The role of body size and fighting experience in predicting contest behaviour in the black field cricket, <Emphasis Type="Italic">Teleogryllus commodus</Emphasis>

Body size strongly predicts fighting behaviour and outcome in many species, with the larger opponent usually winning contests. However, recent fighting experience can have a strong influence on the establishment of dominance hierarchies, with recent winners being more likely to win subsequent contests, while recent losers are more likely to lose. Recent fighting experience therefore has the potential to modify the effect of body size in determining contest behaviour and outcome. Here, we investigate whether recent fighting experience weakens the role of body size in predicting contest behaviour in the black field cricket, Teleogryllus commodus. We compared the role of body size in determining contest outcome during initial non-physical encounters and escalated aggressive physical encounters (grapples), as well as the probability of escalation occurring, in contests involving either a naïve or experienced smaller male against a naïve larger male. We found that recent fighting experience only affected contest outcome during non-physical encounters. Once a contest had escalated into grappling, the effect of previous wins and losses was no longer apparent and body size strongly predicted contest outcome. Thus, once males can directly assess their opponent's fighting ability, recent fighting experience did not alter the effect of body size on contest behaviour and outcome.     

Do the elongated eye stalks of Diopsid flies facilitate rival assessment?

The elongated eye span of male Diopsid flies is a sexually selected character that scales positively with body size. Previously, the duration of agonistic contests was found to increase as rival body size and eye span disparities decreased. Hence, along with its role in mate choice, eye span seems to facilitate mutual assessment of rival size. However, such results are also expected in the absence of rival assessment, when each individual persists according to its own size-dependent internal threshold. Here, we reanalyze these contests to distinguish between these two hypotheses using two measures of size: body length and eye span. Mutual assessment predicts that contest duration should increase with loser size and decrease with winner size. In contrast, our results were more consistent with self-assessment: We found a positive relationship between loser size and contest duration, whereas winner size did not affect contest duration. Thus, flies did not appear to assess the size of their rivals, indicating that the mutual assessment function of eye span elongation may be less important than previously suspected. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Male contests in the scarlet rosefinch (Carpodacus erythrinus) in relation to asymmetries in resource holding power and pairing status

Summary Inter-male contests in relation to asymmetries in their pairing status and resource holding power were studied in a population of the Scarlet rosefinch. Since males in this species do not defend territories before nest building, and since most males have full adult plumage, possession of territory and age are unlikely to be used as cues in the settlement of contests. Rather, two other features are used, namely size and pairing status. In this study, contests between two unpaired males were usually short displacements, whereas 56% of contests between two paired males were escalated. In the first case, the smaller male withdrew in all contests, whereas in the second case there was an equal probability of withdrawal with regard to size. There was no relationship between size difference and the probability of escalation in the contests. Most of the fights between paired and unpaired males escalated, namely in 60% of the cases where the unpaired male was smaller and in 75% of the cases where he was larger. The unpaired male withdrew in 18 out of 23 contests, suggesting that pairing status is more important than size in determining the outcome of a contest. There was no variance in size differences between contestants in short and escalated contests. This study demonstrates that there is a high competition between males for females in this population.     

How do fighting ability and nest value influence usurpation contests in <Emphasis Type="Italic">Polistes</Emphasis> wasps?

Most research on animal contests has focused on the factors that influence the intensity and outcome of aggressive contests within nonsocial species, while relatively little is known about contests in social taxa. Here, we examine contests among queens of the social paper wasp, Polistes dominulus. Queens use multiple reproductive strategies, including nesting alone, usurping established colonies, and cooperatively joining other queens. We stage contests between a nesting queen and a challenger to test how resource value (RV) and resource holding potential (RHP) influence (a) who occupies the nest at the end of the contest and (b) the extent of conflict between the queen and challenger. We found that RHP, as measured by individuals’ facial patterns and body size, influenced the outcome of the contest. Challengers with high RHP were more likely to successfully usurp the nest than challengers with low RHP. Interestingly, queens with relatively high RHP were more likely to form a cooperative association with the challenger than queens with lower RHP, suggesting that queens may evict individuals that are an aggressive threat. RV influenced the intensity of conflict. There was more aggressive conflict over large nests than over small nests. Overall, social taxa have complex contest dynamics with important parallels to contests in nonsocial taxa. Studying contests in social taxa provides an important perspective on the factors that influence individual decisions about conflict versus cooperation.     

