Analysis of the finescale timing of repeated signals: does shell rapping in hermit crabs signal stamina?

Hermit crabs, Pagurus bernhardus, sometimes exchange shells after a period of shell rapping, when the initiating or attacking crab brings its shell rapidly and repeatedly into contact with the shell of the noninitiator or defender in a series of bouts. Bouts are separated by pauses, and raps within bouts are separated by very short periods called 'gaps'. Since within-contest variation is missed when signals are studied by averaging performance rates over entire contests, we analysed the fine within-bout structure of this repeated, aggressive signal. We found that the pattern is consistent with high levels of fatigue in initiators. The duration of the gaps between individual raps increased both within bouts and from bout to bout, and we conclude that this activity is costly to perform. Furthermore, long pauses between bouts is correlated with increased vigour of rapping in the subsequent bout, which suggests that the pause allows for recovery from fatigue induced by rapping. These between-bout pauses may be assessed by noninitiators and provide a signal of stamina. Copyright 2000 The Association for the Study of Animal Behaviour.     

Flexing the abdominals: do bigger muscles make better fighters?

Animal contests often involve the use of repeated signals, which are assumed to advertise stamina, and hence fighting ability. While an individual may be predicted to give up once it has crossed an energetic threshold, costs inflicted by its opponent may also contribute to the giving-up decision. Therefore, physical strength should be of key importance in contests, allowing high signal magnitude as well as potentially inflicting costs. We investigated this using hermit crab shell fights, which employ a 'hybrid signal' of shell rapping, which advertises stamina but also imposes potentially deleterious consequences for the receiver. We examined the links between contest outcomes and two proxies for strength; the protein content and relative mass of hermit crab abdominal muscles, the main muscle group used in shell rapping. Our results indicate that there was no difference in muscle protein between winners and losers, whereas winners had significantly greater muscle mass : body mass ratios. Thus, while stamina has been assumed by theory to be an important determinant of agonistic success, the present results demonstrate the importance of muscle size and thereby strength.     

Power of shell-rapping signals influences physiological costs and subsequent decisions during hermit crab fights

Understanding the costs of signals used in fights is the key to understanding decisions made by contestants. Hermit crabs use shell rapping. This is a clearly defined agonistic signal, which can be quantified in temporal terms and in the power of the key shell-rapping signal component. We examine the relationship between the power expended by attacking hermit crabs and their consequent lactate levels. High power expenditure over the whole fight leads to high lactate, and attackers give up when lactate is high. Some defenders give up early in fights, particularly if the power of raps in early bouts they receive is high. These defenders and those not allowed to fight have low glucose, but those that successfully resist eviction have high glucose. Glucose is mobilized in an attempt to resist; nevertheless, some defenders that attempt resistance are still evicted by persistent attackers. Thus, early power of the signal is a major determinant of success for attackers, albeit at a cost. These data show the link between power, repetition of a signal, metabolic consequences and decisions of contestants in fights. The different activities, decisions and costs of the two roles are not adequately described by existing models of contests.     

Analysis of repeated signals during shell fights in the hermit crab Pagurus bernhardus

Shell exchanges between hermit crabs may occur after a period of shell rapping, when the initiating or attacking crab brings its shell rapidly and repeatedly into contact with the shell of the non-initiator or defender, in a series of bouts. There are two opposing models of hermit crab shell exchange and the function of shell rapping. The negotiation model views shell exchange as a mutualistic activity, in which the initiator supplies information about the quality of its shell via the fundamental frequency of the rapping sound. The aggression model views shell rapping as either detrimental to the defending crab, or as providing it with information about the initiator''s ability or motivation to continue, or both. The negotiation model makes no predictions about the temporal pattern of rapping, but under the aggression model it would be expected that crabs that rapped more vigorously would be more likely to effect an exchange. Repeating the signal could be expected under either model. Crabs that achieve an exchange rap more vigorously, rapping is more persistent when a clear gain in shell quality may be achieved, and the vigour is greater when the relative resource-holding potential (or ''fighting ability'') is high. These findings support the aggression model rather than the negotiation model. Contrary to the predictions of game theory, crabs that do not effect an exchange appear to signal that they are about to give up. The data suggest that rapping is performed repeatedly because the accumulation of all of the performances acts as a signal of stamina.     

