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.

Intrasexual and intersexual selection for neck musculature in men

Over 150 years ago, Darwin (1871) argued that humans evolved to inflict and resist damage in violent combat. Until now, however, no research has examined the claim that humans have evolved anatomical structures to both better inflict and resist damage in real-world agonistic exchanges. Human neck musculature has long been theoretically implicated in force output and concussion resistance capacities, and we proposed that human neck musculature evolved to increase damage resistance (knockout resistance) and damage infliction (knockout power) in real-world violent combat. Larger neck musculature was indeed associated with greater fighting success, knockout resistance, and knockout power capabilities across 715 professional combatants, after comprehensively controlling for weight, height, age, fighter's debut date, sex, reach, leg reach, and facial structure (Study 1). We then discovered that human neck musculature is one of the most sexually dimorphic human anatomical features, when compared against over 70 other human anatomical features (Study 2; N = 6000). We then found that human psychological systems have evolved to generate social perceptions in response to neck musculature. Men with greater physiological neck strength are perceived as stronger fighters (Study 3, after controlling for weight, height, age, beardedness, and torso, arm, bicep, and lower body strength). Study 4 specifically showed that men with larger sternocleidomastoid and upper trapezius muscles -- muscles implicated in damage resistance and infliction, respectively -- are perceived as more dominant (i.e., strong, masculine, anger-prone, and aggressive) and attractive (i.e., short and long-term attractiveness). Our results comprehensively demonstrate that human neck musculature evolved to increase resource-holding potential in violent combat, for which human psychological systems have consequently evolved an attraction. Given that human neck musculature is novel to the psychological and biological sciences, implications are discussed for the research areas of formidability assessment, contest competition, violence, face perception, emotion recognition, attraction and relationship research, political and organisational psychology, sports performance, and biomedical research.

Roads and Road-Posts as an Ecological Trap for Cavity Nesting Desert Birds

While road-side productivity attracts wildlife, roads are also a major cause of mortality. Thus, roads are potentially an attractive sink. We investigated whether roads in a desert environment in southern Israel act as an ecological trap for the territorial mourning wheatear (Oenanthe lugens). We applied an individual-based mechanistic approach to compare the apparent survival of individually-marked wheatears between roadside territories and territories in natural habitats farther away from the road, and determined directionality in territorial shifting to and from the road. Analysis was based on mark-resight techniques and multi-model inference in a multi-strata approach (program MARK). Wheatear survival in road-side territories was too low to be compensated by the maximum possible recruitment, but shifted territories from natural habitat toward the roadside habitat as these territories were vacated by mortality. Vacated territories along the road were re-occupied faster than vacated territories in natural habitat. Thus, the roadside habitat in our study area fulfilled all conditions for an ecological trap. Roads may act as widespread ecological traps and their impact, therefore, may extend well-beyond the existing perception of narrow dissecting elements causing local mortality and/or animal avoidance. In species where habitat selection is based on contest competition (e.g., territorial species) and contest success has a genetically heritable component, ecological traps will induce a paradoxical selection process.

Feeding competition inferred from patch depletion in a supergroup of Rwenzori black-and-white colobus monkeys (Colobus angolensis ruwenzorii) in Rwanda

Competition for food is often a cost associated with living in a group, and can occur in an indirect (scramble) or direct (contest) form. We investigated feeding competition in a supergroup of Rwenzori black-and-white colobus monkeys (Colobus angolensis ruwenzorii) in Rwanda, with the aim of establishing whether freedom from scramble competition allows these monkeys to form supergroups. We used the patch depletion method, measuring intake rate coupled with movement rate, to assess if food patches become depleted over the occupancy period. Resource depletion was evident when the colobus fed on young leaves, but not when feeding on mature leaves. Scramble competition was inferred from a negative correlation between group size and change in intake rate over patch occupancy. Between-group contest competition was inferred from displacement from patches. Although feeding competition exists for select resources, limited competition for mature leaves may enable Rwenzori colobus to live in a supergroup of hundreds of individuals in this montane forest.

How socio-ecological factors influence the differentiation of social relationships: an integrated conceptual framework

The extent of differentiation of social relationships within groups is a means to assess social complexity, with greater differentiation indicating greater social complexity. Socio-ecological factors are likely to influence social complexity, but no attempt has been made to explain the differentiation of social relationships using multiple socio-ecological factors. Here, we propose a conceptual framework based on four components underlying multiple socio-ecological factors that influence the differentiation of social relationships: the extent of within-group contest competition to access resources, the extent to which individuals differ in their ability to provide a variety of services, the need for group-level cooperation and the constraints on social interactions. We use the framework to make predictions about the degree of relationship differentiation that can be expected within a group according to the cumulative contribution of multiple socio-ecological factors to each of the four components. The framework has broad applicability, since the four components are likely to be relevant to a wide range of animal taxa and to additional socio-ecological factors not explicitly dealt with here. Hence, the framework can be used as the basis for the development of novel and testable hypotheses about intra- and interspecific differences in relationship differentiation and social complexity.

On the evolution of baboon greeting rituals

To balance the trade-offs of male co-residence, males living in multi-male groups may exchange ritualized greetings. Although these non-aggressive signals are widespread in the animal kingdom, the repertoire described in the genus Papio is exceptional, involving potentially harmful behaviours such as genital fondling. Such greetings are among the most striking male baboon social interactions, yet their function remains disputed. Drawing on the comprehensive analysis from our own research on wild Guinea baboons, combined with a survey of the literature into other baboon species, we review the form and function of male–male ritualized greetings and their relation to the various social systems present in this genus. These ritualized signals differ between species in their occurrence, form and function. While ritualized greetings are rare in species with the most intense contest competition, the complexity of and risk involved in greeting rituals increase with the degree of male–male tolerance and cooperation. The variety of societies found in this genus, combined with its role as a model for human socioecological evolution, sheds light on the evolution of ritualized behaviour in non-human primates and rituals in humans. This article is part of the theme issue ‘Ritual renaissance: new insights into the most human of behaviours’.