Rats Benefit from Winner and Loser Effects

Prior fighting experience of opponents can influence the outcome of conflicts. After a victory, animals are more likely to win subsequent contests, whereas after a defeat animals are more likely to lose, regardless of the identity of opponents. The underlying mechanisms and the adaptive significance of these winner and loser effects are as yet unknown. Here, we tested experimentally whether agonistic behavior of male wild‐type Norway rats is influenced by social experience, and we investigated whether this might reduce fighting costs (duration of contest, risk of injury) in subsequent encounters. Rats were randomly assigned to receive either a losing or a winning experience and subsequently tested with unfamiliar, naïve opponents. We found that most rats with a winning experience won the subsequent encounter, and all rats with a losing experience lost the next contest. Previous winners attacked more rapidly in the subsequent encounter and reduced their aggressive behavior sooner; the contests were decided more quickly, which saved time and behavioral effort to the winner. Previous losers received less aggression in the next encounter, despite emitting fewer submissive ultrasonic calls than in the preceding contest, thereby reducing the risk of being injured by the opponent. Thus, anonymous social experience influenced rats’ subsequent behavior toward size‐matched, naïve, unknown social partners. Furthermore, apparently, they benefit from showing winner and loser effects in intraspecific contests by saving time, energy, and risk.     

The effects of competition and implicit power motive on men's testosterone,emotion recognition,and aggression

A contribution to a special issue on Hormones and Human Competition.We investigated the effects of competition on men's testosterone levels and assessed whether androgen reactivity was associated with subsequent emotion recognition and reactive and proactive aggression. We also explored whether personalized power (p Power) moderated these relationships. In Study 1, 84 males competed on a number tracing task and interpreted emotions from facial expressions. In Study 2, 72 males competed on the same task and were assessed on proactive and reactive aggression. In both studies, contrary to the biosocial model of status (Mazur, 1985), winners' testosterone levels decreased significantly while losers' levels increased, albeit not significantly. Personalized power moderated the effect of competition outcome on testosterone change in both studies. Using the aggregate sample, we found that the effect of decreased testosterone levels among winners (compared to losers) was significant for individuals low in p Power but not for those with medium or high p Power. Testosterone change was positively related to emotion recognition, but unrelated to either aggression subtype. The testosterone-mediated relationship between winning and losing and emotion recognition was moderated by p Power. In addition, p Power moderated the direct (i.e., non-testosterone mediated) path between competition outcome and emotion recognition and both types of aggression: high p-Power winners were more accurate at deciphering others' emotions than high p-Power losers. Finally, among high p-Power men, winners aggressed more proactively than losers, whereas losers aggressed more reactively than winners. Collectively, these studies highlight the importance of implicit power motivation in modulating hormonal, cognitive, and behavioral outcomes arising from human competition.     

Failure to find sex differences in testosterone activated aggression in two strains of rats

The effect of testosterone (testosterone propionate: TP) on intraspecific aggression in males and females of two strains of rats—WEzob and S3—was examined. Pairs of these rats, gonadectomized and treated either with oil or with testosterone propionate (TP), were tested in three different combinations: OIL against OIL, OIL against TP, and TP against TP-treated animals. Subsequently the effects of TP treatment of the subject and for the opponents interaction with sex and strain on the occurrence of diverse social + aggression behavioral parameters were determined. The results of the S3 strain indicate that testosterone treatment of either the subject or the opponent stimulates aggression in both males and females. No sex difference could be determined with respect to the incidence of aggression. In the WEzob strain a stimulatory effect of TP was shown in females but not in males. The absence of a clear stimulatory effect of TP in WEzob males in terms of changes in the total time spent on aggression, however, could wrongly suggest that TP does not affect aggression in these animals. The possibility of TP having an effect on these males in terms of increasing the intensity of fighting is discussed.     

Seoul virus infection increases aggressive behaviour in male Norway rats

In natural populations of rodents, males are more likely to engage in aggression and be infected with hantaviruses than females. Whether the relationship between hantavirus infection and aggression is due to host- or parasite-mediated mechanisms is unknown. The aim of this study was to determine whether hantavirus infection causes an increase in aggression in male rats and whether these behavioural changes are due to infection of the central nervous system or peripheral tissues. Male laboratory rats were infected with Seoul virus and tested for aggression in a resident-intruder paradigm 15 and 30 days postinoculation (p.i.). Males tested 30 days p.i. (i.e. during the persistent phase of infection) spent more time engaged in aggression than either uninfected males or males tested during the acute phase of infection (i.e. 15 days p.i.). Males that engaged in aggression for a longer duration had more virus present in lung, kidney and testis than males that spent less time engaged in aggression. Infected males shed virus in saliva, faeces, and urine; virus shedding, however, was not correlated with aggression and neither wounding nor transmission of virus to intruder males occurred during behavioural tests. Infection with Seoul virus did not alter either testosterone or corticosterone concentrations. Seoul virus antigens were not detected in the brains of infected rats. These data suggest that hantavirus infection leads to elevated aggression in infected males and may be a by-product of increased virus replication in peripheral tissues.     

