless, we found consistent support for the previously speculated claim that ājudgesā ruling should be analogous to the decision the loser would eventually make for themselves if the fight were to continueā 11.
While there are a number of strengths to the present work, there are also several limitations. First, the UFC contains weight categories, which can limit the effect of morphological features on fighting performance23,47. Future research could examine the few championships in the world that do not have weight restrictions (e.g., Road Fighting Championship). Second, the UFC contains the most elite fighters in the world23,47. This might reduce variance in fighting skill and vigour in contests, but it is currently unknown how fighting skill and vigour function in amateur contests. While the UFC is the only organisation to collect fight-specific performance metrics, future research might wish to manually code fighting performance metrics from the contests in other championships. Third, in line with previous research, we measured perceived resource-holding power using judgesā evaluations of fighting performance11. UFC judges have extensive experiencing in evaluating contests, and it is currently unknown how naĆÆve raters track fighting skill and vigour. Future research might wish to present online participants with recorded fights, providing participants with a slider that allows them to report their evaluations of fighting performance as the fight progresses.
Our findings stimulate several future lines of enquiry into how human contest competition conform to patterns in the broader animal competition literature. Future research will benefit from empirically examining whether our findings that different indicators of fighting ability in humans reflect unique paths to victory, and hence different forms of RHP, predicts outcomes in nonhuman animal contests. For example, nonhuman animal contests result in either death, which is equivalent to knockout in human contests (11; knockouts can result in death:41,42) or submission or retreat, considered equivalent to submission in human contests11. Consequently, our argument that different measures of RHP are associated with unique paths to victory, that different RHP indicators are distinguished by a unique path to victory, and that third-party observers accurately track fighting vigour and skill along their unique paths to victory represents an exciting new line of enquiry beyond human contest competition into the many domains of animal contest theory.
Material and methods
We used a publicly available dataset of UFC fights collated from UFCstats.com50, which included data from 2765 unique fights (women: 246; men: 2519) held between March 21st, 2010, and 11th July, 2020, from 1392 unique fighters (women: 144; men: 1248). Following Lane and Briffa11, we only used data containing fight-specific data (i.e., fighterās skill and vigour for the specific fight, as opposed to the fighterās lifetime skill and vigour averaged across all fights). Compared to Lane and Briffaās11 dataset, our sample twice was the size of unique fighters and five times as many unique fights. To our knowledge, this is the largest fight-specific dataset to date across the sports and biological sciences.
Our analyses included: (1) the percent significant strikes landed (termed by the UFC as striking accuracy) as our measure of striking skill (as defined in11,33); (2) number of strikes attempted per second (calculated as the total number of strikes attempted divided by fight duration) as our measure of striking vigour (as defined in11,33); (3) focal outcome (lose, win) as our measure for the likelihood of victory; (4) KO/TKO (coded as ā1ā) versus submission (coded as ā2ā), to determine whether fighting abilities were KO/TKO or submission-typical manoeuvres; (5) method of resolution (1: decision; 2: KO/TKO; 3: submission), to examine whether third-party observersā valuations significantly differed from, or were analogous to, a particular avenue to victory; (6) sex (female, male); and (7) fighter ID. We also used fight-specific information for: (8) percent takedowns landed (termed by the UFC as grappling accuracy) as our measure of grappling skill (as defined in33); and (9) number of takedowns attempted per second (calculated as the total number of takedowns attempted divided by fight duration) as our measure of grappling vigour (as defined in33).
Our focal fighter data comprised 234,643 significant strikes attempted, 101,311 significant strikes landed, 7949 attempted takedowns, 2998 landed takedowns, which ensures statistical stability to the larger variables to which they belong (i.e., skill and vigour) and emphasises the advantages of using UFC data in contest competition research. The UFC (a billion-dollar organisation) is the only organisation to collect fight-specific performance data, and across a large sample of contests, which would ordinarily be extremely expensive to collect through other sampling methods (e.g., simple random sampling).
Lane and Briffa11 argued that a fighterās skill and vigour are likely dependent on their opponentās fighting behaviour; to address this, they used from one āfocalā individual per fight. In line with Lane and Briffa11 then, we used data from one āfocalā individual per fight with āfightā as the level of replication. Focal fighters were randomly assigned by generating random numbers for the red and blue fighters to avoid potential confounding effects of fighter colour on fighting success11. We used Excelās RANDBETWEEN function to randomly generate numbers ā1ā or ā2ā for each of the 2765 fights, with red fighters being selected as the focal fighter for ā1ā and blue fighters selected as the focal fighter for ā2ā.
