Recognition of Dominance in the Big-Clawed Snapping Shrimp (Alpheus heterochaelis Say 1818) Part II: Analysis of Signal Modality

Alpheus heterochaelis is able to recognise the dominance status of an opponent (see Part I of this series). A former loser does not fight against a former winner but rather escapes immediately after a contact. However, if a former loser meets an inexperienced opponent, the loser fights against it. Here we investigated the signal used for dominance recognition. Two groups of snapping shrimp that had lost a fight on Day 1, intact animals, and shrimp with cut lateral antennular filaments (i.e. without chemosensory aesthetascs), fought against the same winner on Day 2. Intact losers showed escape behaviour, while losers without aesthetascs showed almost the same aggressive behaviour as on Day 1. The main signal in dominance recognition is therefore a chemical one, possibly the urine or a substance carried by it. The main receptor organs for this signal are the lateral filaments of the antennules carrying the aesthetascs.     

The modality of the dominance signal in snapping shrimp (Alpheus heterochaelis) and the corresponding setal types on the antennules

Previous behavioural experiments showed that snapping shrimp lacking lateral antennular filaments, i.e. without chemosensory aesthetascs, lose the ability to distinguish between conspecifics that are inexperienced in fighting and former winners. A chemosensory dominance signal was assumed to be present, although other receptors unique to the lateral filaments may have been responsible for the behavioural changes. In the present study, the antennules of snapping shrimp were examined for differences between the lateral and medial antennule filaments to identify the modality of the dominance signal. We found six different types of setae and two types of pores. A new probably bimodal setal type is described, the broad long simple seta. Only the chemosensory aesthetascs and their associated hydrosensory companion setae are unique to the lateral filament. Thus we conclude that the dominance signal is chemical, because a hydrodynamic signal would be also received by the simple setae distributed on both filaments.     

The modality of the dominance signal in snapping shrimp (Alpheus heterochaelis) and the corresponding setal types on the antennules

Previous behavioural experiments showed that snapping shrimp lacking lateral antennular filaments, i.e. without chemosensory aesthetascs, lose the ability to distinguish between conspecifics that are inexperienced in fighting and former winners. A chemosensory dominance signal was assumed to be present, although other receptors unique to the lateral filaments may have been responsible for the behavioural changes. In the present study, the antennules of snapping shrimp were examined for differences between the lateral and medial antennule filaments to identify the modality of the dominance signal. We found six different types of setae and two types of pores. A new probably bimodal setal type is described, the broad long simple seta. Only the chemosensory aesthetascs and their associated hydrosensory companion setae are unique to the lateral filament. Thus we conclude that the dominance signal is chemical, because a hydrodynamic signal would be also received by the simple setae distributed on both filaments.     

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.     

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.     

Effects of previous experience on the agonistic behaviour of male crickets, Gryllus bimaculatus

Male solitary animals frequently enter aggressive interactions with conspecific individuals to protect their territory or to gain access to females. After an agonistic encounter, the loser (subordinate individual) changes its behaviour from aggression to avoidance. We investigated agonistic interactions between pairs of male crickets to understand how dominance is established and maintained. Two na?ve males readily entered into agonistic interactions. Fights escalated in a stereotyped manner and were concluded with the establishment of dominance. If individuals were isolated after the first encounter and placed together 15 minutes later, subordinate crickets tended to avoid any further contact with the former dominant opponent. Moreover, subordinate males also avoided unfamiliar dominant and na?ve opponents. They displayed aggressive behaviour only towards unfamiliar subordinate opponents. This suggests that the subordinate male change their behaviour depending on the dominance status of the opponent. Dominant crickets, in contrast, displayed aggressive behaviour towards familiar as well as unfamiliar opponents. If the interval between the first and second encounter was longer than 30 minutes, the former subordinate male showed aggressive behaviour again. However, if the subordinate cricket was paired with the same opponent three consecutive times within 45 minutes, it avoided the former dominant opponent for up to 6 hours following the third encounter. Our results suggest that the maintenance of dominance in male crickets depends largely on the behavioural change of subordinate individuals. Possible mechanisms to maintain dominance are discussed.     

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”.     

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.     

Assessment Strategies and the Effects of Fighting Experience on Future Contest Performance in the Green Anole (Anolis carolinensis)

Social experiences can be useful sources of information for animals charged with making fitness‐related decisions. Fighting experience can alter an animal's perception of its fighting ability possibly leading to changes in future contest decisions, which may increase/decrease their probability of winning future contests. Winner and loser effects have been revealed in a wide array of animals, but studies using reptilian models are rare. This study investigated the impact of fighting experience on future contest performance and outcome in the green anole lizard and investigated the assessment strategies used by anoles during contests of different intensities. To determine whether the green anole expresses winner or loser effects, focal animals engaged in a primary contest with a smaller (larger) opponent to gain a winning (losing) experience; opponent size asymmetries were a significant predictor of contest outcome. Focal individuals were isolated for 2 d before being given a secondary contest with a size‐matched, naïve opponent. We found no evidence of winner or loser effects 2 d following a previous contest. Although previous contest outcome did not dictate future contest success, dynamics of the previous contest did. Highly aggressive primary contest losers won a significant proportion of the secondary contests, while less aggressive losers were more apt to lose the secondary contest. Secondary contest success of prior winners was not influenced by earlier contest performance. Further analyses of contest dynamics reveal that individuals may use different assessment strategies depending on the intensity of the contest. Our results demonstrate that future contest success may be driven more by individual performance in a prior contest and less by prior contest outcome.     

