Influence of seeing a female on the male–male interactions of a jumping spider, Hypoblemum albovittatum

An important prediction from game theory is that resource value influences the level to which competitors will escalate conflict. An earlier study considered whether this prediction applies to the male–male interactions of Hypoblemum albovittatum, a jumping spider (Salticidae) from New Zealand. The males of this species escalated conflicts in the presence of a moving mount made from a conspecific female. However, because the control was only a similar-sized motionless cork, an alternative hypothesis (that the cue for escalation is seeing movement of any female-size object, rather than seeing specifically a female) was not ruled out. Here we show that a moving cork, without a mount present, is indeed sufficient to cause males to escalate, but a moving mount (made from a conspecific female) causes males to escalate further. The level of escalation in the presence of a moving mount made from prey (housefly) or from a rival (conspecific male) did not differ significantly from the level of escalation when only a moving cork was present. These findings suggest that, although seeing a moving object similar in size to a conspecific female is sufficient for priming males to escalate, males can also discern by sight that specifically a female is present and, when they have this precise information, they make strategic decisions to escalate conflict further.     

Aggression and RHP in the Northern Swordtail Fish, Xiphophorus cortezi: The Relationship Between Size and Contest Dynamics in Male–Male Competition

Current theory predicts that contest outcome, as well as decisions on whether to initiate a contest, escalate during a contest or retreat are decided by asymmetries in resource holding potential (RHP) and/or expected payoffs between contestants. In this investigation, dyadic contests were staged between male swordtail fish (Xiphophorus cortezi) where individuals were paired based on cumulative fight records and were ranked at the end of the trials in order to approximate RHP. Size was the only asymmetry that I did not attempt to control for and as a result, I was able to determine the relationships between size, contest initiation, escalation and outcome. Individuals changed their contest initiation strategy based on their size relative to that of their opponents, and contrary to predictions, the smaller of the two males in each contest was more likely to initiate the conflict than was the larger male. However, the larger of the two males was more likely to win and standard length proved to be a moderate predictor of an individual's final rank. Regardless of size, initiators fared poorly, winning only 31% of the contests. In instances where the smaller males won the contests, they were no more likely to initiate the encounter than was the larger male. However, when small males did win, fights lasted longer, suggesting that in some cases smaller males may be able to outlast their opponents.     

Canine Reconciliation and Third‐Party‐Initiated Postconflict Affiliation: Do Peacemaking Social Mechanisms in Dogs Rival Those of Higher Primates?

Conflicts over food, access to mates, or other limited resources can sometimes escalate into aggression. In species that form social groups, these aggressive conflicts can jeopardize the benefits of group living, such as enhanced access to valued resources, necessitating the development of behavioural mechanisms that either mitigate conflicts, prevent aggressive escalation or resolve disputes. Two important mechanisms for managing the effects of disputes involve postconflict (PC) affiliative behaviour, either between the former opponents (called ‘reconciliation’) or between one opponent and a third party. Even though numerous studies have tested reconciliatory and third‐party affiliation tendencies in primates, for non‐primate animals little systematic data are available. We performed behavioural observations on three groups of captive domestic dogs (Canis familiaris) and used a PC/MC method to test the existence of reconciliation and third‐party affiliation in this species. The results of this study clearly indicate that both reconciliation and third‐party PC interactions are present in the domestic dog, and form important social mechanisms of the domestic dog. Furthermore, familiar individuals showed a significantly higher proportion of reconciled conflicts than did unfamiliar individuals, and also displayed fewer conflicts. Finally, we show that most of the third‐party affiliations involve the victim of a conflict and that victim‐directed affiliation outweighs victim‐initiated third‐party affiliation in these PC interactions.     

