Winner and loser effects and the structure of dominance hierarchies

In the literature on dominance hierarchies, "winner" and "loser"effects usually are denned as an increased probability of winningat time T, bated on victories at time T-l, T-2, etc, and anincreased probability of losing at time T, based on losing atT-1, T-2, etc., respectively. Despite some early theoreticalwork on winner and loser effects, these factors and how theyaffect the structure of dominance hierarchies have not beenexamined in detail. I developed a computer simulation to examinewinner and loser effects when such effects are independent ofone another (as well as when they interact) and when combatantsassess each other's resource-holding power. When winner effectsalone were important, a hierarchy in which all individuals heldan unambiguous rank was found. When only loser effects wereimportant, a dear alpha individual always emerged, but the rankof others in the group was often unclear because of the scarcityof aggressive interactions. Increasing winner effects for agiven value of the loser effect increase the number of individualswith unambiguous positions in a hierarchy and the converse istrue for increasing the value of the loser effect for a givenwinner effect Although winner and loser effects have been documentedin a number of species, no study has documented both winnerand loser effects (using some controlled, pairwise testing system)and the detailed nature of behavioral interactions when individualsare in groups. I hope the results of this model will spur suchstudies in the future.     

Bystander effects and the structure of dominance hierarchies

Prior modeling work has found that pure winner and loser effects(i.e., changing the estimation of your own fighting abilityas a function of direct prior experience) can have importantconsequences for hierarchy formation. Here these models areextended to incorporate "bystander effects." When bystandereffects are in operation, observers (i.e., bystanders) of aggressiveinteractions change their assessment of the protagonists' fighting abilities (depending on who wins and who loses). Computer simulations demonstrate that when bystander winner effects alone are atplay, groups have a clear omega (bottom-ranking individual),while the relative position of other group members remainsdifficult to determine. When only bystander loser effects arein operation, wins and losses are randomly distributed throughout a group (i.e., no discernible hierarchy). When pure and bystanderwinner effects are jointly in place, a linear hierarchy, inwhich all positions (i.e., to when N = 4) are clearly defined,emerges. Joint pure and bystander loser effects produce thesame result. In principle one could test the predictions fromthe models developed here in a straightforward comparativestudy. Hopefully, the results of this model will spur on suchstudies in the future.     

The ghost of social environments past: dominance relationships include current interactions and experience carried over from previous groups

Dominance hierarchies pervade animal societies. Within a static social environment, in which group size and composition are unchanged, an individual's hierarchy rank results from intrinsic (e.g. body size) and extrinsic (e.g. previous experiences) factors. Little is known, however, about how dominance relationships are formed and maintained when group size and composition are dynamic. Using a fusion-fission protocol, we fused groups of previously isolated shore crabs (Carcinus maenas) into larger groups, and then restored groups to their original size and composition. Pre-fusion hierarchies formed independently of individuals' sizes, and were maintained within a static group via winner/loser effects. Post-fusion hierarchies differed from pre-fusion ones; losing fights during fusion led to a decline in an individual's rank between pre- and post-fusion conditions, while spending time being aggressive during fusion led to an improvement in rank. In post-fusion tanks, larger individuals achieved better ranks than smaller individuals. In conclusion, dominance hierarchies in crabs represent a complex combination of intrinsic and extrinsic factors, in which experiences from previous groups can carry over to affect current competitive interactions.     

Self-organizing dominance hierarchies in a wild primate population

Linear dominance hierarchies, which are common in social animals, can profoundly influence access to limited resources, reproductive opportunities and health. In spite of their importance, the mechanisms that govern the dynamics of such hierarchies remain unclear. Two hypotheses explain how linear hierarchies might emerge and change over time. The ‘prior attributes hypothesis’ posits that individual differences in fighting ability directly determine dominance ranks. By contrast, the ‘social dynamics hypothesis’ posits that dominance ranks emerge from social self-organization dynamics such as winner and loser effects. While the prior attributes hypothesis is well supported in the literature, current support for the social dynamics hypothesis is limited to experimental studies that artificially eliminate or minimize individual differences in fighting abilities. Here, we present the first evidence supporting the social dynamics hypothesis in a wild population. Specifically, we test for winner and loser effects on male hierarchy dynamics in wild baboons, using a novel statistical approach based on the Elo rating method for cardinal rank assignment, which enables the detection of winner and loser effects in uncontrolled group settings. Our results demonstrate (i) the presence of winner and loser effects, and (ii) that individual susceptibility to such effects may have a genetic basis. Taken together, our results show that both social self-organization dynamics and prior attributes can combine to influence hierarchy dynamics even when agonistic interactions are strongly influenced by differences in individual attributes. We hypothesize that, despite variation in individual attributes, winner and loser effects exist (i) because these effects could be particularly beneficial when fighting abilities in other group members change over time, and (ii) because the coevolution of prior attributes and winner and loser effects maintains a balance of both effects.     