We used the R code provided by Lane and Briffa11 to conduct identical generalized linear mixed effects models (GLMMs) via backwards elimination (to remove non-significant terms) for striking skill and vigour, which was then adapted for grappling skill and vigour, with outcome (win vs. lose) as our outcome variable. These GLMMs also included sex and the method of resolution, with the latter entered as a factor to examine Hypotheses 5 and 6. To examine Hypotheses 3 and 4, we ran similar GLMMs but with KO/TKO versus submission as a predictor variable on the likelihood on victory, with a significant positive interaction between fighting skill or vigour and KO/TKO versus submission indicating that the fighting manoeuvre was a submission-typical strategy, a significant negative interaction indicating a KO/TKO-typical strategy, and no significant interaction indicating an all-round strategy to victory (an interpretation predicated on the initial finding that the fighting manoeuvre is associated with the likelihood of victory; as we have demonstrated below, striking and grappling skill and vigour are all associated with the likelihood of victory). There was no strong pattern in the residuals nor any indication of overdispersion in our results. In line with Lane and Briffa11, random intercepts were included to account for the ID of both fighters (red and blue) per fight. Owing to their different scales, striking and grappling skill and vigour were Z-standardised prior to analysis. Our R code and data is available on the Open Science Framework (https://osf.io/be85y/).
Data availability
Our R code and data is publicly available on the Open Science Framework (https://osf.io/be85y/).
References
Andersson, M. Sexual Selection (Princeton University Press, 1994).
Emlen, D. J. The evolution of animal weapons. Annu. Rev. Ecol. Evol. Syst. 39, 387ā413. https://doi.org/10.1146/annurev.ecolsys.39.110707.173502 (2008).
McCullough, E. L., Miller, C. W. & Emlen, D. J. Why sexually selected weapons are not ornaments. Trends Ecol. Evol. 31, 742ā751. https://doi.org/10.1016/j.tree.2016.07.004 (2016).
Dixson, A., Dixson, B. & Anderson, M. Sexual selection and the evolution of visually conspicuous sexually dimorphic traits in male monkeys, apes, and human beings. Annu. Rev. Sex Res. 16(1), 1ā19. https://doi.org/10.1080/10532528.2005.10559826 (2005).
Grueter, C. C., Isler, K. & Dixson, B. J. Are badges of status adaptive in large complex primate groups?. Evol. Hum. Behav. 36(5), 398ā406. https://doi.org/10.1016/j.evolhumbehav.2015.03.003 (2015).
Rico-Guevara, A. & Hurme, K. J. Intrasexually selected weapons. Biol. Rev. 94(1), 60ā101. https://doi.org/10.1111/brv.12436 (2019).
Wong, B. B. & Candolin, U. How is female mate choice affected by male competition?. Biol. Rev. 80(4), 559ā571. https://doi.org/10.1017/S1464793105006809 (2005).
Berglund, A., Bisazza, A. & Pilastro, A. Armaments and ornaments: an evolutionary explanation of traits of dual utility. Biol. J. Lin. Soc. 58(4), 385ā399. https://doi.org/10.1017/S1464793105006809 (1996).
Wiens, J. J. & Tuschhoff, E. Songs versus colours versus horns: what explains the diversity of sexually selected traits?. Biol. Rev. 95(4), 847ā864. https://doi.org/10.1111/brv.12593 (2020).
Darwin, C. The Expression of the Emotions in Man and Animals (J. Murray, London, 1872).
Lane, S. M. & Briffa, M. Perceived and actual fighting ability: Determinants of success by decision, knockout or submission in human combat sports. Biol. Let. 16(10), 20200443. https://doi.org/10.1098/rsbl.2020.0443 (2020).
Carrier, D. R. & Morgan, M. H. Protective buttressing of the hominin face. Biol. Rev. 90(1), 330ā346. https://doi.org/10.1111/brv.12112 (2014).
Morgan, M. H. & Carrier, D. R. Protective buttressing of the human fist and the evolution of hominin hands. J. Exp. Biol. 216, 236ā244. https://doi.org/10.1242/jeb.075713 (2013).
Chagnon, N. A. Yanamamo: The Fierce People (Holt, Rinehart and Winston, New York, 1968).
Morris, J. S., Link, J., Martin, J. C. & Carrier, D. R. Sexual dimorphism in human arm power and force: Implications for sexual selection on fighting ability. J. Exp. Biol. 223(2), jeb212365. https://doi.org/10.1242/jeb.212365 (2020).