Escalated conflict in a social hierarchy

Animals that live in cooperative societies form hierarchies in which dominant individuals reap disproportionate benefits from group cooperation. The stability of these societies requires subordinates to accept their inferior status rather than engage in escalated conflict with dominants over rank. Applying the logic of animal contests to these cases predicts that escalated conflict is more likely where subordinates are reproductively suppressed, where group productivity is high, relatedness is low, and where subordinates are relatively strong. We tested these four predictions in the field on co-foundress associations of the paper wasp Polistes dominulus by inducing contests over dominance rank experimentally. Subordinates with lower levels of ovarian development, and those in larger, more productive groups, were more likely to escalate in conflict with their dominant, as predicted. Neither genetic relatedness nor relative body size had significant effects on the probability of escalation. The original dominant emerged as the winner in all except one escalated contest. The results provide the first evidence that reproductive suppression of subordinates increases the threat of escalated conflict, and hence that reproductive sharing can promote stability of the dominant-subordinate relationship.     

Resource holding potential, subjective resource value, and game theoretical models of aggressiveness signalling

Empirical evidence suggests that aggressiveness (willingness to enter into, or escalate an aggressive interaction) may be more important than the ability to win fights in some species. Both empirical and theoretical traditions treat aggressiveness as a distinct property from the ability (RHP) or motivation (subjective resource value) to win a fight. I examine how these three traits are clearly distinct when modelled using a simple strategic model of escalation. I then examine game theoretical models of agonistic communication and demonstrate that models in which aggressiveness is signalled require: (1) a trait, aggressiveness, which is neither a correlate, nor consequence of RHP or motivation, (2) a handicap which negates any benefit to be gained through the use of a particular signal, and (3) the absence of any other asymmetry which could be used to assign roles to players. I conclude that it is unlikely that these assumptions are ever met, and that empirical examples of "aggressiveness" are far more likely to represent long-term differences in subjective resource value.     

Delayed song maturation and territorial aggression in a songbird

Asymmetries in competitive ability can determine the outcome of social interactions in animals and are often expressed through differences in sexual traits. Competitive ability (resource holding potential, RHP), trait expression and ultimately reproductive success may vary with an individual's age or experience. In some species, reproductively mature males delay acquisition of some adult traits and thereby signal their young age. Theory on animal contests predicts that individuals assess the RHP of an opponent relative to their own, such that escalation is more common between evenly matched opponents. Here, we test predictions from this hypothesis that males respond to a territorial intruder based on their RHP relative to the intruder's RHP. We simulated white-crowned sparrows (Zonotrichia leucophrys) intruding into the territory of a recruit or return. Playback of a song repertoire simulating a young male (recruit) elicited a weaker response from established territory holders (return), but a stronger response from recruits. Playback of a single song type simulating an older male elicited the opposite responses. This indicates that males distinguished between simulated young and old intruders based on song, and responded differently depending on their own experience. Our study highlights the possibility that receiver as well as sender traits should be considered when interpreting animal interactions.     

UV-Green Iridescence Predicts Male Quality during Jumping Spider Contests

Animal colour signals used in intraspecies communications can generally be attributed to a composite effect of structural and pigmentary colours. Notably, the functional role of iridescent coloration that is ‘purely’ structural (i.e., absence of pigments) is poorly understood. Recent studies reveal that iridescent colorations can reliably indicate individual quality, but evidence of iridescence as a pure structural coloration indicative of male quality during contests and relating to an individual’s resource-holding potential (RHP) is lacking. In age- and size-controlled pairwise male-male contests that escalate from visual displays of aggression to more costly physical fights, we demonstrate that the ultraviolet-green iridescence of Cosmophasis umbratica predicts individual persistence and relates to RHP. Contest initiating males exhibited significantly narrower carapace band separation (i.e., relative spectral positions of UV and green hues) than non-initiators. Asymmetries in carapace and abdomen brightness influenced overall contest duration and escalation. As losers retreated upon having reached their own persistence limits in contests that escalated to physical fights, losers with narrower carapace band separation were significantly more persistence. We propose that the carapace UV-green iridescence of C. umbratica predicts individual persistence and is indicative of a male’s RHP. As the observed UV-green hues of C. umbratica are ‘pure’ optical products of a multilayer reflector system, we suggest that intrasexual variations in the optical properties of the scales’ chitin-air-chitin microstructures are responsible for the observed differences in carapace band separations.     