Experimental evidence that winning or losing a fight does not affect sperm quality in a field cricket

Fighting is a powerful social experience that can affect male reproductive behavior, including ejaculatory strategies. Whereas winners may monopolize females, losers may instead perceive high sperm competition and limited future mating opportunities, and accordingly enhance ejaculate quality to maximize their reproductive success. In male field crickets Gryllus bimaculatus that fight aggressively for control of breeding territories, winners are known to possess sperm of lower quality (viability) compared to losers, but it remains unclear whether this is due to short‐term fighting consequences. To test if the fighting experience per se (winning or losing) affects male adjustment of sperm viability, we subjected males to winning and losing experiences by staging fights against size‐matched rivals of known fighting ability. These rivals were males that previously won or lost a fight and, due to “winner‐loser effects” kept winning or losing subsequent contests. We sampled sperm prior and after the fight and twice in control males with no fighting experience and found no differences in sperm viability across measures. We conclude that males do not tailor their ejaculate quality following a single fight, or based on its outcome. Intrinsic differences in other attributes between winners and loser phenotypes may explain differences in sperm quality previously described in this system.     

How winner cells cause the demise of loser cells

Recent results show that, during the process known as cell competition, winner cells identify and kill viable cells from a growing population without requiring engulfment. The engulfment machinery is mainly required in circulating macrophages (hemocytes) after the discrimination between winners and losers is completed and the losers have been killed and extruded from the epithelium. Those new results leave us with the question as to which molecules allow winner cells to recognize and impose cell death on the loser cells during cell competition.     

What sets the odds of winning and losing?

Social experience influences the outcome of conflicts such that winners are more likely to win again and losers will more likely lose again, even against different opponents. Although winner and loser effects prevail throughout the animal kingdom and crucially influence social structures, the ultimate and proximate causes for their existence remain unknown. We propose here that two hypotheses are particularly important among the potential adaptive explanations: the 'social-cue hypothesis', which assumes that victory and defeat leave traces that affect the decisions of subsequent opponents; and the 'self-assessment hypothesis', which assumes that winners and losers gain information about their own relative fighting ability in the population. We discuss potential methodologies for experimental tests of the adaptive nature of winner and loser effects.     

Demonstration of Strength and Concealment of Weakness in Escalating Fights of Male Swordtails (Xiphophorus helleri)

In the green swordtail (Xiphophorus helleri) confrontations between strange males regularly escalate to high levels of mutual Bites and Fin Grips, even between males differing greatly in size. The original expectation of early game theory models that the behaviours of the ultimate winners and losers are indistinguishable until shortly before the end of the fight could not be confirmed. A significant characteristic of loser behaviour is that the Biting rates are higher before and after escalation. Conversely, Fin Grips are more frequent in the ultimate winners than the losers. However, such behavioural differences are very poor predictors of the outcome of the fight from the viewpoint of a single fighting individual during the contest. Correct forecasts did not exceed 67%. The escalating fights of swordtails are considered as trials of strength in which the stronger male tries to demonstrate strength by the most costly behaviour pattern available, namely Fin Grips, and the weaker male conceals weakness quite successfully by countering with the same tactic and suppressing signs of weakness such as Avoidance behaviour.     

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.     

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.     

Victory displays: a game-theoretic analysis

Two rationales have been proposed verbally for the functionof victory displays, which are performed by the winners of contestsbut not by the losers. The "advertising" rationale is that victorydisplays are attempts to communicate victory to other membersof a social group that do not pay attention to contests or cannototherwise identify the winner. The "browbeating" rationale isthat victory displays are attempts to decrease the probabilitythat the loser of a contest will initiate a future contest withthe same individual. We formally explore the logic of theserationales with game-theoretic models. The models show thatboth rationales are logically sound; however, all other thingsbeing equal, the intensity of victory displays will be highestthrough advertising in groups where the reproductive advantageof dominance is low and highest through browbeating in groupswhere the reproductive advantage of dominance is high.     

