Pain as a Cause of Aggression

Aggressive behavior can be elicited by aversive stimuli as anunconditioned reflex. Electric shock, heat, physical blows,and intra-cranial stimulation are among the stimuli which willelicit fighting. Various characteristics of the stimulus—suchas frequency of presentation, intensity, and duration, in thecase of shock—and of the environment in general, influencethe rate at which pain-elicited fighting occurs. Characteristicsof the animals—such as sex, age, sensory impairment, species,and social experience— also influence the occurrence ofrespondent aggression. Aggression can be conditioned according to both the operantand the respondent paradigms. Since both operant and respondentaggression can occur concurrently, interesting interactionsresult. Depending upon a number of variables, respondent aggressionexerts a disruptive influence on social behavior controlledby aversive stimuli. The interference of aggression with behaviormaintained by aversive stimuli may cause us to revise our opinionof the efficiency of aversive stimuli in social situations. Although pain is probably a source of human aggression, furtherresearch with infra-human animals, as well as actual researchwith humans, should elucidate the relationship between aversivestimuli and aggression in man.     

Evolutionary Perspectives on Human Aggression

The papers in this volume present varying approaches to human aggression, each from an evolutionary perspective. The evolutionary studies of aggression collected here all pursue aspects of patterns of response to environmental circumstances and consider explicitly how those circumstances shape the costs and benefits of behaving aggressively. All the authors understand various aspects of aggression as evolved adaptations but none believe that this implies we are doomed to continued violence, but rather that variation in aggression has evolutionary roots. These papers reveal several similarities between human and nonhuman aggression, including our response to physical strength as an indicator of fighting ability, testosterone response to competition, a sensitivity to paternity, and baseline features of intergroup aggression in foragers and chimps. There is also one paper tackling the phylogeny of these traits. The many differences between human and nonhuman aggression are also pursued here. Topics here include the impact of modern weapons and extremes of wealth and power on both the costs and benefits of fighting, and the scale to which coercion can promote aggression that acts against a fighter’s own interests. Also the implications of large-scale human sociality are discussed.     

Effects of physical and social experiences and octopamine receptor agonist on fighting behavior of male crickets Velarifictorus aspersus (Orthoptera: Gryllidae)

Fighting commonly occurs among animals and is very important for resolving conflicts between conspecific individuals over limited resources. The plasticity of fighting strategies and neurobiological mechanisms underlying fighting behavior of insects are not fully understood. In the present study, we examined whether physical and social experiences affected the aggressiveness of males of the cricket Velarifictorus aspersus Walker, and whether an octopamine (OA) receptor agonist could affected the aggressiveness of males exposed to different experiences. We found that flight and winning a fight significantly enhanced male aggressiveness, while losing a fight significantly suppressed male aggressiveness, consistent with the findings of existing studies on other cricket species. We also found that female presence had a stronger enhancing effect on male aggressiveness than flight or winning a fight. These findings demonstrated that physical and social experiences can affect the fighting behavior of male V. aspersus. Topical application of a 0.15?M solution of an OA receptor agonist (chlordimeform, CDM) significantly increased male aggression level, suggesting that OA may play an important role as a neuromodulator in controlling fighting behavior of males of this species. Despite displaying a significantly higher aggression level (level 5 or 6), CDM-treated losers did not escalate to physical combat, while fights between courting males usually resulted in physical escalation. It is likely that fighting behavior is only partly regulated by OA, and additional regulatory pathways may be involved in achieving physical combat.     

Cortisol controls the pubertal development of agonistic behavior in male golden hamsters via type II corticosteroid receptors

In male golden hamsters, agonistic behavior undergoes a pubertal transition from play fighting to adult aggression. Previous studies have shown that this aspect of behavioral development is associated with pubertal increases in glucocorticoids and that daily social stress or injections of a synthetic glucocorticoid accelerate the transition. The goals of this study were to confirm the effects of cortisol on the development of agonistic behavior and to investigate the role of type II corticosteroid receptors in this process. First, animals treated with cortisol during early puberty [from postnatal days 31 (P-31) to P-36] showed an accelerated transition from play fighting to adult aggression. In a second experiment, the behavioral effects of cortisol were blocked by a co-treatment with a type II corticosteroid receptor antagonist. These findings are the first to show a facilitating role for type II corticosteroid receptors in the pubertal development of a social behavior. As such, these findings provide new insights into the neuroendocrine mechanisms controlling behavioral development during puberty.     