Reconciliation, consolation and postconflict behavioral specificity in chimpanzees

Conflicts of interest arise regularly in the lives of all group-living animals and may escalate into aggressive conflicts. The costs of aggressive escalation can be reduced through peaceful postconflict interactions. This study investigated the postconflict behavior of 22 adult chimpanzees at Chester Zoo. The occurrence of reconciliation, i.e. the postconflict affiliative reunion between conflict opponents, and consolation, i.e. a postconflict affiliative interaction directed from a third party to the recipient of aggression, were demonstrated. Consolation was more likely to occur in the absence of reconciliation than after reconciliation, and reconciliation was more likely to occur in the absence of consolation than after consolation, supporting the hypothesis that consolation acts as a substitute for reconciliation when the latter fails to occur. Evidence for behavioral specificity, i.e. context-specific use of certain behaviors, was found for both reconciliation and consolation, which, along with high conciliatory tendencies, suggests an explicit style of postconflict behavior in the study subjects.     

Effects of prey–spider odour on intraspecific interactions of araneophagic jumping spiders

An important prediction from game theory is that the value of a resource will influence the level to which an animal is willing to escalate during conflict with conspecific rivals. Here we use this prediction as the rationale for experiments aimed at determining whether escalation decisions made by predators are influenced by the presence of preferred prey. The predators we use are adult individuals of Portia fimbriata, a jumping-spider (Salticidae) species from Queensland (Australia). P. fimbriata is known to prefer other salticid species as prey and can detect the odour of Jacksonoides queenslandicus, this being an especially common salticid species in the Queensland habitat. Here we show that, for P. fimbriata females, the odour of J. queenslandicus primes escalation of vision-based conflict. However, for P. fimbriata males, no comparable effect was found. These findings suggest intrasexual competition for prey is more important to females than to males.     

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.     

The mismeasure of animal contests

Contests between rivals placing similar value on the resource at stake are commonly won by the rival having greater ‘resource holding potential’ (RHP). Mutual assessment of RHP difference between rivals is usually expected as an economical means of resolution; weaker rivals can retreat when they detect their relative inferiority, thereby avoiding costly, futile persistence. Models of contest resolution that entail retreat decisions based on estimates of RHP difference predict that contest duration diminishes as RHP difference between rivals increases because the asymmetry is more readily detected. This prediction appears to have been fulfilled in contests of diverse taxa, generating widespread support for assessment of RHP differences in contests. But few studies have considered alternatives in which each rival simply persists in accord with its own RHP (‘own RHP-dependent persistence’). In contests decided by own RHP-dependent persistence, in which costs accrue only through each rival's own actions, weaker rivals retreat first because they are inherently less persistent, and contest duration depends primarily on the weaker (losing) rival's RHP rather than RHP difference between the rivals. We show here that the analyses most commonly used to detect effects of RHP difference cannot discriminate between these alternatives. Because RHP difference between rivals tends to be correlated with RHP of the weaker rival in a pair, a negative relation between RHP difference and contest duration may be generated even when decisions of retreat are not based on estimated RHP difference. Many studies purporting to show a negative relation between RHP difference and contest duration may actually reflect an incidental association between weaker rival RHP and RHP difference. We suggest statistical and experimental approaches that may help to discriminate between effects of weaker rival RHP and true effects of RHP difference. We also discuss whether ‘true’ negative effects of RHP difference on contest duration always reflect retreat decisions based on estimated RHP differences. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Fight tactics in wood ants: individuals in smaller groups fight harder but die faster