Gray tree frogs, Hyla versicolor, give lower-frequency aggressive calls in more escalated contests

As animal contests escalate, variation in the performance of aggressive signaling behaviors can give important insights into contest dynamics. In anuran amphibians, males of numerous species utilize distinctive aggressive vocalizations during disputes over calling spaces. Little is known, however, about the causes and consequences of variation in aggressive-call characteristics. We analyzed recordings of calls made during staged aggressive interactions between male gray tree frogs, Hyla versicolor, to determine how variation in a key aggressive-call characteristic, dominant frequency, was affected by increasing contest escalation. We found that dominant frequencies of aggressive calls were lower than those of advertisement calls used to attract females. Furthermore, we found that males lowered their aggressive-call frequencies with increases in escalation. Winners tended to have lower-frequency aggressive calls than losers. We conclude that aggressive calls in H. versicolor are similar to the graded aggressive calls that have been described in several other species. This gradation may allow males to balance the energetic costs of producing lower-frequency calls with the benefits of efficiently repelling rival males. Other processes related to motivation and the physiological effects of participating in contests may also be responsible for the observed variation in aggressive-call frequency with contest escalation. Our results demonstrate that detailed experimental studies of aggressive calling behavior in anurans, which to this point have rarely been performed, are feasible and generate important insights relating to general problems in animal contest behavior and animal communication.     

Effects of ownership status,weight asymmetry,and case fit on the outcome of case contests in two populations of Agrypnia pagetana (Trichoptera : Phryganeidae) larvae

Summary The contest behavior of larvae from a dense and a sparse population of the caddisfly Agrypnia pagetana was studied in the laboratory. Fights were generally of short duration, and most fights were won by owners in both populations. Take-over contests lasted longer than non-takeover ones, indicating that owners in general used a more persistent strategy. Ownership was more important in the sparse population than in the dense population, but contest duration did not differ between the two populations. Effect of weight asymmetry was most pronounced in the dense population, and here the larger larvae usually won. However, when the intruding larvae were much larger than the owner, a low number of take-over contests were probably related to the inferior fit (low resource value) of small cases to the intruding larvae. In both populations the frequency of case takeover contests was low when the case fit to the intruder was low. We conclude that A. pagetana larvae use fighting strategies typical for contests over resources of low value relative to contest costs.     

Territorial defence in the great tit (Parus major): Do residents always win?

Summary I removed resident pairs of great tits from their territories for short periods and released them after replacement pairs had occupied the spaces. When two pairs are manipulated in this way into occupying the same territory an escalated contest ensures. Contests between residents and replacements are longer, more likely to involve physical fights and longer fights or displays than contests between established neighbours or residents and intruders. The degree of escalation between residents and replacements is an increasing function of replacement time. It rises to a peak and then diminishes again. The probability that the replacement will defeat the former resident is an increasing function of replacement time, reaching 90% after several days. These results are consistent with the hypothesis that territorial residents win in contests against intruders because of an asymmetry in payoff rather than an asymmetry in resource holding potential or an arbitary convention. A possibly asymmetry in payoff in the great tit is the cost of defending the territory against neighbours. The cost is higher for replacements than for residents.     