The power of shell rapping influences rates of eviction in hermit crabs

Hermit crabs fight for ownership of shells, and shell exchangemay occur after a period of shell rapping, involving the initiatingor attacking crab bringing its shell rapidly and repeatedlyinto contact with the shell of the noninitiator or defender,in a series of bouts. The temporal pattern of rapping containsinformation about the motivation and/or relative resource holdingpotential (RHP) of the initiator and acts as a repeated signalof stamina. Here we investigated the role of the force withwhich the rapping is performed and how this is related to thetemporal pattern of rapping by rubberizing the external surfaceof shells. Initiators that are prevented from rapping withtheir usual level of force persist with the activity for longer over the whole encounter but use fewer raps per bout and areless likely to effect an exchange than those supplied withcontrol shells. The fact that the force of rapping affectsthe likelihood of a crab being victorious suggests that eitherthe force of rapping contains information about motivation orRHP or that force directly affects noninitiators, reducingtheir ability to maintain an adequate grip on their shells.The data suggest that shell rapping is an agonistic signalrather than one that provides information useful to the noninitiator,as has been suggested by the negotiation model of shell exchange.     

Decision rules, energy metabolism and vigour of hermit-crab fights

Aggressive interactions between animals are often settled by the use of repeated signals that reduce the risk of injury from combat but are expected to be costly. The accumulation of lactic acid and the depletion of energy stores may constrain activity rates during and after fights and thus represent significant costs of signalling. We tested this by analysing the concentrations of lactate and glucose in the haemolymph of hermit crabs following agonistic interactions over the ownership of the gastropod shells that they inhabit. Attackers and defenders play distinct roles of sender and receiver that are fixed for the course of the encounter. Attackers perform bouts of 'shell rapping', which vary in vigour between attackers and during the course of the encounter, and are a key predictor of victory. In contrast to the agonistic behaviour of other species, we can quantify the vigour of fighting. We demonstrate, to our knowledge for the first time, an association between the vigour of aggressive activity and a proximate cost of signalling. We show that the lactate concentration in attackers increases with the amount of shell rapping, and that this appears to constrain the vigour of subsequent rapping. Furthermore, attackers, but not defenders, give up when the concentration of lactate is high. Glucose levels in attackers also increase with the amount of rapping they perform, but do not appear to influence their decision to give up. Defenders are more likely to lose when they have particularly low levels of glucose. We conclude that the two roles use different decision rules during these encounters.     

Cumulative or sequential assessment during hermit crab shell fights: effects of oxygen on decision rules

Agonistic interactions between animals are often settled by the use of repeated signals which advertise the resource-holding potential of the sender. According to the sequential assessment game this repetition increases the accuracy with which receivers may assess the signal, but under the cumulative assessment model the repeated performances accumulate to give a signal of stamina. These models may be distinguished by the temporal pattern of signalling they predict and by the decision rules used by the contestants. Hermit crabs engage in shell fights over possession of the gastropod shells that they inhabit. During these interactions the two roles of signaller and receiver may be examined separately because they are fixed for the duration of the encounter. Attackers rap their shell against that of the defender in a series of bouts whereas defenders remain tightly withdrawn into their shells for the duration of the contest. At the end of a fight the attacker may evict the defender from its shell or decide to give up without first effecting an eviction; the decision for defenders is either to maintain a grip on its shell or to release the shell and allow itself to be evicted. We manipulated fatigue levels separately for attackers and defenders, by varying the oxygen concentration of the water that they are held in prior to fighting, and examined the effects that this has on the likelihood of each decision and on the temporal pattern of rapping. We show that the vigour of rapping and the likelihood of eviction are reduced when the attacker is subjected to low oxygen but that this treatment has no effect on rates of eviction when applied to defenders. We conclude that defenders compare the vigour of rapping with an absolute threshold rather than with a relative threshold when making their decision. The data are compatible with the cumulative assessment model and with the idea that shell rapping signals the stamina of attackers, but do not fit the predictions of the sequential assessment game.     

Information transfer during shell exchange in the hermit crab Clibanarius antillensis

The shell exchange behaviour of the hermit crab Clibanarius antillensis was examined for evidence that the non-initiating crab used information about the initiator's shell. Manipulation of the internal volume of the initiator's shell altered the outcome of exchanges in a manner consistent with the negotiations model of resource exchange. It is suggested that the fundamental frequency of shells is detected by crabs during rapping behaviour when the initiator raps its shell against that of the non-initiator.     

