Intergroup Aggression in Chimpanzees and War in Nomadic Hunter-Gatherers

Chimpanzee and hunter-gatherer intergroup aggression differ in important ways, including humans having the ability to form peaceful relationships and alliances among groups. This paper nevertheless evaluates the hypothesis that intergroup aggression evolved according to the same functional principles in the two species—selection favoring a tendency to kill members of neighboring groups when killing could be carried out safely. According to this idea chimpanzees and humans are equally risk-averse when fighting. When self-sacrificial war practices are found in humans, therefore, they result from cultural systems of reward, punishment, and coercion rather than evolved adaptations to greater risk-taking. To test this “chimpanzee model,” we review intergroup fighting in chimpanzees and nomadic hunter-gatherers living with other nomadic hunter-gatherers as neighbors. Whether humans have evolved specific psychological adaptations for war is unknown, but current evidence suggests that the chimpanzee model is an appropriate starting point for analyzing the biological and cultural evolution of warfare.     

Lateralization of Aggression during Reproduction in Male Siamese Fighting Fish

A preference for the left‐eye use during aggressive interactions has been widely reported in the literature, even though in some cases the direction of lateralization varies among individuals within populations. Laterality of aggression in male Siamese fighting fish has been described in a number of studies, yet very little is known about lateralization of aggression during reproduction and/or parental care in fish. Here, we aimed to investigate the relationship between the different reproductive phases and lateralization in eye use during aggressive interactions in males of Siamese fighting fish. Lateralization in eye use is influenced during the early reproductive state, before and after the bubble nest construction stages. We found that nest‐holding males preferentially used the right eye before and after bubble nest construction independent of the sex of the intruder. During the later reproductive phases, aggressiveness increased whereby the direction of lateralization rather than the degree was influenced supporting the hypothesis that reproductive state influences behavioral consistency in Siamese fighting fish.     

The Problem of Adult Play Fighting: A Comparative Analysis of Play and Courtship in Primates

Although play fighting, like play generally, is predominantly a feature of the juvenile phase, such behavior persists in the adults of many species. There are two major contexts in which adults engage in play fighting – with juveniles and with other adults. The least attention has been given to adult–adult play. However, one pattern that has been noted by several authors is that the most commonly occurring context of adult–adult play fighting is during courtship, and that this is more likely to occur in solitary species. Supposedly, such play could function to overcome the aggressiveness of potential pairmates unfamiliar with one another, or as a means of evaluating mate quality by one or both partners. By contrasting the presence and degree of play fighting during courtship with the degree of male–female familiarity, the hypothesis that the former is influenced by the latter is tested. Data on 35 species of primates, from 15 families, were compiled from the literature and compared using a method of independent contrasts that incorporates information on phylogenetic relationships. A significant regression was found, with the degree of male–female familiarity accounting for 40% of the variance in courtship play. Therefore, our data support the hypothesis that play fighting in courtship is influenced by male–female patterns of association. However, the data also indicate that other factors must influence the occurrence of play fighting amongst adults, not only during courtship, but also in nonsexual contexts. The broader context of adult–adult play in mammals is discussed.     

Peering into the Dynamics of Social Interactions: Measuring Play Fighting in Rats

Play fighting in the rat involves attack and defense of the nape of the neck, which if contacted, is gently nuzzled with the snout. Because the movements of one animal are countered by the actions of its partner, play fighting is a complex, dynamic interaction. This dynamic complexity raises methodological problems about what to score for experimental studies. We present a scoring schema that is sensitive to the correlated nature of the actions performed. The frequency of play fighting can be measured by counting the number of playful nape attacks occurring per unit time. However, playful defense, as it can only occur in response to attack, is necessarily a contingent measure that is best measured as a percentage (#attacks defended/total # attacks X 100%). How a particular attack is defended against can involve one of several tactics, and these are contingent on defense having taken place; consequently, the type of defense is also best expressed contingently as a percentage. Two experiments illustrate how these measurements can be used to detect the effect of brain damage on play fighting even when there is no effect on overall playfulness. That is, the schema presented here is designed to detect and evaluate changes in the content of play following an experimental treatment.     

The Evolution of Reliable and Unreliable Badges of Fighting Ability

SYNOPSIS. When a population may be characterized by interferencecompetition for resources, variation in fighting ability amongindividuals, and repeated confrontations between individuals,together with difficulty of individual recognition, badges ofstatus should invade as recognition marks that render good fightersmemorable. Reliability of such badges can be maintained by negativefrequency-dependent selection when individuals of differentappearance (and status) either play mutually beneficial rolesor employ alternate competitive tactics. In territorial socialsystems intraspecific mimicry of recognition badges should evolvebecause, in contrast to group-living situations, the cost toa cheat of being discovered is low when individuals are dispersed.The general result of such mimicry is that good and poor fightersbecome similar in appearance. From the theoretical treatmentof status recognition badges I derive a number of predictionsthat pertain both to interand intraspecific differences in conspicuouscoloration and to the evolution of local song dialects in birds.     