Snca and Bdnf gene expression in the VTA and raphe nuclei of midbrain in chronically victorious and defeated male mice

Background

Alpha-synuclein (α-Syn) is a small neuronal protein that has been found to be expressed throughout the brain. It has been shown that α-Syn regulates the homeostasis of monoamine neurotransmitters and is involved in various degenerative and affective disorders. There is indication that α-Syn may regulate expression of the brain-derived neurotropic factor (BDNF) which plays an important role in the mood disorders.

Methodology/Principal Findings

The study aimed to analyze the mRNA levels of Snca and Bdnf genes in the ventral tegmental area (VTA) and raphe nuclei of the midbrain in male mice that had each won or defeated 20 encounters (20-time winners and 20-time losers, respectively) in daily agonistic interactions. Groups of animals that had the same winning and losing track record followed by a no-fight period for 14 days (no-fighting winners and no-fighting losers) were also studied. Snca mRNA levels were increased in the raphe nuclei in the 20-time losers and in the VTA of the 20-time winners. After no-fight period Snca mRNA levels decreased in both groups. Snca mRNA levels were similar to the control level in the VTA of the 20-time losers and in the raphe nuclei of the 20-time winners. However Snca gene expression increased in these areas in the no-fighting winners and no-fighting losers in comparison with respective mRNA levels in animals before no-fight period. Bdnf mRNA levels increased in VTA of 20-time winners. Significant positive correlations were found between the mRNA levels of Snca and Bdnf genes in the raphe nuclei.

Conclusions/Significance

Social experience affects Snca gene expression depending on brain areas and functional activity of monoaminergic systems in chronically victorious or defeated mice. These findings may be useful for understanding the mechanisms of forming different alpha-synucleinopathies.     

An individual-orientated model of the emergence of despotic and egalitarian societies

Single behavioural differences between egalitarian and despotic animal societies are often assumed to reflect specific adaptations. However, in the present paper, I will show in an individual-orientated model, how many behavioural traits of egalitarian and despotic virtual societies arise as emergent characteristics. The artificial entities live in a homogeneous world and only aggregate, and upon meeting one another and may perform dominance interactions in which the effects of winning and losing are self-reinforcing. The behaviour of these entities is studied in a similar way to that of real animals. It will be shown that by varying the intensity of aggression only, one may switch from egalitarian to despotic virtual societies. Differences between the two types of society appear to correspond closely to those between despotic and egalitarian macaque species in the real world. In addition, artificial despotic societies show a clearer spatial centrality of dominants and, counter-intuitively, more rank overlap between the sexes than the egalitarian ones. Because of the correspondence with patterns in real animals, the model makes it worthwhile comparing despotic and egalitarian species for socio-spatial structure and rank overlap too. Furthermore, it presents us with parsimonious hypotheses which can be tested in real animals for patterns of aggression, spatial structure and the distribution of social positive and sexual behaviour.     

Towards the integration of social dominance and spatial structure

My aim was to show how individual-oriented (or artificial life) models may provide an integrative background for the development of theories about dominance by including effects of spatial structure. Dominance interactions are thought to serve two different, contrasting functions: acquisition of high rank and reduction of aggression. The model I present consists of a homogeneous virtual world inhabited by artificial agents whose actions are restricted to grouping and dominance interactions in which the effects of winning and losing are self-reinforcing. The two functions are implemented as strategies to initiate dominance interactions and the intensity of aggression and dominance perception (direct or memory based) are varied experimentally. Behaviour is studied by recording the same behavioural units as in real animals. Ranks appear to differentiate more clearly at high than at low intensity of aggression and also more in the case of direct than of memory-based rank perception. Strong differentiation of rank produces a cascade of unexpected effects that differ depending on which function is implemented: for instance, a decline in aggression, spatial centrality of dominants and a correlation between rank and aggression. Insight into the origination of these self-organized patterns leads to new hypotheses for the study of the social behaviour of real animals. Copyright 2000 The Association for the Study of Animal Behaviour.     

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.     