Familiarity influences body darkening in territorial disputes between juvenile salmon

Escalated contests between animals are potentially costly because of increased energy expenditure and risk of predation or injury. Hence we would expect selection to favour any mechanism that avoids unnecessary prolonged fighting. One such means of avoiding escalated fights could be the use of information gained through individual recognition. Previous work has shown that a darkening of the body colour is closely associated with submission in contests between juvenile Atlantic salmon, Salmo salar, and it has been hypothesized that this may act as a visual signal to the opponent. We tested the hypothesis that body darkening is used to reduce the cost of contests between familiar fish such that losers darken more quickly when faced with familiar than unfamiliar opponents. In contests between unfamiliar fish, submissive darkening occurred after more escalated contests in which the loser incurred more aggression, whereas the opposite occurred when familiar fish were in conflict. In addition familiar fish either submitted quickly or engaged in protracted conflicts in which neither fish signalled submission, whereas in unfamiliar fish contests were of intermediate duration regardless of whether either fish darkened. We suggest that body darkening is used by familiar fish to signal submission to familiar dominants in order to avert a costly escalated fight, but familiarity can lead to escalation without submission if perceived competitive asymmetries are finely balanced. Copyright 2000 The Association for the Study of Animal Behaviour.     

Indirect genetics effects and evolutionary constraint: an analysis of social dominance in red deer, Cervus elaphus

By determining access to limited resources, social dominance is often an important determinant of fitness. Thus, if heritable, standard theory predicts mean dominance should evolve. However, dominance is usually inferred from the tendency to win contests, and given one winner and one loser in any dyadic contest, the mean proportion won will always equal 0.5. Here, we argue that the apparent conflict between quantitative genetic theory and common sense is resolved by recognition of indirect genetic effects (IGEs). We estimate selection on, and genetic (co)variance structures for, social dominance, in a wild population of red deer Cervus elaphus, on the Scottish island of Rum. While dominance is heritable and positively correlated with lifetime fitness, contest outcomes depend as much on the genes carried by an opponent as on the genotype of a focal individual. We show how this dependency imposes an absolute evolutionary constraint on the phenotypic mean, thus reconciling theoretical predictions with common sense. More generally, we argue that IGEs likely provide a widespread but poorly recognized source of evolutionary constraint for traits influenced by competition.     

A biological validation procedure for the measurements of fecal outputs and fecal cortisol metabolites in male Syrian hamsters

Monitoring fecal outputs and fecal cortisol metabolites (FCM), a noninvasive technique, has been used to investigate physiological responses to stress and relationships between hormones and behavior in an increasing number of species. The aim of this study was to investigate whether measurements of fecal outputs and FCM can be used as indexes to repeatedly and precisely monitor stress levels in male Syrian hamsters using a social defeat as a biological validation method. The feces voided by each animal were collected every 3 h for at least 1 day before and after experiencing a single fighting interaction, and the extracted FCM during the pre- and post-fight phases was quantified by enzyme immunoassays. During the pre-fight baseline phase, both the number of fecal pellets and the FCM levels fluctuated throughout the whole day. Although the number of fecal pellets did not differ between the dark and light cycles, the levels of FCM were significantly higher during the dark cycle than during the light cycle. During the post-fight phase, the experience of fighting did not result in a significant difference in the number of fecal pellets per hour between the winner and loser groups, but did considerably increase the total amount of fecal outputs in both groups. The level of FCM was significantly higher in the loser group than in the winner group during the 1st and 7th 3-h collection periods after the fight, which indicated that the experience of defect affected the behavioral and physiological responses of the losers. Our findings suggest that measurement of FCM is sensitive enough to distinguish the stress levels between winners and losers after experiencing a fight. The measurements of fecal outputs and FCM levels provide new opportunities to longitudinally and frequently monitor behavioral and hormonal responses to stress in hamsters and other small laboratory animals.     

Does Lateral Presentation of the Palmate Antlers During Fights by Fallow Deer (Dama dama L.) Signify Dominance or Submission?

A central aim of the study of animal communication is to identify the mode and content of information transferred between individuals. The lateral presentation of the antler palm between male fallow deer has been described as either a signal of individual quality or an attempt to avoid fighting. In the first case two phenotypic features have been proposed by which transmission of individual quality may be facilitated. These are antler size and antler symmetry. The alternative hypothesis proposes that the lateral presentation of antlers occurs as a consequence of averting a threatening posture and may signify a reluctance to fight. We examined whether mature fallow deer use lateral palm presentation as a display during fights to indicate antler size and symmetry. We found no relationship between presentation rate of the antler and antler size and symmetry. Furthermore, males did not preferentially present their larger antler to their opponent. We also investigated whether the rate at which males presented antlers laterally during a fight was related to their ability to win the fight. Our results show that the male who performed more presentations during a fight was more likely to lose it. There were behavioural differences in the way in which a bout of presentation ended; subsequent losers tended to turn their body away from their opponent and subsequent winners tended to lower their antlers to an opponent which we interpret as an invitation to continue fighting. We conclude that the lateral palm presentation serves to de-escalate fighting between mature fallow deer. It is not a mechanism by which to communicate individual quality but rather an indication that a male is less committed to continuing investment in the current contest.