What Determines the Duration of War? Insights from Assessment Strategies in Animal Contests

Interstate wars and animal contests both involve disputed resources, restraint and giving up decisions. In both cases it seems illogical for the weaker side to persist in the conflict if it will eventually lose. In the case of animal contests analyses of the links between opponent power and contest duration have provided insights into what sources of information are available to fighting animals. I outline the theory of information use during animal contests and describe a statistical framework that has been used to distinguish between two strategies that individuals use to decide whether to persist or quit. I then apply this framework to the analysis of interstate wars. War duration increases with the power of winners and losers. These patterns provide no support for the idea that wars are settled on the basis of mutual assessment of capabilities but indicate that settlement is based on attrition. In contrast to most animal contests, war duration is as closely linked to the power of the winning side as to that of the losing side. Overall, this analysis highlights a number of similarities between animal contests and interstate war, indicating that both could be investigated using similar conceptual frameworks.     

A conflicting-tendency model of spider agonistic behaviour: Hybrid-pure population line comparisons

Contests over web-sites in the spider, Agelenopsis aperta, have been simulated by a model in which the next act of a spider is determined by the value of two ‘drives’ or ‘tendencies’, to attack and to flee. Such a model can explain the fact that F₁ hybrids between two geographical races of A. aperta are not intermediate between their parents, but are more aggressive than either. The model also accounts for the following features of the observed contests: (1) contests consist of a series of bouts, during which the spiders perform increasingly aggressive acts; (2) contests are usually won by the larger spider; (3) if sizes are equal, contests are usually won by the owner; (4) contests over excellent-quality sites are more costly than over poor-quality sites; and (5) transition probabilities for de-escalation, matching and escalation with respect to prior acts show site quality and weight bias trends. It is argued that a two-tendency model is the simplest that would account for these observations. The model also accurately simulates spider behaviour in agonistic contexts not used in its development (i.e. round-robin contests involving opponents from different populations, and a group of contests involving unequally-matched hybrid opponents).The main discrepancy between the model and observed encounters is that the model predicts that, if the owner is larger, the cost of a contest will not vary with the value of the disputed site. This is because the smaller intruder, having no knowledge of site quality (Riechert 1984) is expected to withdraw early in the contest, regardless of site quality. In the actual contests over excellent sites, estimated costs are higher even when the owner is larger and the intruder loses. The change in intruder behaviour associated with site value occurs following the first series of actions, but within the first bout of the interactions: our analyses indicate that any information conveyed by the owner concerning site value is not through overt behaviour (i.e. through the frequencies with which specific acts are performed).     

Sexual dominance in hermit crab shell fights: asymmetries in owner–intruder status,crab size,and resource value between sexes

We studied sexual dominance and seasonal differences in aggressiveness of individuals in intraspecific competition for shells of the hermit crab Pagurus filholi in terms of size of contestants and duration of the attempt to deprive other crabs of their shell. Experiments were conducted using paired intrasexual and intersexual contests in the pre-breeding and post-breeding seasons. Size ratios between contestants were systematically varied to assess the sexual difference in size and owner advantages. In both intrasexual and intersexual contests intruder crabs tended to win the contests more often as their size increased, that is, size advantage overcame owner advantage. Although we did not recognize a sexual difference in size and owner advantages in contest outcomes, male intruder crabs took a shorter time to deprive female owners of a shell than to deprive male owners. Furthermore, male individuals in the pre-breeding season had significantly longer fight durations. Fighting is costly. Thus males can afford to expend more energy and time fighting, indicating that males are dominant over females in shell fights as both intruders and owners. Electronic Publication     

Fighting for shells: how private information about resource value changes hermit crab pre-fight displays and escalated fight behaviour

Pre-fight displays typically provide honest, but sometimes dishonest, information about resource holding potential and may be influenced by assessment of resource value and hence motivation to acquire the resource. These assessments of potential costs and benefits are also predicted to influence escalated fight behaviour. This is examined in shell exchange contests of hermit crabs in which we establish an information asymmetry about a particularly poor quality shell. The poor shell was created by gluing sand to the interior whereas control shells lacked sand and the low value of the poor shell could not be accurately assessed by the opponent. Crabs in the poor shell showed changes in the use of pre-fight displays, apparently to increase the chances of swapping shells. When the fights escalated, crabs in poor shells fought harder if they took the role of attacker but gave up quickly if in the defender role. These tactics appear to be adaptive but do not result in a major shift in the roles taken or outcome. We thus link resource assessment with pre-fight displays, the roles taken, tactics used during escalation and the outcome of these contests.     