Caton, N. R., & Lewis, D. M. G. Intersexual and intrasexual selection for neck musculature in men: Attractiveness, dominance, and actual fighting success. https://doi.org/10.31234/osf.io/yez3t (2021, November 3)
Caton, N. R., & Lewis, D. M. G. Armed forces: Intrasexual selection for upper limb length in Homo sapiens. https://doi.org/10.31234/osf.io/fw6s9 (2021, August 31).
Archer, J. Does sexual selection explain human sex differences in aggression?. Behav. Brain Sci. 32(3ā4), 249ā266. https://doi.org/10.1017/S0140525X09990951 (2009).
Puts, D. A. Beauty and the beast: Mechanisms of sexual selection in humans. Evol. Hum. Behav. 31(3), 157ā175. https://doi.org/10.1016/j.evolhumbehav.2010.02.005 (2010).
Jones, B. C. et al. To which world regions does the valenceādominance model of social perception apply?. Nat. Hum. Behav. 5(1), 159ā169. https://doi.org/10.1038/s41562-020-01007-2 (2021).
Fink, B., Neave, N. & Seydel, H. Male facial appearance signals physical strength to women. Am. J. Hum. Biol. 19, 82ā87. https://doi.org/10.1002/ajhb.20583 (2007).
Zilioli, S. et al. Face of a fighter: Bizygomatic width as a cue of formidability. Aggress. Behav. 41(4), 322ā330. https://doi.org/10.1002/ab.21544 (2015).
Caton, N. R., Hannan, J. & Dixson, B. J. W. Facial width-to-height ratio predicts fighting success: A direct replication and extension of Zilioli et al. (2014). Aggress. Behavior. https://doi.org/10.1002/ab.22027 (in press).
Sell, A. et al. Human adaptations for the visual assessment of strength and fighting ability from the body and face. Proc. R. Soc. B Biol. Sci. 276(1656), 575ā584. https://doi.org/10.1098/rspb.2008.1177 (2009).
Caton, N. R., Lewis, D. M. G., Al-Shawaf, L. & Evans, K. C. Human intersexual courtship, in The Oxford Handbook of Evolutionary Psychology and Romantic Relationships (eds Shackelford, T. & Mogilski, J.) (Oxford University Press, in press).
Sell, A., Lukazsweski, A. W. & Townsley, M. Cues of upper body strength account for most of the variance in menās bodily attractiveness. Proc. R. Soc. B Biol. Sci. 284(1869), 20171819. https://doi.org/10.1098/rspb.2017.1819 (2017).
Geniole, S. N., Denson, T. F., Dixson, B. J., CarrƩ, J. M. & McCormick, C. M. Evidence from meta-analyses of the facial width-to-height ratio as an evolved cue of threat. PLOS ONE 10(7), e0132726. https://doi.org/10.1371/journal.pone.0132726 (2015).
Todorov, A., Mandisodza, A. N., Goren, A. & Hall, C. C. Inferences of competence from faces predict election outcomes. Science 308(5728), 1623ā1626. https://doi.org/10.1126/science.1110589 (2005).
Wang, D., Nair, K., Kouchaki, M., Zajac, E. J. & Zhao, X. A case of evolutionary mismatch? Why facial width-to-height ratio may not predict behavioral tendencies. Psychol. Sci. 30(7), 1074ā1081. https://doi.org/10.1177/0956797619849928 (2019).
Hill, A. K. et al. Quantifying the strength and form of sexual selection on menās traits. Evol. Hum. Behav. 34(5), 334ā341. https://doi.org/10.1016/j.evolhumbehav.2013.05.004 (2013).
Kordsmeyer, T. L., Hunt, J., Puts, D. A., Ostner, J. & Penke, L. The relative importance of intra-and intersexual selection on human male sexually dimorphic traits. Evol. Hum. Behav. 39(4), 424ā436. https://doi.org/10.1016/j.evolhumbehav.2018.03.008 (2018).
Loehr, J. & OāHara, R. B. Facial morphology predicts male fitness and rank but not survival in Second World War Finnish soldiers. Biol. Lett. 9(4), 20130049. https://doi.org/10.1098/rsbl.2013.0049 (2013).
Briffa, M. & Lane, S. M. The role of skill in animal contests: A neglected component of fighting ability. Proc. R. Soc. B Biol. Sci. 284(1863), 20171596. https://doi.org/10.1098/rspb.2017.1596 (2017).
Lane, S. M. & Briffa, M. Skilful mating? Insights from animal contest research. Anim. Behav. https://doi.org/10.1016/j.anbehav.2021.03.006 (2021).