A method for quantifying aggression in male Drosophila melanogaster

Aggressive behavior is a complex social behavior that is difficult to measure. Here, we describe a simple method for the quantitative analysis of aggression in male Drosophila melanogaster. Traditional measurements of aggressive behavior have relied on a territorial context with a food territory and a female as factors that induce or enhance aggression. The protocol described here is devoid of a food territory or a female, making it simpler than most existing methods used to measure aggressive behavior. Multiple pairs of males are tested simultaneously to obtain an average fighting score. Four parameters are used to quantify the behavior: frequency, index, latency and intensity of fighting based on unambiguous offensive fighting behaviors. The assay takes 15 min, during which time a frequency score is obtained for 20-35 pairs simultaneously. More in-depth analysis, including latency, index and intensity, can be performed on the videotaped record of the experiment. The assay is highly reproducible and requires limited resources in a simple setup.     

Stress and the development of agonistic behavior in golden hamsters

Aggressive behavior can be studied as either offensive or defensive responses to a stimulus. The studies discussed in this review are focused on the peripubertal development of offensive aggression in male golden hamsters and its responsiveness to repeated social stress. Quantitative and qualitative changes in offensive responses were analyzed during this period. Quantitative changes in offensive responses were observed as decreased frequency of attacks. Qualitative changes were observed as changes in attack types, as animals reorient their attacks gradually from the face to the lower belly and rump. These developmental changes were altered by repeated exposure to social stress during early puberty. Daily exposure to aggressive adults during early puberty accelerated the qualitative development of offensive responses and the onset of adult-like offensive responses. In contrast, social stress had little effect on the quantitative changes associated with early puberty. However, social stress was associated with higher attack frequency during adulthood. These effects of stress during early puberty contrast with those observed with animals in late puberty. At that time, repeated exposure to aggressive adults inhibits offensive aggression. These data constitute the basis for a new theory on the development of agonistic behavior that includes the following hypotheses. First, it is hypothesized that mid-puberty is marked by a change in responsiveness to repeated social stress. As such, differences in stress responsiveness from social interactions are interpreted as a basic distinction between play fighting and adult aggression. Second, it is also hypothesized that a common neural circuitry mediates the activation of offensive responses during play fighting and adult aggressive interactions.     

The relationship between violent video games,acculturation, and aggression among Latino adolescents

Multiple factors are involved in the occurrence of aggressive behavior. The purpose of this study was to evaluate the hypotheses that Latino middle school children exposed to higher levels of video game playing will exhibit a higher level of aggression and fighting compared to children exposed to lower levels and that the more acculturated middle school Latino children will play more video games and will prefer more violent video games compared to less acculturated middle school Latino children. This study involved 5,831 students attending eight public schools in Texas. A linear relationship was observed between the time spent playing video games and aggression scores. Higher aggression scores were significantly associated with heavier video playing for boys and girls (p < 0.0001). The more students played video games, the more they fought at school (p < 0.0001). As Latino middle school students were more acculturated, their preference for violent video game playing increased, as well as the amount of time they played video games. Students who reported speaking more Spanish at home and with their friends were less likely to spend large amounts of time playing video games and less likely to prefer violent video games (p < 0.05).     

Neurobiological correlates premeditated (learned) aggression: seeking new experimental approaches]

Theoretical possibility of experimental modeling of learned (premediated) aggression developing in human after experience of aggression is considered. The sensory contact technique increases aggressiveness in male mice and allows aggressive type of behavior to be formed as a result of repeated experience of victories in daily agonistic confrontations. Some behavioral domains confirm the development of learned aggression in males similar to those in humans. The features are: repeated experience of aggression reinforced by victories; elements of learned behavior after period of confrontations; intent, measured by increase of the aggressive motivation prior agonistic confrontation; decreased emotionality estimated by parameters of open field behavior. Relevant stimuli provoke demonstration of aggression. This review summarized data on the influence of positive fighting experience in daily intermale confrontations on the behavior, neurochemistry and physiology of aggressive mice (winners). This sort of experience changes many characteristics in individual and social behaviors, these having been estimated in different tests and in varied situations. Some physiological parameters are also changed in the winners. Neurochemical data confirm the activation of brain dopaminergic systems and functional inhibition of serotonergic system in winners under influence of repeated experience of aggression. The expression of the neurochemical and behavioral changes observed in winners has been found dependent on the mouse strain and on the duration of their agonistic confrontations. Similarities in mechanisms of learned aggression in humans and mice are considered.     