When social animals engage in inter-group contests, the outcome is determined by group sizes and individual masses, which together determine group resource-holding potential ('group RHP'). Individuals that perceive themselves as being in a group with high RHP may receive a motivational increase and increase their aggression levels. Alternatively, individuals in lower RHP groups may increase their aggression levels in an attempt to overcome the RHP deficit. We investigate how 'group RHP' influences agonistic tactics in red wood ants Formica rufa. Larger groups had higher total agonistic indices, but per capita agonistic indices were highest in the smallest groups, indicating that individuals in smaller groups fought harder. Agonistic indices were influenced by relative mean mass, focal group size, opponent group size and opponent group agonistic index. Focal group attrition rates decreased as focal group relative agonistic indices increased and there was a strong negative influence of relative mean mass. The highest focal attrition rates were received when opponent groups were numerically large and composed of large individuals. Thus, fight tactics in F. rufa seem to vary with both aspects of group RHP, group size and the individual attributes of group members, indicating that information on these are available to fighting ants.     

How cross-modality effects during intraspecific interactions of jumping spiders differ depending on whether a female-choice or mutual-choice mating system is adopted

An important prediction from game theory is that the value of a resource influences the level to which conflict escalates. Here we use jumping spiders (Salticidae) to consider this prediction in the context of species adopting different mating systems (‘female mate-choice’ and ‘mutual mate-choice’). Our experiments are designed for determining whether the odour of conspecific females, more than the odour of heterospecific females, primes males to escalate conflict with a potential same-sex rival and also whether the odour of conspecific males, more than the odour of heterospecific males, primes females to escalate conflict with a potential same-sex rival. Four species were studied: Evarcha culicivora, a species in which mutual mate-choice is pronounced, and Portia fimbriata, Portia africana, and Jacksonoides queenslandicus, more conventional salticids in which female mate-choice and male–male competition appear to be dominant. Our hypothesis is that, for all four species, there is strong competition between males for access to females and that, for E. culicivora, but not for the other three species, there is also strong competition between females for access to males. Our findings are consistent with this hypothesis, as we show that, although the odour of conspecific females primes escalation of vision-based male–male conflict for all four species, E. culicivora is the only species for which there is evidence of odour from conspecific males priming the escalation of female–female conflict.     

Assessment strategy and the evolution of fighting behaviour

The view is examined that the adaptive value of conventional aspects of fighting behaviour is for assessment of relative RHP (resource holding power) of the combatants. Outcomes of aggressive disputes should be decided by each individual's fitness budget available for expenditure during a fight (determined by the fitness difference between adoption of alternative strategies, escalation or withdrawal without escalation) and on the rate of expenditure of the fitness budget if escalation occurs (determined by the RHPs of the combatants). Thus response thresholds for alternative strategies (“assessments”) will be determined by natural selection on a basis of which opponent is likely to expend its fitness budget first, should escalation occur. This “loser” should retreat (before escalation) and the winner should stay in possession of the resource. Many aggressive decisions depend on whether one is a resource holder, or an attacker. Assuming the RHP of the combatants to be equal, there are many instances of fitness pay-off imbalances between holder and attacker which should weight the dispute outcome in favour of one or other opponent by allowing it a greater expendable fitness budget. Usually the weighting favours the holder; the attacker therefore needs a correspondingly higher RHP before it may be expected to win. This is not invariably the case, and much observed data fits the predictions of this sort of model. If assessments are perfect and budget expenditure rates exactly predictable, then there would never seem to be any case for escalation. Escalation can be explained in terms of injury inflictions (expenditures) occurring as discrete events; i.e. as “bouts” won or lost during fighting. Assessment can give only a probabilistic prediction of the outcome of a bout. A simple model is developed to investigate escalation situations. Each combatant assesses relative RHP; this correlates with an absolute probability of winning the next bout (c_abs). The stake played for is infliction of loss of RHP and is determined by the fitness budgets of the opponents. (Each individual plays for the withdrawal of its opponent.) This defines a critical probability of winning (c_crit) for each combatant, above which escalation is the favourable strategy (c_abs > c_crit) and below which withdrawal is favourable (c_abs < c_crit). Escalation should occur only where c_abs-c_crit is positive for both combatants. This model gives predictions compatible with the observations, indicating that RHP loss alone can be adequate to explain withdrawal: escalation behaviour. Withdrawal tendency will be increased by low searching costs. Escalations should be restricted to closely matched RHP opponents if RHP disparity is the major imbalance. Outside the “escalation range” of a given individual, the higher RHP individual wins and the lower one loses (i.e. it should withdraw after conventional display). RHP disparity and holder: attacker imbalance should both interact to shape the observed pattern, though their relative importances will depend on species and situation. In some instances selection may favour immediate withdrawal from an occupied territory even without assessment of RHP.     