Briffa, M. & Fortescue, K. J. Motor pattern during fights in the hermit crab Pagurus bernhardus: Evidence for the role of skill in animal contests. Anim. Behav. 128, 13ā20. https://doi.org/10.1016/j.anbehav.2017.03.031 (2017).
Lane, S. M. & Briffa, M. The role of spatial accuracy and precision in hermit crab contests. Anim. Behav. 167, 111ā118. https://doi.org/10.1016/j.anbehav.2020.07.013 (2020).
McCullough, E. L., Tobalske, B. W. & Emlen, D. J. Structural adaptations to diverse fighting styles in sexually selected weapons. Proc. Natl. Acad. Sci. 111, 14484ā14488. https://doi.org/10.1073/pnas.1409585111 (2014).
Moreno, E. The society of our āout of Africaā ancestors (I) the migrant warriors that colonized the world. Commun. Integrat. Biol. 4(2), 163ā170. https://doi.org/10.4161/cib.4.2.14320 (2011).
Kirk, C. Does anthropometry influence technical factors in competitive mixed martial arts?. Hum. Mov. 19(2), 46ā59. https://doi.org/10.5114/hm.2018.74059 (2018).
Stellpflug, S. J., Menton, W. H. & LeFevere, R. C. Analysis of the fight-ending chokes in the history of the Ultimate Fighting Championshipā¢ mixed martial arts promotion. Phys. Sportsmed. 50(1), 60ā63. https://doi.org/10.1080/00913847.2020.1866958 (2022).
HĆ„nell, A. & Rostami, E. How can a punch knock you out?. Front. Neurol. 11, 570566. https://doi.org/10.3389/fneur.2020.570566 (2020).
Flynn, A., Halsey, M. & Lee, M. Emblematic violence and aetiological cul-de-sacs: On the discourse of āone-punchā(non) fatalities. Br. J. Criminol. 56(1), 179ā195. https://doi.org/10.1093/bjc/azv039 (2016).
Lailvaux, S. P., Herrel, A., VanHooydonck, B., Meyers, J. J. & Irschick, D. J. Performance capacity, fighting tactics and the evolution of lifeāstage male morphs in the green anole lizard (Anolis carolinensis). Proc. R. Soc. Lond. Ser. B Biol. Sci. 271(1556), 2501ā2508. https://doi.org/10.1098/rspb.2004.2891 (2004).
Aquiloni, L., BuÅiÄ, M. & Gherardi, F. Crayfish females eavesdrop on fighting males before choosing the dominant mate. Curr. Biol. 18(11), R462āR463. https://doi.org/10.1016/j.cub.2008.04.006 (2008).
Oliveira, R. F., McGregor, P. K. & Latruffe, C. Know thine enemy: fighting fish gather information from observing conspecific interactions. Proc. R. Soc. Lond. Ser. B Biol. Sci. 265(1401), 1045ā1049. https://doi.org/10.1098/rspb.1998.0397 (1998).
Earley, R. L. & Dugatkin, L. A. Eavesdropping on visual cues in green swordtail (Xiphophorus helleri) fights: a case for networking. Proc. R. Soc. Lond. Ser. B Biol. Sci. 269(149), 943ā952. https://doi.org/10.1098/rspb.2002.1973 (2002).
Caton, N. R., Pearson, S. G. & Dixson, B. J. W. Is facial structure an honest cue to real-world dominance and fighting ability in men? A pre-registered direct replication of TÅebickĆ½ et al. (2013). Evol. Hum. Behav. 43(4), 311ā324. https://doi.org/10.1016/j.evolhumbehav.2022.04.002 (2021).
Pinker, S. The Better Angels of Our Nature: The Decline of Violence in History and Its Causes. Penguin: UK (2011).
Dixson, B. J., Sherlock, J. M., Cornwell, W. K. & Kasumovic, M. M. Contest competition and menās facial hair: Beards may not provide advantages in combat. Evol. Hum. Behav. 39, 147ā153. https://doi.org/10.1016/j.evolhumbehav.2017.11.004 (2018).
Dabbert, M. Ultimate UFC dataset. Retrieved from https://www.kaggle.com/mdabbert/ultimate-ufc-dataset (2021).
Author information
Authors and Affiliations
Contributions
N.R.C. designed the study, and analysed and interpreted the data. N.R.C. drafted the manuscript and B.J.W.D. added critical revisions.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Additional information
Publisher's note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Caton, N.R., Dixson, B.J.W. Human third-party observers accurately track fighting skill and vigour along their unique paths to victory. Sci Rep 12, 14841 (2022). https://doi.org/10.1038/s41598-022-19044-4
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41598-022-19044-4
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.