The role of progesterone in pregnancy-induced aggression in mice

A series of six experiments was performed in order to explore the potential involvement of progesterone (P) in pregnancy-induced aggression (PIA) displayed by Rockland-Swiss mice toward adult male intruders. In Experiment 1, circulating levels of P and aggression were low on gestation Days 6 and 10 while both the behavior and the steroid reached peak levels by gestation Day 14. By gestation Day 18 (the day prior to parturition), serum P was at its lowest level yet aggressive behavior was still intense. Also, individual differences in the display of fighting behavior by pregnant females were not related to circulating P. Experiments 2 and 3 showed that supplemental P treatment to early pregnant female mice did not advance the onset of aggression. Experiment 4 showed that P treatment promoted the onset and elevated the incidence of aggression in virgin mice, but only in those females with intact ovaries. Experiment 5 showed that the aggressive behavior of P-stimulated virgin females was qualitatively and quantitatively different from that exhibited by pregnant mice in that the former exhibited fewer attacks and lunges than the latter. Finally, Experiment 6 showed that the removal of P from aggressive, P-stimulated virgins dramatically attenuated levels of the behavior. This contrasts sharply with the continued fighting behavior observed in late pregnant P-deficient mice. Thus, although P augments aggression in female mice it apparently is not a sufficient stimulus for producing pregnancy-like aggressive behavior.     

Rhythms,rhythmicity and aggression

The relationships between biological rhythms and human aggressive behavior are addressed and discussed in this article: First, circadian rhythms and aggression are considered. Studies of sleep/waking cycle disturbances in aggression are reported. Severe aggression is associated with profound changes in sleep architecture. Causal link is difficult to establish given that sleep disturbance and aggressive behavior could be the symptoms of the same disorder. Specific aggressive behavior developed during sleep is also described. In addition, hormonal circadian rhythm studies are reported. Thus, low cortisol levels, in particular low cortisol variability, are associated with aggressive behavior, suggesting an inhibitory role of cortisol. Testosterone has daily and seasonal fluctuations, but no link with aggression has been established. Neurophysiological underlying mechanisms are discussed in the last part of this article, with a focus on the relationship between brain rhythm and aggression. Increase of slow-wave EEG activities is observed in individuals with aggressive behavior. Epilepsy, as a disease of brain rhythm could be associated with aggressive behavior, in pre, post and inter ictal periodes. Incidence of aggression is not likely more prevalent in epileptic individuals compared to those with other neurological conditions. Ictal changes take the form of profound behavioral changes, including aggressive behavior which has been interpreted as the emergence of “archeical” or innate motor patterns. In this multidisciplinary approach, the main difficulty is the categorization of the differents types of aggression. Finally, taken together, these studies suggest that biological rhythms, especially circadian rhythms, could provide therapeutic benefits to human aggressive behavior. Biological rhythymicity seems to be a necessary permanent training offering interesting perspectives for the adaptation to changes in the field of aggression.     

Repeated exposure to social stress alters the development of agonistic behavior in male golden hamsters

In male golden hamsters, exposure to social stress during puberty alters aggressive behavior. Interestingly, agonistic behavior undergoes two major transitions during puberty: a decline in attack frequency and a shift from play fighting to adult-like aggression. Based on previous observations, we developed an approach for characterizing offensive responses as play fighting or adult-like. The present studies had two aims. First, we validated our approach by looking at the development of attack types during puberty. Second, we looked at the effects of repeated social stress on the development of agonistic behavior by repeatedly exposing individuals to aggressive adults during puberty. In the first phase of the study, our results point to three different developmental periods. Initially, animals engage in agonistic behavior though attacks targeted at the face and cheeks. This period lasts from Postnatal Day 20 (P-20) to P-40 (early puberty). This phase corresponding to play fighting is followed by a transitional period characterized by attacks focused on the flanks (from P-40 to P-50, mid-puberty). Afterward, animals perform adult-like aggression characterized by attacks focused on the belly and rear. Our data also show that repeated exposure to aggressive adults has two separate effects on the development of agonistic behavior. Repeated social stress accelerated the onset of adult-like agonistic responses. Furthermore, attack frequency, while decreasing during puberty, remained at a higher level in early adulthood in stressed animals. These results show that repeated exposure to social stress during puberty alters the development of agonistic behavior both qualitatively and quantitatively.     