Ferocious Fighting between Male Grasshoppers

Contests among individuals over mating opportunities are common across diverse taxa, yet physical conflict is relatively rare. Due to the potentially fatal consequences of physical fighting, most animals employ mechanisms of conflict resolution involving signalling and ritualistic assessment. Here we provide the first evidence of ubiquitous escalated fighting in grasshoppers. The chameleon grasshopper (Kosciuscola tristis) is an Australian alpine specialist, in which males engage in highly aggressive combat over ovipositing females. We describe discrete agonistic behaviours including mandible flaring, mounting, grappling, kicking and biting, and their use depending on the individual’s role as challenger or defender. We show that male role predicts damage, with challengers being more heavily damaged than males defending females (defenders). Challengers also possess wider mandibles than defenders, but are similar in other metrics of body size. Our data suggest that fights escalate between males matched in body size and that mandibles are used as weapons in this species. This system represents an exciting opportunity for future research into the evolution of costly fighting behaviour in an otherwise placid group.     

Non‐sex‐biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

Males and females commonly compete for limited resources. When interaction costs are similar for both sexes and there are no sexual differences in resource value estimation, a non‐sex‐biased dominance is expected. Moreover, only non‐sex‐biased assessment of contenders fighting ability (Resource Holding Potential, RHP) should influence contest decisions. To test these predictions, we evaluated non‐breeding agonistic intra‐ and intersexual dyadic interactions in the weakly electric fish, Gymnotus omarorum. During the non‐breeding season, resource value is not expected to depend on individuals’ reproductive status and should thus be equal for males and females. In addition, as G. omarorum presents no sexual differences in body size, interaction costs can be considered symmetric between sexes. We confirmed that body size differences, but not individuals’ gender, is the best predictor of dominance. We correlated RHP asymmetries with contest duration and evidenced that body size but not sex influences assessment in intrasexual and intersexual encounters. All dyads tested engaged in agonistic interactions (N = 33) in which a clear dominant emerged. The analysis of conflict phases evidenced the submissive role of electric displays. Electric organ discharge (EOD) interruptions appear early in the contest as an electric hiding attempt, whereas chirps are post‐resolution signals of subordinate status. Interestingly, the decision of interrupting the EOD was also influenced by RHP asymmetries, whereas chirping activity was influenced by the intensity of the attacks received. Our results confirm that body size is the best RHP proxy in non‐breeding intra‐ and intersexual contests of this monomorphic species and demonstrated a sequential pattern of submissive signalling by means of two different electric displays.     

Arms Races, Conflict Costs and Evolutionary Dynamics

In conflicts and fights, the winner is often determined by the difference in resource-holding potential, e.g. size, weaponry, strength (RHP). I model the evolution of RHP in a symmetric game with continuous strategies. I show that there is a convergence stable ESS level of RHP if the cost of the trait increases faster than linearly, and that this is the only solution if the cost increases fast enough with RHP. Otherwise with slowly increasing cost, the solution is a cyclically fluctuating level of RHP. It is also shown that if the cost increases linearly with RHP, the only solution to the game is neutrally stable cycles, the amplitude determined by the initial conditions. The cycles come about because in a population drawn into an arms race of RHP, individuals after a while suffer costs so high that mutants with the lowest possible armament level may invade. Copyright 1999 Academic Press.     