A lover, not a fighter: mating causes male crickets to lose fights

Both resource-holding potential (RHP) and experience in aggressive contests are known to affect future aggressive behaviour. However, few studies have examined the effects of mating experience on agonistic behaviour, despite the fact that dominant males usually acquire more matings. We investigated the effect of mating experience on male aggressive behaviour including the relationship between RHP and fighting success in the fall field cricket, Gryllus pennsylvanicus. We formed pairs of size- and age-matched males that varied in RHP (relative weapon size) and conducted two experiments. In the first, we varied male mating experience by allowing one male in a pair to either be (a) ‘mated’: court, be mounted and copulate with a virgin female or (b) ‘experienced’: court, be mounted, but prevented from copulating. The second experiment varied postcopulatory experience where the male was allowed (‘contact’) or prevented from (‘no-contact’) continued contact with his recent mate. Following treatment, experimental males engaged in an aggressive contest with the naïve size- and age-matched male. In our first experiment, we found that mated and experienced males were equally likely to escalate contests to combat with a naïve opponent, but mated males were less likely than experienced males to win. There was no effect of mating on the relationship between RHP and fighting success. In our second experiment, we found no effect of maintaining contact with the female on the tendency to escalate or the probability of winning. As in the first experiment, males with relatively larger heads again won more fights and this relationship was unaffected by male experience. These results suggest that mating is itself detrimental to male success in aggressive contests, but that this effect is not sufficient to eliminate the effect of RHP on fighting success.     

Investment in territorial defence depends on rearing environment in brown trout (Salmo trutta)

In many animals, territoriality will arise or cease depending on environmental factors such as intruder rate and resource availability. We investigated the effect of rearing environment on territorial behaviour in ~1.5-month-old brown trout. In the laboratory, wild-caught (reared at a low density) and hatchery-reared (high density) trout were allowed to defend a territory against a size-matched intruder reared in the same or the other environment. Because territorial behaviour should be relaxed at high-rearing densities, we hypothesized that hatchery-reared trout should value their territories less and therefore invest less in defence compared with wild-caught trout. However, in all cases, territory owners were more likely to win the contest and hatchery-reared trout were just as likely as wild-reared to win mixed contests. Furthermore, pairs of hatchery-reared trout initiated contests sooner, fought longer and were more aggressive during the contest compared with pairs of wild trout. When hatchery-reared owners met wild intruders, the contest ended sooner compared with when the roles were reversed. We conclude that territorial behaviour in brown trout is largely innate, but that the hatchery environment has promoted more aggressive individuals. These results suggest that hatchery-reared trout invest more time and energy to obtain the same contest success as wild trout. In conclusion, the lack of experience of territorial defence in a high-density rearing environment seems to reduce the efficiency of territorial behaviour. In turn, this may have negative consequences for the performance of released hatchery fish in the wild.     

Fighting without weaponry: a review of male-male contest competition in butterflies

The males of many butterfly species compete via pair-wise intrasexual contests, yet lack any obvious morphological traits conventionally associated with animal aggression. The evolution of contest behavior in this group is therefore interesting because the means by which individuals are able to inflict costs upon each other during combat are unclear. Here we review the relevant empirical information within a game-theoretical context in order to analyze contest-related costs and mechanisms of dispute settlement. Territorial butterfly contests carry obvious role asymmetries, and individuals in the 'resident' role are more likely to win across all studied species. We use this phenomenon as a framework for gaining further insight into the evolution of contest settlement in this group. Four principal hypotheses are evaluated: (1) that residency is used as an arbitrary convention, (2) that residents stand to gain a greater payoff if successful, (3) that residency bestows higher resource-holding potential (RHP) on males in that role, and (4) that residents are males of intrinsically higher RHP. We found most support for the latter explanation (the 'superior-competitor' hypothesis), partly because the alternatives require a restrictive set of ecological conditions, but also due to positive support for the idea of intrinsic RHP. The strongest evidence is provided by species in which repeated interactions (between the same individuals) always have the same outcome, regardless of changes in residency status. However, we also found a consistent effect on contest duration due to apparent 'confusion over residency,' which suggests a conventional element to contest behavior. Although butterfly contests must be costly in some way, the literature provides limited insight into how costs accrue, and hence the determinants of contest RHP remain obscure. We suggest that careful experimentation, using 'natural' manipulations where possible, should prove most beneficial in identifying costs and exploring schedules of contest settlement in this group. Moreover, future investigations should be sensitive to the fact that butterfly territoriality and associated male-male interactions are seated within a broader ecological and life-historical context.     