Individual recognition, dominance hierarchies and winner and loser effects

Winner and loser effects are defined as an increased probability of winning an aggressive interaction at time T, based on victories at time T-1, T-2, etc., and an increased probability of losing at time T, based on losses at time T-1, T-2, etc., respectively. Prior theoretical work on dominance hierarchy formation has demonstrated that when players are not capable of individual recognition, loser effects always produce a clear top-ranked (alpha) individual, but all other ranks in a group remain unclear; whereas winner effects always produce strict linear hierarchies in which the rank of each individual is clear. Paradoxically, however, when individual recognition--a phenomenon long thought to stabilize hierarchies--is possible, winner and loser effects have no impact on the probability of forming strict linear hierarchies.     

Merging social hierarchies: Effects on dominance rank in male green swordtail fish (Xiphophorus helleri)

When the same set of individuals are placed in different social contexts, some groups members often experience a change in dominance status. We examined the context-dependence of social status using a group fusion protocol in male green swordtail fish (Xiphophorus helleri). Six individuals were matched for size and separated into two groups of three fish. Each triad established a stable hierarchy after which time the two subgroups were merged into one larger assemblage. The maintenance of within- and between-group rank relationships was examined. Relative rank was preserved within each subgroup across social contexts but we found no evidence that familiarity with dominant animals assists individuals of one subgroup in achieving higher rank (coat-tail effects). Dominant individuals from the pre-fusion groups were significantly likely to obtain high status in the merged group and vice versa for subordinate pre-fusion animals. These results demonstrate that social rank in swordtails is relatively impervious to changes in social context, but we address some deviations from this trend. Small differences in standard length were a significant predictor of the most dominant rank in the post-fusion hierarchy, with the largest animals tending to occupy the alpha position. We discuss our results in terms of the potential factors involved in within- and between-group rank maintenance, including individual recognition, winner and loser effects, or asymmetries in dominance-related characteristics.     

Coalition formation in animals and the nature of winner and loser effects

Coalition formation has been documented in a diverse array of taxa, yet there has been little formal analysis of polyadic interactions such as coalitions. Here, we develop an optimality model which examines the role of winner and loser effects in shaping coalition formation. We demonstrate that the predicted patterns of alliances are strongly dependent on the way in which winner and loser effects change with contestant strength. When winner and loser effects decrease with the resource-holding power (RHP) of the combatants, coalitions will be favoured between the strongest members of a group, but not between the weakest. If, in contrast, winner and loser effects increase with RHP, exactly the opposite predictions emerge. All other things being equal, intervention is more likely to prove worthwhile when the beneficiary of the aid is weaker (and its opponent is stronger), because the beneficiary is then less likely to win without help. Consequently, intervention is more probable when the impact of victory on the subsequent performance of a combatant increases with that individual's strength because this selects for intervention in favour of weaker combatants. The published literature on hierarchy formation does not reveal how winner and loser effects actually change with contestant strength and we therefore hope that our model will spur others to collect such data; in this light we suggest an experiment which will help to elucidate the nature of winner and loser effects and their impact on coalition formation in animals.     

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.     

Why do winners keep winning? Androgen mediation of winner but not loser effects in cichlid fish

Animal conflicts are influenced by social experience such that a previous winning experience increases the probability of winning the next agonistic interaction, whereas a previous losing experience has the opposite effect. Since androgens respond to social interactions, increasing in winners and decreasing in losers, we hypothesized that socially induced transient changes in androgen levels could be a causal mediator of winner/loser effects. To test this hypothesis, we staged fights between dyads of size-matched males of the Mozambique tilapia (Oreochromis mossambicus). After the first contest, winners were treated with the anti-androgen cyproterone acetate and losers were supplemented with 11-ketotestosterone. Two hours after the end of the first fight, two contests were staged simultaneously between the winner of the first fight and a naive male and between the loser of first fight and another naive male. The majority (88%) of control winners also won the second interaction, whereas the majority of control losers (87%) lost their second fight, thus confirming the presence of winner/loser effects in this species. As predicted, the success of anti-androgen-treated winners in the second fight decreased significantly to chance levels (44%), but the success of androgenized losers (19%) did not show a significant increase. In summary, the treatment with anti-androgen blocks the winner effect, whereas androgen administration fails to reverse the loser effect, suggesting an involvement of androgens on the winner but not on the loser effect.     