Serotonin alters decisions to withdraw in fighting crayfish, Astacus astacus: the motivational concept revisited

The biogenic amine serotonin is thought to play an important role in aggression in many species, including man. This paper summarizes experimental approaches which attempt to link this neuromodulator with fighting in a crayfish model for which the complex agonistic behavior is well characterized. Based on a quantitative analysis of fighting we demonstrate that the infusion of small amounts of serotonin into freely-moving crayfish alters fighting behavior by specifically interfering with the timing of a treated animal's decision to withdraw from an encounter. In the presence of added serotonin, fights last considerably longer compared to controls, but no changes were detected in the rules of escalation, the likelihood of initiating an interaction, or its eventual outcome. Attempts to dissect the underlying neuronal mechanisms pharmacologically hinged on fluoxetine as a potent inhibitor of serotonin re-uptake. Although no behavioral changes were associated with acute infusion of fluoxetine alone, in combination with serotonin it effectively prevented the previously observed fight-enhancing effects. Our data strongly support the significance of functional amine re-uptake mechanisms for behavior and continued use of this invertebrate model should prove a promising route to unravel further the complex bases of aggression. Accepted: 11 October 1997     

The coevolution of juvenile play–fighting and adult competition

Although play–fighting is widespread among juvenile mammals, its adaptive significance remains unclear. It has been proposed that play is beneficial for developing skills to improve success in adult contests (motor‐training hypothesis), but the links between juvenile play–fighting and adult aggression are complex and not well understood. In this theoretical study, we investigate the coevolution between juvenile play–fighting and adult aggression using evolutionary computer simulations. We consider a simple life history with two sequential stages: a juvenile phase in which individuals play–fight with other juveniles to develop their fighting skills; and an adult phase in which individuals engage in potentially aggressive contests over access to resources and ultimately mating opportunities, leading to reproductive success. The simulations track genetic evolution in key traits affecting adult contests, such as the level of aggression, as well as juvenile investment in play–fighting, capturing the coevolutionary feedbacks between juvenile and adult decisions. We find that coevolution leads to one of two outcomes: a high‐play, high‐aggression situation with highly aggressive adult contests preceded by a prolonged period of juvenile play–fighting to improve fighting ability, or a low‐play, low‐aggression situation in which adult contests are resolved without fighting and there is minimal investment in play–fighting before individuals mature. Which of these outcomes is favoured depends on the mortality costs and on the type of societal structure: societies with strong reproductive skew, favouring monopolization of resources, show high levels of adult aggression and high investment in juvenile play–fighting, whereas societies with low reproductive skew have both low adult aggression and low levels of play–fighting. A review of empirical evidence, particularly in the primate genus Macaca, highlights some limitations of our model and suggests that other, complementary functional explanations are needed to account for the full range of competitive and cooperative forms of play–fighting. Our study illustrates the power of evolutionary simulations to shed light on the long‐standing puzzle of animal play.     

The implicit rules of combat reflect the evolved function of combat: An evolutionary-psychological analysis of fairness and honor in human aggression

Aggression was a powerful selection pressure for our human ancestors. Aggressive contests would have selected not only for anatomical features (e.g., superior upper-body strength in adult males), but also for behavior-regulation adaptations for the conditional and cost-effective deployment of aggression. Here, we explore two concepts that appear to be part of the neurocognitive suite that regulates aggression. The first is the concept of a “fair fight”, which tags whether an aggressive bout can provide an unbiased estimate of the relative fighting ability of the combatants. The second concept is the “honorability” of a fighter, which tags whether an individual can be trusted to not inflict aggression in excess of what is needed to establish the relative fighting ability of the combatants. Eight predictions were derived from these hypotheses and tested experimentally with vignettes that varied relevant aspects of a fight. The results support the predictions. Fights are perceived as “fair” to the extent that the fighters avoid biased tactics that improve their probability of winning absent superior fighting ability (e.g., not attacking when the opponent is unprepared). Further, fighters are perceived as “honorable” when they minimize the danger to both combatants (e.g., not attacking after the opponent surrenders). We conclude by listing additional implications of the theory.     

Behavioral Syndromes Break Down in Urban Song Sparrow Populations

Animals in urban habitats face a number of unique stresses, including the necessity of dealing with high levels of human activity. Growing research suggests that: (1) inherent traits, as opposed to learned behavior, influence which species invade urban habitats, and (2) individuals exhibit behavioral syndromes that limit behavioral flexibility. As a result, perhaps only animals with inherently bold personalities successfully settle in areas of high human activity, and such animals may also exhibit correlated variation in other behavioral traits, such as territorial aggression. In this study, we examine boldness and aggression in several urban and rural populations of song sparrows (Melospiza melodia). We found that urban birds were both bolder toward humans and also showed higher levels of aggression. We found a correlation between boldness and aggression in all populations combined, but no correlation within urban populations. Our results agree with other recent studies of song sparrow behavior, suggesting that greater boldness and aggression are general features of urban song sparrow populations, and a lack of a correlation between boldness and aggression in urban habitats is a general phenomenon as well. Urban habitats may select for bold and aggressive birds, and yet the traits can vary independently. These results add to a small number of studies which find that behavioral syndromes break down in potentially high quality habitats.     

