Human third-party observers accurately track fighting skill and vigour along their unique paths to victory

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 (N = 2,765), 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.

Cues of upper body strength account for most of the variance in men's bodily attractiveness

Evolution equips sexually reproducing species with mate choice mechanisms that function to evaluate the reproductive consequences of mating with different individuals. Indeed, evolutionary psychologists have shown that women's mate choice mechanisms track many cues of men's genetic quality and ability to invest resources in the woman and her offspring. One variable that predicted both a man's genetic quality and his ability to invest is the man's formidability (i.e. fighting ability or resource holding power/potential). Modern women, therefore, should have mate choice mechanisms that respond to ancestral cues of a man's fighting ability. One crucial component of a man's ability to fight is his upper body strength. Here, we test how important physical strength is to men's bodily attractiveness. Three sets of photographs of men's bodies were shown to raters who estimated either their physical strength or their attractiveness. Estimates of physical strength determined over 70% of men's bodily attractiveness. Additional analyses showed that tallness and leanness were also favoured, and, along with estimates of physical strength, accounted for 80% of men's bodily attractiveness. Contrary to popular theories of men's physical attractiveness, there was no evidence of a nonlinear effect; the strongest men were the most attractive in all samples.

Aggression towards unfamiliar intruders by male lizards Eurolophosaurus nanuzae depends on contestant’s body traits: a test of the dear enemy effect

A reduction of territory owners’ aggression towards their neighbours in relation to the intrusion of strangers characterises the dear enemy phenomenon. Supposedly, the disparity in aggression levels of territory owners is due to a higher threat imposed by strangers compared to the threat imposed by neighbours. To evaluate the occurrence of the phenomenon in males of the small-sized lizard Eurolophosaurus nanuzae we performed a field manipulative study. We considered three models to run intrusions in males’ territories: neighbour, tailed stranger (unfamiliar) and tailless stranger intruders. Our results lend support to the presence of dear enemy for this species as residents acted more aggressively towards strangers than to neighbours. In addition, the information we provide supports the relative threat hypothesis as territory owners were more aggressive towards tailed stranger intruders than to tailless stranger intruders. In this sense, tail condition can represent a trait that signals the ‘resource holding power’ (RHP) of a lizard. Therefore, we show that beyond neighbourhood recognition, residents are able to evaluate the potential threat of stranger intruders in general, thereby extending the evolutionary gains of the dear enemy by saving energy even in the context they are expected to acts with higher costs.

Extrinsic effects, estimating opponents' RHP, and the structure of dominance hierarchies

We examined the impact of winner and loser effects on dominance hierarchy formation when individuals are capable of estimating their opponent's resource holding power (RHP). The accuracy of such estimates was a variable in our simulations, and we considered cases in which all individuals err within the same bounds, as well as cases in which some individuals consistently overestimate, while others consistently underestimate their opponent's fighting RHP. In all cases, we found a clearly defined linear hierarchy. In most simulations, the vast majority of interactions were ‘attack–retreats’, and the remainder of interactions were almost all ‘fights’. Error rates had no effect on the linearity of the hierarchy or the basic attack–retreat nature of interactions, and consistent over and underestimation did not affect the ultimate position of an individual in a hierarchy.

Food fights in house crickets, Acheta domesticus, and the effects of body size and hunger level

Animals often compete directly with conspecifics for food resources, and fighting success can be positively related to relative resource-holding power (RHP) and relative resource value (i.e., motivation to fight). Despite the ease of manipulating resource value during fights over food (by manipulating hunger levels), most studies have focused on male fighting in relation to gaining access to mates. In this study, pairwise contests over single food items were used to examine the effects of being the first to acquire a resource, relative body mass, relative body size (femur length), and relative level of food deprivation (i.e., hunger) on competitive feeding ability in male and female house crickets, Acheta domesticus. Only when the food pellet was movable did acquiring the resource first improve fighting success. When the pellet was fastened to the test arena, increased relative hunger level and high relative body mass both increased the likelihood of a takeover. However, the effects of body mass disappeared when scaled to body size. When the attacker and defender were equally hungry, larger relative body size increased takeover success but, when the attacker was either more or less hungry, body size had little effect on the likelihood of a takeover. Thus fight outcomes were dependent on an interaction between RHP and motivational asymmetries and on whether the resource was movable or stationary. Contest duration was not related to the magnitude of morphological differences between opponents, suggesting that assessment of fighting ability may be brief or nonexistent during time-limited animal contests over food items.