Dynamics of intersexual conflict over precopulatory mate guarding in two populations of the isopod Idotea baltica

Aggressiveness during intersexual conflicts is predicted to depend on its costs, the value of winning and the power asymmetry of the contestants, all of which may vary between populations. In the marine isopod Idotea baltica (Pallas) a conflict occurs as females resist the attempts by males to start precopulatory mate guarding. We analysed contest dynamics with respect to female maturity stage, that is, to time left to reproductive moult, with which the payoffs of guarding for males and females change. We did this in two populations that differ in synchrony of reproduction, sex ratio and the degree of sexual dimorphism. The intensity and dynamics of contests differed between populations: in the more size-dimorphic population, females, the smaller sex, resisted less by forceful flexing but more by hooking their body than in the other population. Male aggressiveness stayed at a constant level with respect to female maturity. In the less size-dimorphic population, female resistance by flexing was intense and it decreased, while male persistence increased, with the approaching reproductive moult. Contests were more intense with small than with large males. These results fit well with the predictions from models of conflict behaviour. Assessment of the payoffs of winning versus contest costs, coadaptation of the level of aggressiveness to the other traits affecting contest outcome, and counteradaptations by the sexes to each other largely explain the dynamics and between-population differences of these contests. Copyright 2000 The Association for the Study of Animal Behaviour.     

Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes

Experience in aggressive contests often affects behaviour during, and the outcome of, later contests. This review discusses evidence for, variations in, and consequences of such effects. Generally, prior winning experiences increase, and prior losing experiences decrease, the probability of winning in later contests, reflecting modifications of expected fighting ability. We examine differences in the methodologies used to study experience effects, and the relative importance and persistence of winning and losing experiences within and across taxa. We review the voluminous, but somewhat disconnected, literature on the neuroendocrine mechanisms that mediate experience effects. Most studies focus on only one of a number of possible mechanisms without providing a comprehensive view of how these mechanisms are integrated into overt behaviour. More carefully controlled work on the mechanisms underlying experience effects is needed before firm conclusions can be drawn.Behavioural changes during contests that relate to prior experience fall into two general categories. Losing experiences decrease willingness to engage in a contest while winning experiences increase willingness to escalate a contest. As expected from the sequential assessment model of contest behaviour, experiences become less important to outcomes of contests that escalate to physical fighting.A limited number of studies indicate that integration of multiple experiences can influence current contest behaviour. Details of multiple experience integration for any species are virtually unknown. We propose a simple additive model for this integration of multiple experiences into an individual's expected fighting ability. The model accounts for different magnitudes of experience effects and the possible decline in experience effects over time.Predicting contest outcomes based on prior experiences requires an algorithm that translates experience differences into contest outcomes. We propose two general types of model, one based solely on individual differences in integrated multiple experiences and the other based on the probability contests reach the escalated phase. The difference models include four algorithms reflecting possible decision rules that convert the perceived fighting abilities of two rivals into their probabilities of winning. The second type of algorithm focuses on how experience influences the probability that a subsequent contest will escalate and the fact that escalated contests may not be influenced by prior experience. Neither type of algorithm has been systematically investigated.Finally, we review models for the formation of dominance hierarchies that assume that prior experience influences contest outcome. Numerous models have reached varied conclusions depending on which factors examined in this review are included. We know relatively little about the importance of and variation in experience effects in nature and how they influence the dynamics of aggressive interactions in social groups and random assemblages of individuals. Researchers should be very active in this area in the next decade. The role of experience must be integrated with other influences on contest outcome, such as prior residency, to arrive at a more complete picture of variations in contest outcomes. We expect that this integrated view will be important in understanding other types of interactions between individuals, such as mating and predator-prey interactions, that also are affected significantly by prior experiences.     

All by myself? Meta‐analysis of animal contests shows stronger support for self than for mutual assessment models

Since the 1970's, models based on evolutionary game theory, such as war of attrition (WOA), energetic war of attrition (E‐WOA), cumulative assessment model (CAM) and sequential assessment model (SAM), have been widely applied to understand how animals settle contests. Despite the important theoretical advances provided by these models, empirical evidence indicates that rules adopted by animals to settle contests vary among species. This stimulated recent discussions about the generality and applicability of models of contest. A meta‐analysis may be helpful to answer questions such as: (i) is there a common contest rule to settle contests; (ii) do contest characteristics, such as the occurrence of physical contact during the fight, influence the use of specific contest rules; and (iii) is there a phylogenetic signal behind contest rules? To answer these questions, we gathered information on the relationship between contest duration and traits linked to contestants' resource holding potential (RHP) for randomly paired rivals and RHP‐matched rivals. We also gathered behavioural data about contest escalation and RHP asymmetry. In contests between randomly paired rivals, we found a positive relationship between contest duration and loser RHP but did not find any pattern for winners. We also found a low phylogenetic signal and a similar response for species that fight with and without physical contact. In RHP‐matched rivals, we found a positive relationship between contest duration and the mean RHP of the pair. Finally, we found a negative relation between contest escalation and RHP asymmetry, even though it was more variable than the other results. Our results thus indicate that rivals settle contests following the rules predicted by WOA and E‐WOA in most species. However, we also found inconsistencies between the behaviours exhibited during contests and the assumptions of WOA models in most species. We discuss additional (and relatively untested) theoretical possibilities that may be explored to resolve the existing inconsistencies.     