Neurosteroids, GABAA receptors, and escalated aggressive behavior

Aggressive behavior can serve important adaptive functions in social species. However, if it exceeds the species-typical pattern, it may become maladaptive. Very high or escalated levels of aggressive behavior can be induced in laboratory rodents by pharmacological (alcohol-heightened aggression), environmental (social instigation), or behavioral (frustration-induced aggression) means. These various forms of escalated aggressive behavior may be useful in further elucidating the neurochemical control over aggression and violence. One neurochemical system most consistently linked with escalated aggression is the GABAergic system, in conjunction with other amines and peptides. Although direct stimulation of GABA receptors generally suppresses aggression, a number of studies have found that positive allosteric modulators of GABAA receptors can cause increases in aggressive behavior. For example, alcohol, benzodiazepines, and many neurosteroids are all positive modulators of the GABAA receptor and all can cause increased levels of aggressive behavior. These effects are dose-dependent and higher doses of these compounds generally shift from heightening aggressive behavior to being sedative and anti-aggressive. In addition, these modulators interact with each other and can have additive effects on the GABAA receptor and on behavior, including aggression. The GABAA receptor is a heteropentameric protein that can be constituted from various subunits. It has been shown that subunit composition can affect sensitivity of the receptor to some modulators and that subunit composition differentially affects the sedative vs anxiolytic actions of benzodiazepines. Initial studies targeting alpha subunits of the GABAA receptor point to their significant role in the aggression-heightening effects of alcohol, benzodiazepines, and neurosteroids.     

What can animal aggression research tell us about human aggression?

Research on endocrinological correlates of aggression in laboratory animals is implicitly motivated by an expectation that the results of such studies may be applicable to human aggression as well. Research with a focus on the stimulus antecedents of aggression, its response characteristics, and its outcomes suggests a number of detailed correspondences between offensive aggression in laboratory rodents and human angry aggression. These include resource (including status and territory) competition as motives that are particularly elicited by conspecific challenge situations and, when the aggression is successful, outcomes of reduction of challenge and enhancement of resource control and status. Although the response characteristics of human aggression have been dramatically altered by human verbal, technological, and social advancements, there is some evidence for targeting of blows, similar to a well-established pattern for offensive aggression in many nonhuman mammals. Finally, for people as well as for nonhuman mammals, fear of defeat or punishment is a major factor inhibiting the expression of offensive aggression. While defensive aggression has been very little researched in people, it may represent a different phenomenon than angry aggression, again providing a parallel to the offense-defense distinction of laboratory rodent studies.     

The effect of pharmacologic substances on intraspecies aggression]

It is found, that aggression which occures in rats under inevitable painful stimulation is coursed as excessive excitation of the rats. This aggression may be used for revealing the sedative effect of drugs. The method of fighting for the stool which is caused by motivated fighting for the territory was worked out. This motivated aggression may be used for revealing the tranquillizing effect of drugs. Using these methods it is found that stelasine, haloperidol, amitriptyline, imipramine, chlordiazepoxide, diazepam and benactyzine in small doses have a tranquillizing effect, while pentobarbital and chlorpromazine have primarily a sedative effect.     

Male Siamese Fighting Fish, Betta splendens, Increase Rather than Conceal Courtship Behavior when a Rival is Present

The impact of social environment on mating success is especially pronounced in species where both intraspecific and interspecific selection influence reproduction, such as the Siamese fighting fish. Males alter male–male interactions when either a male or female audience is present, but how males change their behavior toward a female when a rival male is present is unknown. This study addresses whether males alter their behavior toward a female in a way that would prevent a rival male from interrupting courtship. The behavior of male Siamese fighting fish toward a dummy female was examined under various degrees of visual cover, both in the presence and absence of a rival male, to investigate whether males use concealment provided by the structural environment to their advantage. While males did not use barriers to conceal courtship as hypothesized, males altered their behavior by increasing courtship and monitoring their nest when a rival was visible. This increase in courtship is in contrast to most studies on courtship in the presence of a rival that find a reduction in courtship behavior. Males spent more time opercular gill flaring when no barriers were present, suggesting that males may be trying to court the female and communicate to the rival simultaneously. There was also a trend for aggression toward the female and the rival to decrease as screen length increased. Thus, males compensate for the presence of a rival by adjusting their courtship and aggressive behaviors, which could have important implications for courtship success.