Breaking up fights between others: a model of intervention behaviour

To examine when and why animals break up fights between others in their group, I modelled whether ''winner'' and ''loser'' effects might be one element driving the evolution of intervention behaviour. I considered one particular type of intervention: when the intervener simply breaks up fights between two others, but does not favour either party in so doing. When victories at time T + 1 are more likely given a victory at time T (i.e. winner effects), intervention is often favoured. Intervention is favoured in these circumstances because the intervening party in essence stops others from ''getting on a roll'' and climbing up any hierarchy that exists. However, when loser effects alone are at work (defeats at time T + 1 are more likely given a defeat at time T), breaking up fights between others is never selected. If both winner and loser effects are operating simultaneously, then the likelihood of intervention behaviour evolving is a function of the relative strength of these two effects. The greater the winner effect relative to the loser effect, the more likely intervention behaviour is to evolve.     

Winner and loser effects in human competitions. Evidence from equally matched tennis players

Animals winning an agonistic encounter are more likely to win their next encounter while losers are less likely, even when controlling for motivation and physical size. Do these winner and loser effects exist in human competitions? Drawing on a large database of professional tennis matches, we were able to control for players' ability and thereby test for winner and loser effects. We narrowed the database to matches between players who on average did not differ significantly in rank, and further to matches in which the first set was fought to a long tie-break. These closely fought matches present a natural experiment because players are assigned to treatment conditions – winning or losing a set – despite similar ability and performance. We found that among men, the winner of a closely fought tie-break had an approximate 60% chance of winning the second set, the loser a 40% chance. These effects did not exist among women, a finding consistent with the hypothesis that androgens mediate winner and loser effects. Our results may help in the design of competitions in sport as well as in work environments, where it may prove useful to either encourage winner effects or to attenuate their occasional adverse consequences.     

On the evolution of pure winner and loser effects: A game-theoretic model

The persistence of linear dominance hierarchies is often attributed to higher probabilities of a win after a win or a loss after a loss in agonistic interactions, yet there has been no theory on the evolution of such prior-experience effects. Here an analytic model, based on the idea that contests are determined by subjective perceptions of resource-holding potential (RHP) which animals may revise in the light of experience, demonstrates that winner and loser effects can evolve through round-robin competition among triads of animals drawn randomly from their population, and that the probability of a hierarchy increases with the strength of the combined effect. The effects are pure, in the sense that a contestant observes neither its own RHP nor its opponent’s RHP or RHP perception or win—loss record; and so the strength of an effect is unmodified by the RHPs of particular individuals, but depends on the distribution of RHP among the population at large. The greater the difference between an individual’s and its opponent’s RHP perception, the more likely it is to win a contest; however, if it overestimates its RHP, then the cost of fighting increases with the overestimate. A winner or loser effect exists only if the fitness gain of the beta individual in a hierarchy, relative to that of the alpha, is less than 0.5. Then a loser effect can exist alone, or it can coexist with a winner effect; however, there cannot exist a winner effect without a loser effect.     

A model of coalition formation in animals

A simple three-player model is presented for the evolution of coalitions. The model demonstrates that, under certain conditions, ''winner'' and ''loser'' effects both favour coalition formation. Winner effects are defined as an increased probability of winning at time T + 1, given a victory at time T, whereas loser effects entail an increased probability of losing at time T + 1, given a loss at time T. Increasing the strength of loser effects or winner effects, or the strength of an individual''s position in the hierarchy, makes coalition formation in general more likely, whereas increasing the costs of giving aid does the opposite. The model does not assume any form of reciprocity, but rather examines whether some form of reciprocity or pseudoreciprocity emerges from the model itself. When either winner or loser effects exist, reciprocal coalition formation (e.g. i helps j against k, and j helps i against k) between β (second-ranked individual) and α (highest-ranked individual) or between α and γ (lowest-ranked individual) was possible, but reciprocal aid-giving between γ and β was never favoured. Thus, we have the counterintuitive result that although a coalition between the two lowest members of a hierarchy against the dominant individual is possible (as selection may favour γ aiding β against α), such a coalition is not predicted to be reciprocal in kind. Interpopulational comparisons examining winner/loser effects and coalition formation would allow for a test of many of the model''s most basic predictions. Unfortunately, most work on coalitions has been undertaken in primates, whereas work on winner and loser effects has focused on rodents, and more recently, in fish and birds. Hopefully, the model presented here will spur future work that will look at all of these factors simultaneously in many taxa.     