Recognition of Dominance in the Big-Clawed Snapping Shrimp ( Alpheus Heterochaelis Say 1818) Part I: Individual or Group Recognition?

The snapping shrimp Alpheus heterochaelis uses its snapper claw to produce fast water jets in fights. Here we investigate whether these shrimp are able to recognise their opponents (individually or on a group level) in order to reduce fighting costs and risk of injury. Losers meeting familiar or unfamiliar winners differ from those meeting inexperienced opponents by showing immediate escapes after a contact as well as hardly any aggressive behaviours (e.g. water jets). There is a significant decrease in aggressiveness in these losers and this is maintained for days. In contrast, losers meeting inexperienced opponents show a slow stepwise decrease in aggressiveness after losing on several consecutive days. Thus, snapping shrimp can recognise the dominance status of the opponent. The lack of behavioural differences in losers meeting familiar or unfamiliar winners favours recognition on a group level rather than individual recognition.     

Changes in Heart Rate Associated with Contest Outcome in Agonistic Encounters in Lobsters

Agonistic contests between lobsters housed together in a confined space progress through encounters of increasing intensity until a dominance relationship is established. Once this relationship is established, losing animals continually retreat from the advances of winners.These encounters are likely to consume much energy in both winning and losing animals. Therefore, one might expect involvement of many physiological systems before, during and after fights. Here, we report effects of agonistic encounters on cardiac frequency in winning and losing adult lobsters involved in dyadic interactions.The results show that: (i) small but significant increases in heart rate are observed upon chemical detection of a conspecific; (ii) during agonistic interactions, further increases in heart rate are seen; and (iii) ultimate winners exhibit greater increases in heart rate lasting longer periods of time compared to ultimate losers. Heart rate in winners remains elevated for at least 15 min after the contests have ended and animals have been returned to their home tanks. Reduced effects are seen in second and third pairings between familiar opponents.The sustained changes in heart rate that we observe in winning lobsters may result from hormonal modulation of cardiac function related to the change in social status brought about by contest outcome.     

Is boldness a resource-holding potential trait? Fighting prowess and changes in startle response in the sea anemone,Actinia equina

Contest theory predicts the evolution of a stable mixture of different strategies for fighting. Here, we investigate the possibility that stable between-individual differences in startle-response durations influence fighting ability or 'resource-holding potential' (RHP) in the beadlet sea anemone, Actinia equina. Both winners and losers showed significant repeatability of pre-fight startle-response durations but mean pre-fight startle-response durations were greater for eventual losers than for eventual winners, indicating that RHP varies with boldness. In particular, individuals with short startle responses inflicted more attacks on their opponent. Both repeatability and mean-level responses were changed by the experience of fighting, and these changes varied with outcome. In losers, repeatability was disrupted to a greater extent and the mean startle-response durations were subject to a greater increase than in winners. Thus, following a fight, this behavioural correlate of RHP behaves in a way similar to post-fight changes in physiological status, which can also vary between winners and losers. Understanding the links between aggression and boldness therefore has the potential to enhance our understanding of both the evolution of animal personality and the 'winner and loser effects' of post-fight changes in RHP.     

Gonadal hormones modulate the display of conditioned defeat in male Syrian hamsters

It has been widely reported that gonadal hormones influence the display of aggression in Syrian hamsters; conversely, much less is known about whether gonadal hormones modulate submissive/defensive behaviors in these animals. Following social defeat, male hamsters no longer display normal territorial aggression but instead display submissive/defensive behavior in the presence of a smaller opponent, a phenomenon we have termed conditioned defeat (CD). The purpose of the present study was to examine the effect of gonadal hormones on the display of CD in male hamsters. In Experiment 1, males were castrated or sham-operated. The castrated males were significantly more submissive following social defeat relative to their intact counterparts. The increased submissive behavior in the castrated males during CD testing was particularly surprising, given the fact that they were attacked significantly less during CD training. In Experiment 2a, males were castrated and given hormone replacement. Castrated males treated with testosterone or dihydrotestosterone displayed significantly less submissive behavior following social defeat than did those treated with cholesterol or estradiol. Finally, in Experiment 2b, there was no effect of hormone replacement on aggressive behavior in non-defeated hamsters suggesting that the decrease in submissive behavior in males treated with dihydrotestosterone or testosterone is specific to being previously defeated. Taken together the data indicate that the presence of androgens reduces the display of submission in defeated male hamsters. More importantly, these findings suggest that androgens may have a protective effect against the development of depression-like or anxiety-like behaviors following exposure to an ethologically relevant stressor.     