Intruder Pressure Affects Territory Size and Foraging Success in Asymmetric Contests in the Water Strider Gerris lacustris

The theory of evolutionarily stable strategies (ESS) in asymmetric contests predicts that the propensity of an individual to expose itself to risk during contests depends on the individual's resource-holding-potential (RHP) and on the value of the disputed resource (V) for the individual compared with that for an opponent. If encounters of a territory owner with individuals of high RHP and high food demands (V) increase in frequency, one should expect a decrease in total aggressiveness of the territory owner, and in consequence a decrease of its territory size. Such a decrease should result in a lower amount of food consumed by the territory owner. Using natural variability in RHP and V in Gerris lacustris, I experimentally tested these predictions. The average prey item has higher value (V) for reproductive female water striders (which probably transform most of their food into eggs), than for nonreproductive females and for males. Because males are smaller, they have lower RHP than females, as RHP depends on size. Thus the reproductive females are the class of individuals of high RHP and high food demands (high V). Most nonreproductive females defend food-based territories. I observed two groups of water striders in a seminatural laboratory setting. As predicted, there was a negative correlation between the rate of encounter with reproductive females and size of the territory, and a positive correlation between territory size and number of Drosophila flies consumed by the owner. Territories were smaller in the group with high rates of encounter between territory owners and reproductive females. Territory owners caught the same number of Drosophila flies as non-territorial individuals in this group. In contrast, in the group with fewer encounters between territory owners and reproductive females, territories were larger, and territory owners gained more food than non territorial water striders.     

Male contest investment changes with male body size but not female quality in the spider Nephila clavipes

Animals use rules to adjust their level of investment in a contest. We evaluate male strategies during contests over females in the golden orb-web spider Nephila clavipes. We tested whether male behaviour changes with female value, and found that contests were similar in intensity and outcome whether the female was a juvenile or adult, virgin or non-virgin, or whether one male had invested sperm in the female. We found evidence that males use a self-assessment strategy when deciding to withdraw from a contest. Loser body size and contestant size difference were correlated with a higher frequency of contest escalation, and fights involving two large males were more likely to escalate than a fight in which one male was small. A multiple regression showed that loser body size had a stronger effect on contest escalation than contestant body size difference. More importantly, the size of the winning male had no effect on contest escalation, a key prediction of a self-assessment strategy. In N. clavipes, body size is the primary factor that determines the outcome of male contests, and males do not appear to assess their opponent or the quality of the resource when deciding to withdraw from the fight.     

Origins of behavioural variability: categorical and discriminative assessment in serial contests

Can social behaviour be probabilistic? Classical results in evolutionary game theory seem to suggest that recognizable asymmetries between individuals in the likelihood of winning fights (i.e. in resource holding power, RHP) rule out the stable probabilistic behaviour associated with mixed strategies. Here, using a variant of the hawk-dove game, I show that these mixed strategies can be common in asymmetric contests, because opponents of similar RHP benefit by ignoring discernable differences to form RHP equivalence categories. This process of categorizing (i.e. opting not to distinguish an opponent's discernably different RHP from one's own) can be adaptive when assessment is imprecise or expensive and mistakes can lead to dangerous combat. For large RHP differences between opponents, discriminating by acting on the discernable RHP difference is evolutionarily stable. For moderate RHP differences, typically neither categorizing nor discriminating is stable, but categorizing predominates if social interactions are frequent. More frequent interactions imply that the social status ultimately achieved is more important in accumulating fitness; this reduces the probability that a loser in combat will accept the submissive role without additional fighting, where this probability of ‘surrender’ expresses another mixed strategy. These patterns appear to fit the establishment of social relationships in some species for which combat is dangerous, and the analysis represents a step towards understanding the emergence of dominance hierarchies.