Eavesdropping on visual cues in green swordtail (Xiphophorus helleri) fights: a case for networking

Aggressive contests probably occur in networking environments where information about fighting ability is conveyed both to an opponent and to individuals peripheral to the fight itself, the bystanders. Our primary aim was to investigate the relative influences of eavesdropping and prior social experience on the dynamics of aggressive contests in Xiphophorus helleri. A bystander's ability to witness an encounter was manipulated using clear, one-way mirror, and opaque partitions. After watching (or not watching) the initial contest, the bystander encountered either the winner or loser of the bout. Treatment comparisons of bystander-winner or bystander-loser contest dynamics indicated the presence or absence of winner, loser, or eavesdropping effects. Winner and loser effects had negligible influences on bystander contest dynamics. Eavesdropping significantly reduced the bystander's propensity to initiate aggression, escalate, and win against seen winners regardless of whether the watched bout had escalated or not. Though eavesdropping had relatively little effect on bystander-loser contest dynamics, bystanders were less prone to initiate aggression and win against losers that had escalated in the witnessed bout. Thus, bystanders appear to preferentially retain and utilize information gained about potentially dangerous opponents (winners or persistent losers). Our data lend clear support for the importance of eavesdropping in visually based aggressive signalling systems.     

Persistence of Winner and Loser Effects Depends on the Behaviour Measured

Recent contest experience can influence an individual’s behaviour in subsequent contests. When the probability of winning a subsequent contest is used to quantify experience effects, a loser effect usually lasts longer than a winner effect. This conclusion, however, may be caused by this probability understating the persistence of the influence of a winning experience on contest decisions. Using Kryptolebias marmoratus, a mangrove killifish, as the study organism, we investigated whether different conclusions about the relative persistence of winning and losing experiences would be reached when different aspects of contest behaviour (probability of initiating attacks, probability of winning non‐escalated and escalated contests, escalation rate and contest duration) were measured. The results indicated that the apparent persistence of the effect of winning or losing experiences varied with the behaviour studied. When the likelihood to initiate attacks was used, no winner effect was detected while the loser effect lasted for <1 d. When escalation rate was used, the winner effect lasted for 2–4 d, while the loser effect lasted for 1–2 d. When the probability of winning non‐escalated contests was used, the winner effect was detectable for <1 d, while the loser effect lasted for 2–4 d. And, when contest duration was used, the winner effect was detectable for 2–4 d, but no loser effect was detectable. These results show that (1) the probability of winning a subsequent contest understated the persistence of the influence of a winning experience on the fish’s contest decisions, (2) the measures most effective at detecting winner effects are different from those most effective at detecting loser effects and (3) in K. marmoratus, both effects can be detected 2 d after the completion of experience training but both dissipate in 4 d.     

Post-contest behaviour in black-capped chickadees (<Emphasis Type="Italic">Poecile atricapillus</Emphasis>): loser displays,not victory displays,follow asymmetrical countersinging exchanges

Victory displays are behaviours that occur after the conclusion of a signaling contest, performed solely by the contest winner. Victory displays may reinforce the dominance of the winner either to the loser or to other conspecifics within signaling range. Victory displays are poorly studied despite the significant consequences that post-conflict behaviour may have on the individuals involved. We examined the period immediately following 50 territorial countersinging contests between males in 10 neighbourhoods of black-capped chickadees (Poecile atricapillus) of known dominance rank. We characterized the post-contest singing behaviour of chickadees and evaluated whether post-contest behaviour is consistent with victory displays. Using a 16-microphone acoustic location system to simultaneously record entire neighbourhoods of breeding chickadees, we isolated 50 dyadic countersinging contests and measured the vocal behaviour of the contestants in the minutes following each interaction. Eighty-six percent of contests were followed by a period of solo singing by one of the contestants, while 14% were followed by silence. The post-contest singer was most often the contestant who held a subordinate dominance position in the previous winter’s dominance hierarchy; dominant males performed post-contest song bouts significantly less often. Asymmetry in overlapping between contestants did not predict which bird sang a post-contest bout. However, in a significant majority of cases, the post-contest singer was pitch-matched by his opponent during the contest more than he pitch-matched his opponent. Our results indicate that male chickadees do not perform acoustic victory displays after countersinging contests. In contrast, the post-contest behaviour of territorial chickadees is more consistent with a “loser display”.     