The spider and fly revisited: ploy–counterploy behavior in a unique predator–prey system

Earlier studies have shown that the sarcophagid fly Arachnidomyia lindae is the principal egg-sac predator of the colonial orb-weaving spider Metepeira incrassata, and that risk of egg-sac loss increases with group size. Observations of specialized behaviors for attack (flies) and defense (spiders) suggest that this predator-prey relationship may incorporate elements of ploy and counterploy behavior. Here we explore this relationship in detail and test hypotheses regarding efficacy of attack and defense behaviors. Egg-sac guarding by the spider includes defensive behaviors specific to this fly predator, which were observed during experimental "attacks" with tethered A. lindae, but not with similar presentations of non-predatory Musca domestica. Experimental studies also show that Metepeira incrassata recognizes this predatory fly by airborne cues (i.e., the signature frequency of wing-beats), and can distinguish between this predator and other flies (potential prey) on the basis of wing-beat frequency differences. Removal of female spiders results in a significantly higher probability of unguarded egg-sacs being parasitized, demonstrating the adaptive value of spider defensive behaviors. We also present evidence that A. lindae utilizes a behavioral ploy for circumventing spider guarding behavior (aggressive mimicry - producing vibrations of captured prey in the web), and that Metepeira incrassata, in turn, exhibits a counter-ploy behavior (signal thread cutting) to eliminate this potentially distracting vibratory information. While previous studies have shown that this colonial web-building spider uses a number of general attack-abatement mechanisms against a diversity of predators and parasitoids, results of this study suggest that selection pressures from a highly specialized predator may also result in evolution of predator-specific prey responses.     

Inter-sexual contests in the hermit crab <Emphasis Type="Italic">Pagurus bernhardus</Emphasis>: females fight harder but males win more encounters

Contests often occur between members of the same sex when they compete for access to mates, but inter-sexual contests may occur over access to other essential resources such as food or shelter. Despite the possibility that such contests are common, most studies focus on male fighting, and very few have analysed fights between males and females. Because males and females differ physically and physiologically, fighting ability or resource-holding potential (RHP) may also be subject to inter-sexual variation. In this study, we investigate size-controlled inter-sexual contests over the ownership of empty gastropod shells in the hermit crab Pagurus bernhardus. During these fights, there are two roles, attacker and defender, and we use a fully orthogonal experimental design to compare the performance of males and females in each role, when fighting either a male or female opponent. Although females fight more intensely, male attackers have an advantage when compared to females playing the attacker role, as they are more likely to evict the defender from its shell and thus win the resource. Further, in the defender role, male defenders are subject to shorter attacks than female defenders. The differences in agonistic performance could not be attributed to differences in perceived resource value between the sexes or to differences in body or weapon size. There are clear differences in the agonistic behaviour of males and females, and this possibility should be incorporated into models of contest behaviour. In particular, evolutionarily stable strategies may be expected to vary with sex ratios.     

The relationship between conspecific superparasitism and the outcome of in vitro contests staged between different larval instars of the solitary endoparasitoid Venturia canescens

When endoparasitoid wasps oviposit into hosts which have already been parasitized (= superparasitism), this results in potentially lethal interlarval competition. For solitary species, the decision to lay additional eggs should therefore be based on the probability of superparasite survival in any superparasitized host. In this study, in vitro contests staged between three larval instars of Venturia canescens Grav. (Hymenoptera: Ichneumonidae) reveal that the age difference between competitors affects the outcome of interlarval competition. Three parameters were used to assess parasitoid performance: the number of fights initiated, the number of bites inflicted, and the duration of each bite. When fighting takes place between two first instars, then both competitors were found to be evenly matched. However, at greater age differences, first-instar competitors appeared to win more fights than their larger second instar rivals. The advantage shown by younger competitors is most pronounced in contests staged between first and third instar larvae. These findings are consistent with the increasingly high levels of conspecific superparasitism shown by V. canescens in the first 5 days after initial host attack, suggesting that this parasitoid can deploy her use of deliberate superparasitism in an adaptive way. Received: 13 December 1995/Accepted after revision: 5 March 1996     

Male signaling behavior and sexual selection in a wolf spider (Araneae: Lycosidae): a test for dual functions