Effects of repeated aggressive encounters on approach to a female and plasma testosterone in male mice

Effects of repeated experience of aggression accompanied by social victories or social defeat in 10 daily agonistic confrontations on testosterone levels in and the behavioral response of CBA/Lac male mice exposed to a receptive female from behind a perforated transparent partition have been examined. Testosterone levels were not changed significantly in the mice that had consistently been victorious over 10 days (winners) or in the mice that had consistently been defeated over 10 days (losers). Losers and controls (mice that had been caged individually for 5 days) responded with increased levels of behavioral activity near the partition and elevated testosterone. Winners showed a significantly poorer behavioral and hormonal response. It is concluded that the repeated display of aggression by male mice led to a reduction in both their behavioral and neuroendocrine responses to an estrous female.     

Further evidence for the role of nitric oxide in maternal aggression: effects of L-NAME on maternal aggression towards female intruders in Wistar rats

It has been shown that nitric oxide (NO) increases aggression in male mice, whereas it decreases aggression in lactating female mice and prairie voles. It is also known that aggression can be exhibited at different levels in rodent species, strain or subtypes. The aims of this study were to investigate the proportion of aggressiveness in Wistar rats, the effect of intraperitoneally administered nonspecific nitric oxide synthase (NOS) inhibitor L-NAME (NG-nitro L-arginine methyl ester) on maternal aggression towards female intruders, and whether these effects are due to NO production or not. Rats were given saline intraperitoneally on the postpartum Day 2 and aggression levels were recorded. The same rats were given 60 mg/kg L-NAME or D-NAME (NG-nitro D-arginine methyl ester) on the postpartum Day 3 and their effects on aggression levels were compared to saline. While L-NAME administration did not cause any differences in the total number of aggressive behavior, aggression duration and aggression intensity, it reduced the proportion of animals showing aggressive behavior. In addition, the latency of the first aggression was significantly increased by L-NAME. In the D-NAME group, however, no significant change was found. Our results have shown that L-NAME reduces maternal aggression towards female intruders in Wistar rats through inhibition of NO production. These results suggest that the role of NO in offensive and defensive maternal aggression shares neural mechanisms.     

The effect of central serotonin administration on the neurogenic damage to the gastric mucosa due to chronic social stress in inbred mice]

A reiterated negative experience of intermale confrontations for 3 and 10 days resulted in aggravation of neurogenic ulceration of gastric mucosa in defeated males from all strains of mice under study, the number of mucosa erosions being 2-3-fold greater than in winners or control animals. Administration of serotonine into the lateral ventricles increased the number of erosions in intact mice of all genotypes. In experimental groups, a considerable diversity was found in respect to the effects of exogenous serotonine on the gastric mucosa. Following 10 days of the stress, both in winners and losers, a decrease of the gastric mucosa sensitivity to central serotonine was revealed.     

The development of catatonic reactions in male mice of CBA/Lac strain: the effect of repeated experience of aggression and submission

The development of catatonic reactions with rigid muscle tension due to stimulation of the skin at the scruff (catatonia-"pinch" test) and wax muscle plasticity (repeated pinch-induced catalepsy displayed on the parallel bars--BAR-test) was investigated in aggressive and submissive CBA/Lac male mice with repeated experiences of social victories (winners) or defeats (losers), accordingly. The expression of catatonic-like state in "pinch" test was significantly more in the losers after 20 daily agonistic confrontations in comparison with the winners. The catalepsy in the BAR-test was increased in animals with experience of agonistic confrontation in comparison with the controls, however expression of catalepsy reaction depended on kind and duration of agonistic interactions. The pronounced freezing predominated in the free behavior of the losers and, on the contrary, the winners demonstrated the abnormal undirected jumping. It was suggested that two contrast forms of catatonic syndrome accompanying by development of akinesia- or hiperkinesia-like states, are developed in the defeated and victorious (accordingly) mice of cataleptic CBA/Lac strain.     

The effect of DAGO,selective milli-opioid receptor agonist,on hostile and anxious behaviors in male mice with different experience of aggression

Effects of mu-opioid receptor agonist DAGO (2.0 mg/kg, s.c.) on anxioUs, hostile, and aggressive behaviors of male mice with repeated 3- and 20-day experience of aggression accompanied by victories (T3 and T20 winners, respectively) were stUdied. T20 winners showed lower aggression (attacking and biting) and hostile behavior and were more anxioUs (estimated by plUs-maze test) than T3 winners. In the plUs-maze test DAGO prodUced anxiogenic effects in intact males and was ineffective in T3 and T20 winners testifying to a decrease in mu-receptor sensitivity Under the inflUence of repeated aggression. In agonistic confrontation test, DAGO increased aggressive grooming in T20 winners, decreased hostile behavior (digging and throwing partner's litter) in T3 winners, and did not inflUence attacks in both groUps. It is sUggested that mu-opioid receptors are involved into forming the aggressive behavioral type in mice, and DAGO effects may be conditioned by emotional backgroUnd of these behavioral forms.     