Gender effects on aggression, dominance rank, and affiliative behaviors in a flock of captive adult cockatiels (Nymphicus hollandicus)

Agonistic and affiliative behaviors and spatial positioning were studied in a small psittacine species, Nymphicus hollandicus. Subjects studied were flock-housed breeders including five hens and seven cocks. Fifteen-minute focal animal samples were collected for the entire flock in a randomly distributed order during mate selection and the onset of the breeding season. All agonistic behaviors were recorded, including the winner and loser of each interaction, along with allopreening and copulation behavior. Point samples were recorded every 60s to determine social spacing. Rates of aggression were significantly higher for male cockatiels than female cockatiels. Results based on dyadic agonistic interactions showed males to rank significantly higher in the social hierarchy than females. Associations within the flock were not random. Individual birds associated more with specific birds than would be predicted by chance. Analysis of spatial data revealed that there were both same-sex and opposite-sex preferred associations within the flock. Male cockatiels engaged in allopreening behavior with females significantly more than with other males. Observations of breeding and nesting behaviors revealed pairs, a triad, and extra-pair mating.     

Examination of prior contest experience and the retention of winner and loser effects

In many animal taxa, prior contest experience affects future performance such that winning increases the chances of winning in the future (winner effect) and losing increases the chances of losing in the future (loser effect). It is, however, not clear whether this pattern typically arises from experience effects on actual or perceived fighting ability (or both). In this study, we looked at winner and loser effects in the jumping spider Phidippus clarus. We assigned winning or losing experience to spiders and tested them against opponents of similar fighting ability in subsequent contests at 1-, 2-, 5-, and 24-h intervals. We examined the strength of winner and loser effects, how long effects persist, as well as how experience affected perceived and actual fighting ability. Our results demonstrate that winner and loser effects are of approximately the same magnitude, although loser effects last longer than winner effects. Our results also demonstrate that previous experience alters actual fighting ability because both the assessment and escalation periods were affected by experience. We suggest that the retention time of experience effects depends on expected encounter rates as well as other behavioral and ecological factors. In systems with short breeding seasons and/or rapidly fluctuating populations, context-dependent retention of experience effects may allow males to track their status relative to the fluctuating fighting ability of local competitors without paying the costs necessary to recall or assess individual competitors.     

Traits of winner and loser species indicate drivers of herb layer changes over two decades in forests of NW Germany

Questions: What are the most likely environmental drivers for compositional herb layer changes as indicated by trait differences between winner and loser species? Location: Weser‐Elbe region (NW Germany). Methods: We resurveyed the herb layer communities of ancient forest patches on base‐rich sites of 175 semi‐permanent plots. Species traits were tested for their ability to discriminate between winner and loser species using logistic regression analyses and deviance partitioning. Results: Of 115 species tested, 31 were identified as winner species and 30 as loser species. Winner species had higher seed longevity, flowered later in the season and more often had an oceanic distribution compared to loser species. Loser species tended to have a higher specific leaf area, were more susceptible to deer browsing and had a performance optimum at higher soil pH compared to winner species. The loser species also represented several ancient forest and threatened species. Deviance partitioning indicated that local drivers (i.e. disturbance due to forest management) were primarily responsible for the species shifts, while regional drivers (i.e. browsing pressure and acidification from atmospheric deposition) and global drivers (i.e. climate warming) had moderate effects. There was no evidence that canopy closure, drainage or eutrophication contributed to herb layer changes. Conclusions: The relative importance of the different drivers as indicated by the winner and loser species differs from that found in previous long‐term studies. Relating species traits to species performance is a valuable tool that provides insight into the environmental drivers that are most likely responsible for herb layer changes.     

Previous experience and contest outcome: winner effects persist in absence of evident loser effects in a parasitoid wasp

Abstract The experience of a previous conflict can affect animals' performance during a later contest: a victory usually increases and a defeat usually decreases the probability of winning a subsequent conflict. These winner and loser effects could result from a reassessment by contestants of their perceived fighting abilities. Game-theoretic models based on this assumption predict that a loser effect can exist alone or in the presence of a winner effect, but a winner effect cannot persist alone, at least when contestants are young and without experience of contest. Moreover, when both effects coexist, the loser effect is expected to be of a greater magnitude and last longer than the winner effect. To date, these predictions have been supported by empirical evidence. Here we show for the first time that a winner effect can exist in the absence of any evident loser effect in a parasitoid wasp, Eupelmus vuilleti, when fighting for hosts. This finding consequently raises questions about the possible mechanisms involved and challenges the main assumption of previous theoretical models. We suggest an alternative explanation for the evolution of only winner effects that is based on the modification of contestants' subjective value of the resource rather than on a reestimation of their fighting abilities.