Male signaling behaviors are often studied in a single context but may serve multiple functions (e.g., in male–male competition and female mate choice). We examined the issue of dual function male signals in a wolf spider species Schizocosa ocreata (Hentz) that displays the same species-specific signaling behaviors in both male–male and male–female contexts. These signaling behaviors have been described as either aggression or courtship according to the context observed. We tested the possibility of dual functions by comparing the relationship between behaviors and outcome of male–male contests (winner/loser) and male–female mating encounters (mating success). Frequency, rate, and mean duration of signaling behaviors did not vary with outcome of male–male contests, which appears instead to be based upon relative size and body mass. Winners of contests had significantly greater body mass than losers, and greater mass relative to opponents was significantly associated with probability of winning. Overall, signaling rates were much higher in male–female interactions than in male–male contests and were higher for males that successfully mated than for those that did not mate. Mean duration of some male displays was also greater for males that successfully mated. However, male size was not associated with probability of mating. Taken together, results suggest an intersexual selection context for the current function of male signals in these wolf spiders and that increased display vigor is associated with male mating success.     

Twilight fighting in the evening brown butterfly,<Emphasis Type="Italic"> Melanitis leda</Emphasis> (L.) (Nymphalidae): age and residency effects

Butterflies are conspicuous among animals that fight for mating opportunities because it is not clear how contest costs could accrue. Nevertheless, the bulk of research in this group suggests that contests are settled on the basis of asymmetries in fighting ability (the superior competitor hypothesis). There is also a consistent effect on contest escalation due to apparent 'confusion' over residency, which could result from resident butterflies playing different tactics than non-residents, or from a causal link between residency and fighting ability. I addressed these hypotheses by studying contest behavior in the crepuscular species Melanitis leda (L.) (Nymphalidae) over a 5-month period in tropical Australia. Males competed via conspicuous two-stage maneuvers of a form unique among butterflies. Prior residents won 77% of all contests. Non-resident males arrived and perched in occupied sites until challenged by incumbent residents, and the aggressiveness of these interlopers increased as a function of the time before they were detected. Contest winners also tended to be younger than losers, and contest escalation was negatively related to both the age of the losing male and the magnitude of the between-combatant age asymmetry. These results are consistent with the superior competitor hypothesis based upon age as a determinant (or correlate) of fighting ability, but also suggest residency has a unique influence on contest behavior. Further research is clearly required to unravel the effects of age, residency and other potential biophysical determinants of fighting ability in this species.     

Carotenoid-based status signalling in red-shouldered widowbirds (<Emphasis Type="Italic">Euplectes axillaris</Emphasis>): epaulet size and redness affect captive and territorial competition

In breeding plumage, the African male red-shouldered widowbirds (Euplectes axillaris) are black except for red carotenoid-based epaulets ('shoulder patches'), similar to the well-studied American red-winged blackbirds (Agelaius phoeniceus). To experimentally test the signal function of the red epaulets in male red-shouldered widowbirds, we manipulated epaulet colour and size (within natural variation), and observed the effects in two competitive contexts. First, in captivity, unfamiliar males with epaulets of different size or redness were staged in dyadic contests over food. Only epaulet manipulations significantly affected contest outcome; red epaulet males out-competed orange and control males, which in turn dominated blackened (no epaulet) males. Epaulet size manipulations had particularly strong effects; enlarged epaulet males vigorously defeated smaller epaulet males within colour treatments, whereas the independent effect of 'redness' (i.e., within size treatments) was much weaker. Second, in a field experiment, epaulets were manipulated prior to territory establishment. Males with enlarged epaulets acquired territories, whereas most males with blackened or reduced signals failed to obtain territories. Furthermore, among the territorial males, birds with enlarged epaulets defended larger territories and spent less time in boundary signalling, yet they received fewer intrusions from conspecifics. In contrast, epaulet manipulations had no effect on either female visitation rates or subsequent female settlement. These results concur with recent studies and suggest that the carotenoid-based epaulets of red-shouldered widowbirds are status signals, sexually selected through male contest competition for territories.     

