Contest Resolution Research Articles

Wait or fight? Ownership asymmetry affects contest behaviors in a parasitoid wasp

Contests for indivisible resources are often settled on the basis of the asymmetry that exists between contestants in their ownership status: Owners (i.e., individuals already possessing a resource) usually retain the resource when facing intruder competitors. This owner advantage is predicted to be more pronounced as the asymmetry in ownership status accentuates: when the residency period increases, the probability of owners keeping their resource should increase and contestant aggressiveness should be reduced. In the present study, we tested this prediction in a parasitoid wasp, Eupelmus vuilleti, where females fight for hosts, larvae of Callosobruchus maculatus, on which to lay their eggs. To do so, we experimentally varied the arrival times of 2 conspecific females on a host, generating different asymmetries in ownership status between them. We further explored the strategies adopted by intruder females according to the stage reached by the owners in their host exploitation sequence. When the residency period was short, both females fought and contests were won by the female valuing more greatly the host, rather than by the owner per se. The loser left the host vicinity and the winner exploited the host alone. In contrast, when the residency period was long (the owners were ready to parasitize the host), the intruders did not leave but instead waited for the departure of the resident female in order to superparasitize the host. This “waiting strategy” seems particularly adaptive in this species, in which females tend to prefer already parasitized hosts to healthy hosts. Key words: contest resolution, Eupelmus vuilleti, female egg load, ownership status, resource value. [Behav Ecol]

Influence of previous residency and body mass in the territorial contests of the butterfly Hermeuptychia fallax (Lepidoptera: Satyrinae)

Although many studies have evaluated the possible rules used by males to settle territorial contests, few have simultaneously considered the possibility of mutual or self-assessment of fighting ability and the effect of prior residence on the chances of winning. Here, we experimentally investigated the influence of previous residency on the development and outcome of contests between males of the butterfly Hermeuptychia fallax. By comparing the frequency of resident victories in nonmanipulated disputes versus interactions in which both rivals were induced to behave as residents, we showed that prior residency may interact with body mass in contest resolution. In nonmanipulated disputes, resident males showed high winning probabilities, although they were similar to losers in body mass. However, when both males behaved as residents, the disputes were longer and were settled in favor of the heavier male. This indicates that, although residency is an important component of male willingness to fight, it is not the only factor influencing conflict resolution. Despite the correlation between body mass and territorial success, the weight of neither winners nor losers correlated with fighting duration. We conclude that residence asymmetry may sometimes conceal male traits important to contest resolution and that it may obscure the identification of the assessment strategy used in contests.

An experimental test of the effect of signal size and performance capacity on dominance in the green anole lizard

Summary1. Many animals use signals to resolve disputes over resources. Some signals act as reliable indicators of other traits, such as whole‐organism performance or body condition, which may also be important for resolving disputes. Because of the correlations inherent in reliable signals, it is challenging to determine which variables are directly relevant for resolving aggressive interactions.2. We examined the relationships among dewlap size, bite force and condition, all traits that may be important to conflict resolution in male green anole lizards (Anolis carolinensis). Using a large sample of wild‐caught animals, we showed significant positive correlations between dewlap size and maximum bite force capacity, when each trait is corrected for its correlation with body size.3. We tested the relative importance of each trait to the outcome of interactions in a subset of our sample. We staged dominance encounters between size‐matched male green anoles after surgically reducing the dewlap size of one competitor.4. We show that reducing the size of the dewlap does not significantly change the outcome of staged interactions. Rather, males with higher values of bite force capacity were more likely to win fights. We hypothesize that during close‐proximity aggressive interactions, male green anoles use more direct means of assessing one another and that dewlap size functions as a signal of bite force primarily during long‐distance territorial displays. Body condition was correlated with bite force, but did not differ significantly between winners and losers.5. Our results show how an experimental approach can decouple reliable signals from their correlated traits to test which factors influence male contest resolution.

Body morphologies shape territorial dominance in the satyrine butterfly Lethe diana

Male butterflies compete over mating territories via aerial interactions. It has often been suggested that flight performance, and thus flight-related traits, would play an important role in butterflies’ contest resolution; however, most studies have not demonstrated a clear relationship between these traits and territory ownership in butterflies. Males of the satyrine butterfly Lethe diana compete over territories via linear chases, a behavior that is uncommon among butterflies: rather, contests of most butterfly species consist of circling flights. In the present study, some morphological and physiological traits of L. diana were compared between territory owners and intruders to search for traits correlated with resource holding power (RHP). Territory owners had higher flight-muscle ratio (FMR) and were heavier than intruders, indicating that flight performance plays a key role in their contest resolution. Contest duration was not affected by the owners’ FMR or body mass, and thus I obtained no evidence supporting the hypothesis that contestants assess their relative RHP before making a decision to retreat. I speculate that the fact that these traits had an effect on territorial status in L. diana was a result of the characteristics of the contest behavior of this butterfly. During their contests, males L. diana fly faster in their linear flights than males of most other butterflies, which engage in slower circling flights. Therefore, males of L. diana probably need higher flight performance capabilities in order to win their contests.