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.     

Same resource,different benefits: hermit crab shell structure advantages owners,but not intruders in agonistic interactions

Resources may confer advantages by enhancing their owners’ fighting ability (resource holding potential; RHP). While the resource-correlated RHP hypothesis has been recognized as a determinant of agonistic success in different taxa, this has mostly been based on assessment of either the intruder or the owner, but only rarely in both contestants. We tested whether the internal structure of shells affects hermit crabs’ RHP, both as owners defending the shell against eviction and as intruders attempting to gain access to an occupied shell. We conducted contests (n?=?60) to compare the success in shell eviction by intruders in intact shells vs. in shells with the columella artificially reduced, and the success of shell retention by owners in intact shells vs. shells with the columella reduced. The internal configuration of the shell showed different resource-correlated RHP effects depending on the individual’s role in the fight. The presence of a columella in the intruder’s shell did not affect the likelihood that they would evict their opponents. However, owners resisted more evictions in shells with intact columella than those in shells with reduced columella. Our results demonstrate that the same resource can offer different RHP advantages to owners and intruders during an agonistic interaction.

     

Ultraviolet signals fighting ability in a lizard

Ultraviolet (UV) signals are used in female mate choice in numerous taxa; however, the role of UV signals in male contests remains relatively unexplored. We experimentally reduced throat UV of free-ranging lizards (Platysaurus broadleyi) to test whether UV acts as a signal of fighting ability during male contests. We found that UV-reduced males were more likely to be challenged than control males. However, contest outcome was not influenced by UV-reduction, and this was despite other obvious asymmetries between opponents, such as body size and residency. Throat UV was confirmed as a signal of fighting ability because contests were more likely to escalate when one contestant had reduced UV. Therefore, throat UV, not body size or residency, was used during the initial stage of opponent assessment, but this did not influence contest outcome. The results suggest that UV overrides other traits that could function as signals during rival assessment.     

Song rate as a signal of male aggressiveness during territorial contests in the wood warbler

Aggressive signaling is an important component in animal communication, as it provides an efficient mechanism for settling conflicts over resources between competitors. In songbirds, a number of singing behaviors have been proposed to be aggressive signals used in territory defense, including song rate. Although aggressive signaling in songbirds has received considerable research attention, adequate evidence for most putative aggressive signals is not available. In this study, we experimentally investigated whether the song rate of male wood warblers Phylloscopus sibilatrix is a signal of their aggressive intent in male–male interactions. We found that males responded differentially to simulated territorial intrusions depending on the song rate of an intruder. Moreover, males that continued to sing during territorial contests increased their song rates, and this behavior predicted the strength of aggressive escalation by the signaler. These results suggest that song rate is an aggressive signal during male–male interactions in the wood warbler. We also found high intra‐individual repeatability in the strength of aggressive response to simulated intrusions, likely reflecting differences in personality (aggressiveness) or quality of male wood warblers. We conclude that changes in singing rate may be an efficient mechanism of signaling immediate shifts in motivation of signalers during territorial contests, especially in species that lack large repertoires or have simple songs.     

Shell fights in the hermit crabPagurus geminus: Effect of cheliped use and shell rapping

In shell fights of the hermit crab,Pagurus geminus, frequently it is observed that large crabs (attackers) grasp the thoracic appendage of small crabs (defenders) with the major cheliped and pull the smaller crabs out of their shells. If this is a standard occurrence and result, then the interaction should not be called a “negotiation” (Hazlett 1978). The role of cheliped use by the attckers in the eviction of defenders was therefore studied using crabs with tubes on their chelipeds, and the effect of shell rapping, which is thought to be necessary for eviction, was studied using crabs without shells. The experimental crabs evicted the defenders but fighting was significantly prolonged. Therefore, the negotiation model cannot be rejected. Specific aspects of shell fights in hermit crabs are discussed.