Residence duration influences the outcome of territorial conflicts in brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>)

From the value asymmetry hypothesis, we predicted that increasing residence duration should increase perceived territory value, which should (1) increase the motivation of a replacement owner to defend the territory, (2) decrease the probability that the original owner regains the territory and (3) increase contest duration and aggression between the original and replacement owner. These predictions were tested on young brown trout. First, individual fish were allowed to establish territories for 3 days. These original owners were then moved to an adjacent territory, whereupon replacement owners were allowed to take up their territories for either 2 or 4 days. After 4 days, the original owners were returned to their old territories, now containing a new size-matched resident. The resulting conflict was observed and the initiator, duration, aggression levels and the winner of the contest were determined. Replacement trout that had been resident for 4 days initiated and won more contests against original owners than did 2-day replacements. This suggests that the motivation to defend the territory increases with residence time, which will determine the outcome of territorial contests between opponents of similar resource-holding potential (i.e. size). Moreover, contests with 4-day replacements were longer and aggression levels were higher than in contests with 2-day replacement trout. These results are consistent with the value asymmetry hypothesis, extending its validity for explaining the behaviour of territorial animals.     

Loud calls as indicators of dominance in male baboons (Papio cynocephalus ursinus)

Adult male baboons (Papio cynocephalus) give loud two-syllable 'wahoo' calls during dawn choruses, interactions between groups, when chasing females, and in aggressive interactions with other males. These 'contest' wahoos are acoustically different from 'alarm' wahoos given to predators. In a study of free-ranging baboons in the Okavango Delta, Botswana, we found no significant correlations between the acoustic features of wahoos and adult male size; however, acoustic features were correlated with male dominance rank, age, and calling bout length. Here we show that other measures of calling behavior also appear to function as honest indicators of stamina and competitive ability. High-ranking males were more likely than middle- or low-ranking males to participate in wahoo bouts. They called at significantly higher rates, and their bouts were longer and contained more calls. All males were significantly more likely to participate in wahoo bouts with another male if their opponent's rank was similar to their own. Bouts involving males of similar ranks were longer, contained more wahoos, and involved calling at higher rates, than other bouts. In contests between males of similar ranks, the subordinate and dominant were equally likely to end the bout, whereas in contests involving males of disparate ranks, subordinates were significantly more likely to end the bout. Bouts involving males of similar rank were significantly more likely than others to escalate and result in physical fighting.     

The genetic mating system of a sea spider with male-biased sexual size dimorphism: evidence for paternity skew despite random mating success

Male-biased size dimorphism is usually expected to evolve in taxa with intense male–male competition for mates, and it is hence associated with high variances in male mating success. Most species of pycnogonid sea spiders exhibit female-biased size dimorphism, and are notable among arthropods for having exclusive male parental care of embryos. Relatively little, however, is known about their natural history, breeding ecology, and mating systems. Here we first show that Ammothella biunguiculata, a small intertidal sea spider, exhibits male-biased size dimorphism. Moreover, we combine genetic parentage analysis with quantitative measures of sexual selection to show that male body size does not appear to be under directional selection. Simulations of random mating revealed that mate acquisition in this species is largely driven by chance factors, although actual paternity success is likely non-randomly distributed. Finally, the opportunity for sexual selection (I _s), an indirect metric for the potential strength of sexual selection, in A. biunguiculata males was less than half of that estimated in a sea spider with female-biased size dimorphism, suggesting the direction of size dimorphism may not be a reliable predictor of the intensity of sexual selection in this group. We highlight the suitability of pycnogonids as model systems for addressing questions relating parental investment and sexual selection, as well as the current lack of basic information on their natural history and breeding ecology.     

Weapon size versus body size as a predictor of winning in fights between shore crabs, Carcinus maenas (L.)

Relative body size (carapace width) and weapon size (chela length) were used as indicators of resource holding potential (RHP) in the agonistic behaviour of male shore crabs, Carcinus maenas (L.). Weapon size was found to be a more reliable predictor of the outcome of pairwise fights than body size. Crabs with longer chelae than their opponents were more likely to win fights than crabs with relatively larger bodies. Body size had less influence on the outcome of fights. Relative body and weapon size did not influence initiation of contests but did affect the likelihood of winning; however, this was significant only for weapon size. Winning crabs had heavier claws with greater surface area than losing crabs. There was no relationship between relative size and fight duration. The frequency of cheliped display increased with chela length and win- ners performed significantly more displays than losers. Received: 5 February 1997 / Accepted after revision: 20 May 1997