Can subjective resource value affect aggressiveness and contest outcome in parasitoid wasps?

Although animal contests for resources are often settled in favour of individuals with the greatest fighting abilities, other factors, such as those affecting the value that contestants place on the resource, can also influence contest resolution and aggressiveness. For example, competitors should fight more fiercely for access to larger than smaller food items. However, this also depends on contestants’ subjective value of the resource: individuals deprived of a given resource should value it more highly than individuals that recently had access to it. They should therefore be more aggressive and win contests more frequently. In some parasitoid species, adult females fight for hosts on which to lay eggs and feed. Using the parasitoid Eupelmus vuilleti, we investigated the effect on contest outcome of two factors affecting females’ subjective host value: the quality of the habitat previously encountered by females (either rich or poor in hosts) and females’ physiological state (i.e. the number of ready-to-lay eggs in their ovaries). Subjective host value should increase with both host scarcity and egg load. We first described the agonistic behaviours displayed by E. vuilleti females during contests and then verified that difference in habitat quality was perceived by females (they modified their host exploitation behaviours accordingly) but did not affect their egg load, so that the effects of both factors could be separated. Both habitat quality experience and egg load affected contest outcomes, with egg load asymmetry being the main factor in this species. Our results support game theoretical predictions.

Self-assessment by males during energetically costly contests over precopula females in amphipods

In animal contests, individuals can either engage in mutual assessment of both their own and their opponent's resource-holding potential (RHP) and adjust their behaviour according to estimated differences, or instead persist in accordance with thresholds determined by assessment of just their own RHP. We examined the predictions of alternative mutual assessment and self-assessment models for decision rules in contest resolution during struggles between males over females in precopula in the amphipod Gammarus pulex. Contest duration was positively related to the weight of the loser but not the weight of the winner. Our results support the hypothesis that males rely on information about their own RHP in determining contest behaviour and do not use information about their opponent. Fighting was energetically costly, and energy reserves were depleted during contests. Contest duration was associated with the physiological state of the loser (but not the winner) at the end of the contest, and to a lesser extent his size, further supporting self-assessment.

Social interactions tune aggression and stress responsiveness in a territorial cichlid fish ( Archocentrus nigrofasciatus)

We examined the relative influences of pre-fight housing condition, contest intensity, and contest outcome in modulating post-fight stress hormone concentrations in territorial male convict cichlids ( Archocentrus nigrofasciatus). Individuals were housed either in isolation or in semi-natural communal tanks. Pairs of male cichlids that differed considerably in body mass were selected from the same housing regime. Pre-fight water-borne cortisol levels were obtained before allowing the dyad to interact until contest resolution, after which time post-fight cortisol levels were obtained from the winner and loser. There were no outcome-related differences in post-fight cortisol concentrations following escalated or non-escalated contests, a result that held true for both housing regimes. Pre-fight cortisol levels were significantly higher than post-fight cortisol levels, suggesting that initial confinement in a beaker for the water-borne hormone samples was a stressor, but that the animals acclimated quickly to confinement. Fights involving previously isolated participants were significantly more intense than those involving group-housed animals, which we explain as being a function of established relationships between social isolation, heightened acute cortisol responsiveness, and the expression of excessive aggressive behavior. Only group-housed losers demonstrated the ability to modulate aggression or hypothalamic–pituitary–interrenal (HPI) activity in a graded fashion to acute increases in cortisol or changes in contest intensity, respectively. We discuss a variety of factors that could disrupt the ability of isolates to appropriately modulate interactions between social behavior and the HPI axis, and we examine a number of functional hypotheses underlying the sensitivity of group-housed losers to changes in contest dynamics.

Fighting behavior in hermit crabs: the combined effect of resource-holding potential and resource value in Pagurus longicarpus

A study was conducted to determine whether asymmetries in both resource-holding potential (RHP) and resource value (RV) influence dominance and fighting behavior in the hermit crab Pagurus longicarpus. A total of 120 groups of three crabs were observed for 10 min in four experiments that tested all diverse combinations of equal/different RHP (i.e. ‘body size’) and equal/different RV (i.e. ‘shell size’ and ‘shell quality’). In a fifth experiment, dominant and subordinate individuals of the same size category (26 groups) were forced to enter shells of opposite quality than those previously occupied, and then the behavior of the reconstituted original groups was observed for additional 10 min. As expected, crabs in lower quality shells were more willing to initiate and to escalate fights. However, their attacks were directed to any crab of the group, independently of the defender's shell quality, and the fight duration did not vary with the different value of the resources at stake. This may indicate that P. longicarpus is unable to assess the quality of the shells available in its social environment but bases its tactical decisions during fights solely on its own resource. This suggestion was confirmed by the change in the fighting behavior of crabs whose shell quality was experimentally altered. This manipulation induced an overall increase in the intensity of aggression, drastic modification of crab behavior, and inversion of the hierarchy even though these crabs have had previous experiences of wins/losses and were familiar to the other members of the group. In this species, large crab size and/or the occupancy of adequate (and oversized) shells appeared to be the most likely determinant of contest resolution. Individuals seemed to retain a memory of the previously held resource and behaved accordingly.