The Payoff of Fighting in House Crickets: Motivational Asymmetry Increases Male Aggression and Mating Success

Motivational asymmetry caused by differences in subjective resource value is a key component of strategic models of aggression. We study the role of motivational asymmetry in determining differential aggression and mating success of male house crickets, Acheta domesticus. We also assess the extent to which mating differences associated with motivational asymmetry are due to direct male–male fighting vs. male–female interactions. We manipulated male motivation to compete for a mating opportunity by providing males with either no access or nightly access to females for 4 d prior to the experiment. As predicted, when males from each treatment had to compete for the female, those with lower prior access were more aggressive and mated more often. In contrast, when males from each treatment were paired individually with females, there was no significant difference in the frequencies with which they pursued, courted or mated with females. We also found no evidence for female choice based on motivational asymmetry; the rate of successful courtship did not differ between treatments. We conclude that prior mate encounter rate can generate motivational asymmetry, leading to differential mating success mediated by direct male–male aggression.     

Intelligence, Coalitional Killing, and the Antecedents of War

Advances in primatological research have recently led to a hypothesis that lethal coalitionary raiding in chimpanzees is the product of an evolutionarily adaptive "dominance drive" that disposes adult males to seek out low-cost opportunities for conspecific killing. This conclusion has been extended into a claim that human warfare and other forms of coalitional killing are outcomes of a hardwired, "demonic male" complex. Reversing this evidential approach, I argue from data on conspecific killing in humans that humans and chimpanzees have an aversion to killing conspecifics. Their lethal violence, I propose, is more parsimoniously explained as the result of a developed intelligence capable of envisioning the future and, when necessary, of disabling this aversion to achieve desired goals.     

Fine‐ and Broad‐Scale Approaches to Understanding the Evolution of Aggression in Crickets

Male field crickets frequently engage in agonistic contests to establish dominance in social interactions and gain access to mate attraction territories. Crickets (Orthoptera: Gryllidae) are often used as a model taxon to study aggression, but limited documentation of aggression in some cricket species hinders our understanding of its evolutionary costs and benefits. Our study investigated cricket aggression at two scales: the within‐species scale for two cricket species, Gryllus assimilis and G. veletis, whose aggression had not been adequately documented and the among‐species scale to detect evolutionary patterns in species’ levels of aggression. In both G. veletis and G. assimilis, winners spent more time being aggressive than losers, but they were not larger or heavier. Collectively, our results reveal that G. veletis males are more aggressive than G. assimilis. Male G. veletis had higher aggression scores that male G. assimilis. The majority of G. veletis contests escalated to grappling (a highly aggressive behavior), while less than one quarter of G. assimilis contests escalated to grappling. Further, G. veletis males transitioned between two of the most aggressive behaviors most often while G. assimilis transitioned between two of the least aggressive behaviors most often. We integrate this new information on aggression for G. assimilis and G. veletis with previously documented aggression data for many cricket species to investigate aggression in a broader evolutionary context than previously possible. Within a phylogenetic context, we test the hypothesis that species whose males use burrows from which to call and attract females are more aggressive than species with non‐burrowing males. We found evidence consistent with this hypothesis; species with burrowing males tended to be more aggressive than species with non‐burrowing males. Together, our study provides fine‐scale understanding of aggression in two cricket species and broad‐scale evolutionary context for aggression across cricket species.     

Intergroup Conflict and Rational Decision Making

The literature has been relatively silent about post-conflict processes. However, understanding the way humans deal with post-conflict situations is a challenge in our societies. With this in mind, we focus the present study on the rationality of cooperative decision making after an intergroup conflict, i.e., the extent to which groups take advantage of post-conflict situations to obtain benefits from collaborating with the other group involved in the conflict. Based on dual-process theories of thinking and affect heuristic, we propose that intergroup conflict hinders the rationality of cooperative decision making. We also hypothesize that this rationality improves when groups are involved in an in-group deliberative discussion. Results of a laboratory experiment support the idea that intergroup conflict –associated with indicators of the activation of negative feelings (negative affect state and heart rate)– has a negative effect on the aforementioned rationality over time and on both group and individual decision making. Although intergroup conflict leads to sub-optimal decision making, rationality improves when groups and individuals subjected to intergroup conflict make decisions after an in-group deliberative discussion. Additionally, the increased rationality of the group decision making after the deliberative discussion is transferred to subsequent individual decision making.     

Trophallaxis and Aggression in the Ponerine Ant,Ponera coarctata: Implications for the Evolution of Liquid Food Exchange in the Hymenoptera

Trophallaxis (i.e. the exchange of alimentary liquid among colony members) plays a major role in the societies of many social Hymenoptera. Food is often not equally distributed among nestmates but instead is directed towards more dominant individuals by means of trophallaxis. Antagonistic behaviour can be associated with the exchange of food, or aggression may trigger the offering of food, even where social food exchange does not normally occur. In orphaned colonies of the ponerine ant, Ponera coarctata, workers interact aggressively at a high frequency. They establish hierarchies soon after the removal of the reproductive queen. One of the consequences of aggression among workers is trophallaxis, which also occurs regularly, although less frequently, in queenright colonies. The connection of trophallaxis and aggression in Ponera coarctata and in many other species of the Hymenoptera is discussed. This study and various other examples show that, besides the nutritional function of trophallaxis, the offering of food may often serve as an appeasement behaviour during aggressive interactions. We speculate that appeasing food offers may have provided the basis for the further evolution and elaboration of trophallaxis in many social Hymenoptera.