Sex and estrous cycle differences in the display of conditioned defeat in Syrian hamsters

We have reported that there is a sex difference in the behavioral response to social defeat in hamsters. While previously defeated male hamsters fail to display normal territorial aggression and instead produce submissive/defensive behavior, a phenomenon that we have termed conditioned defeat (CD), only a small portion of previously defeated females exhibit CD. In Experiment 1, we tested the hypothesis that CD varies over the estrous cycle and found that previously defeated female hamsters tested on diestrus 2 and proestrus were more likely to exhibit CD than were females tested on diestrus 1 and estrus. In Experiment 2, we found that regardless of hormonal status, non-defeated females displayed normal territorial aggression, indicating that the behavioral changes observed in Experiment 1 were not due to a cyclic variation in submissive behavior independent of a previous defeat encounter. In Experiment 3, we found that females tested 4 days after defeat responded similarly to those tested 1 day after defeat suggesting that the hormonal status of females on the day of testing is a more important determinant of the behavioral response to defeat than is the hormonal status on the day of defeat training. Finally, in Experiment 4, we monitored anxiety-like behaviors in diestrous 1 and proestrous females in an open field arena and found that there was no effect of cycle on any of the observed behavioral measures, suggesting that the observed differences in CD are not the result of differences in generalized anxiety-like behaviors across the estrous cycle.     

Behavioural Syndromes in Urban and Rural Populations of Song Sparrows

Animals in urban habitats are often noticeably bold in the presence of humans. Such boldness may arise due to habituation, as urban animals learn, through repeated exposure, that passing humans do not represent a threat. However, there is growing research suggesting that: (1) inherent traits, as opposed to learned behaviour, influence which species invade urban habitats, and (2) individuals exhibit individual personality traits that limit behavioural flexibility, with the possible result that not all individuals would be able to demonstrate an appropriate level of boldness in urban environments. As a result, perhaps only birds with inherently bold personalities could successfully settle in an area of high human disturbance, and further, we might also expect to see the existence of behavioural syndromes, where boldness is correlated with variation in other behavioural traits such as aggression. In this study, we examined boldness and territorial aggression in urban and rural populations of song sparrows. We found that urban birds were bolder towards humans and that urban birds also showed higher levels of territorial aggression. We also found an overall correlation between boldness and territorial aggression, suggesting that urban boldness may be part of a behavioural syndrome. However, we see no correlation between boldness and aggression in the urban population, and thus, more work is needed to determine the mechanisms accounting for high levels of boldness and aggression urban song sparrows.     

Conditioned defeat in male and female Syrian hamsters

A brief exposure to social defeat in male Syrian hamsters (Mesocricetus auratus) leads to profound changes in the subsequent agonistic behavior exhibited by the defeated animals. Following defeat in the home cage of an aggressive conspecific, male hamsters will subsequently fail to defend their home territory even if the intruder is a smaller, nonaggressive male. This phenomenon has been called conditioned defeat. In Experiment 1, we examined the duration of conditioned defeat by repeatedly testing (every 3-5 days) defeated hamsters with a nonaggressive intruder. We found that conditioned defeat occurs in all defeated male hamsters and persists for a prolonged period of time (at least 33 days) in the majority of male hamsters tested despite the fact that these animals are never attacked by the nonaggressive intruders. In Experiment 2, we examined whether conditioned defeat could be induced in female Syrian hamsters. While conditioned defeat occurred in some females, they displayed only low levels of submissive/defensive behavior and, in contrast to males, the conditioned defeat response did not persist beyond the first test. These results suggest that in male hamsters conditioned defeat is a profound, persistent behavioral change characterized by a total absence of territorial aggression and by the frequent display of submissive and defensive behaviors. Conversely, social defeat in female hamsters does not appear to induce long-term behavioral changes. Finally, in Experiment 3, we determined that plasma adrenocorticotropin-like immunoreactivity increases in females following social defeat in a manner similar to that seen in males, suggesting that the disparate behavioral reactions of males and females are not due to sex differences in the release of, or response to, plasma adrenocorticotropin.