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.

Body Size, Leg Autotomy, and Prior Experience as Factors in the Fighting Success of Male Crab Spiders, Misumenoides formosipes

Game theory predicts which characteristics of male crab spiders should affect their success in mate-guarding contests. Previous data on Misumenoides formosipes suggested that body size and leg number influenced fight outcome, but the study had methodological problems. Using improved methods, we corroborated the size advantage but refuted the disadvantage of leg reduction. We also conducted the first test for an invertebrate on the effect that a single contest has on the outcome of subsequent contests. When size and residency status were controlled, prior winners defeated prior losers after only one contest experience. We discuss the importance of controlling for previous experiences in studies of contest resolution and the ramifications for both past and future studies.

Don't count your eggs before they're parasitized: contest resolution and the trade-offsduring patch defense in a parasitoid wasp

Although aggressive conflicts over hosts occur among females in many species of insect parasitoids, few studies have examined the mechanisms by which these contests are resolved. In Trissolcus basalis, a parasitoid of pentatomid bug egg masses, females co-exploiting an egg mass (patch) encounter one another repeatedly and fight for possession of the patch. We investigated the resolution of pairwise contests by experimentally varying the release time of females and the size of the patch. Logistic regression showed that the female arriving first on the patch was more likely to win both the first agonistic encounter and to retain overall possession of the patch. This advantage to the first female suggests a resource-correlated asymmetry in favor of the first female, due to her having invested more offspring in the patch. Although escalations were more common when the asymmetry in arrival times was small, the majority of encounters within all contests were nevertheless resolved without escalated fighting, with the resident attacking and the intruder backing down. Thus contest resolution basically followed a “bourgeois” rule. Residents tolerated intruders more frequently when the patch size was larger and the second female was released later, illustrating the trade-off faced by residents between defending the patch and continuing to exploit it.

Blue Tit (Parus Caeruleus) Agonistic Displays: A Reappraisal

Abstract1. In order to reappraise work by Stokes (1962a, b) and subsequent interpretations of his results, a data set involving 906 dyadic Blue Tit interactions was coIlected and analysed using video playback. 2. A number of shortcomings in Stokes' original approach were highlighted, including his broad definition of attack and a lack of information on the true frequency of behaviours and of combinations of elements at an instance in time. 3. A total of 16 behaviours and actions with a clear agonistic context were identified. Similarity indices (Jaccard Index) were calculated based on the co-occurrence of behaviours at an instance in time. No behaviours were associated at a high enough level to warrant their consideration solely as compound displays. 4. True attack was rare, unpredictable and effective. Escalated fights occurred in < 1% of interactions, with peck attacks which were not escalated into fighting in a further 2% of interactions. The rarity of true attack makes it more useful to look at the relationship between behaviour and interaction outcome (win/lose), than between behaviour and subsequent actions (attack/stay/escape). 5. Seven behaviours were associated with winning (e.g. Wings out, Nape erect) and three (e.g. Crest erect) with losing. However, no behaviour was restricted to only one of these roles, no behaviour precisely predicted winning or losing, and many birds won or lost without showing these behaviours. Crest erect is associated with fear but is not simply a surrender signal. 6. In addition to the displays investigated, several other factors are thought to be involved in contest resolution. These are briefly discussed.

Effects of territory ownership on dominance in the desert clicker (Orthoptera: Acrididae)

Field observations of the territorial acridid Ligurotettix coquilletti, revealed the importance of territory ownership in contest resolution and in determining the dominance of males. An experiment in which males were paired on the territories of one of two matched individuals further substantiated the role of ownership in determining an individual's dominance at a particular site by demonstrating that, in general, (1) each of the two males won the contest according to which was the owner, and (2) the contests escalated if the intruder disregarded the other's ownership, possibly as a result, of intruder-misperception. However, other factors, such as physical strength (resource holding potential) as signalled by visual and acoustic displays, probably influenced the outcomes of contests as well.

Asymmetric contests over resources for survival and migration: a field experiment with bluethroats

The relative importance of prior occupancy and of asymmetries in both value of winning and in resource-holding potential for the outcome of animal contests was tested. Bluethroats, Luscinia s. svecica, were offered food ad libitum in the field. Prior to migration, at a moulting site, their weight increased rapidly and peaked after a few days. Lean birds won significantly more interactions than fat birds, probably owing to higher motivation to fight for the food. Dominance between individuals shifted and was not correlated with size or prior occupancy. During the subsequent migration, at a stopover site, the bluethroats gained weight throughout their stay. Dominance between individuals was constant and positively correlated with size, whereas fat reserves and prior occupancy had no effect on contest resolution. Fat deposition rates were positively correlated with dominance status. The different dominance patterns are explained in terms of different gain curves of fat for birds putting on fat for survival and migration, respectively. Though prior occupancy had no general effect on dominance patterns, such an uncorrelated asymmetry may have been used to settle single contests between fat birds at the moulting site. The fact that dominance (acting through differences in size) influences fat deposition rates may be important for the spatial and temporal pattern of migration in birds that compete for resources at stopover sites.