Intraspecific aggression in the colour morphs of the anemone Phymactis clematis from Chile

The anemone Phymactis clematis exhibits intraspecific agression. An investigation into dominance relationships amongst the 3 colour morphs collected from Vna del Mar, Chile, during June, 1980, revealed them to be equally aggressive. The outcomes of experimental contests appeared to reside solely with size-dependent differences in the threshold for the release of agression. Contests were asymmetric and larger individuals displayed aggression earlier against smaller opponents, and won encounters. An analysis of times for the onset of aggression in contests showed that they were highly variable and were not a good indicator of the absolute aggressive ability of individuals as denoted by dominance rankings. It provided, however, evidence indicating a lack of assessment of opponent size during initial contact. This is discussed in relation to the evolution of aggression in anemones and to the concept of evolutionarily stable strategies in contest behaviour. It is concluded that future work on the apparent size-dependent threshold will most instructively be conducted at the neurophysiological level.     

Direct male-male competition can facilitate invasion of new colour types in Lake Victoria cichlids

The possibility that disruptive sexual selection alone can cause sympatric speciation is currently a subject of much debate. The initial difficulty for new and rare ornament phenotypes to invade a population, and the stabilisation of the resulting polymorphism in trait and preference make this hypothesis problematic. Recent theoretical work indicates that the invasion is facilitated if males with the new phenotype have an initial advantage in male-male competition. We studied a pair of sympatric incipient species of cichlids from Lake Victoria, in which the red (Pundamilia nyererei) and blue males (P. pundamilia) vigorously defend territories. Other studies suggested that red phenotypes may have repeatedly invaded blue populations in independent episodes of speciation. We hypothesised that red coloration confers an advantage in male-male competition, assisting red phenotypes to invade. To test this hypothesis, we staged contests between red and blue males from a population where the phenotypes are interbreeding morphs or incipient species. We staged contests under both white and green light condition. Green light effectively masks the difference between red and blue coloration. Red males dominated blue males under white light, but their competitive advantage was significantly diminished under green light. Contests were shorter when colour differences were visible. Experience of blue males with red males did not affect the outcome of a contest. The advantage of red over blue in combats may assist the red phenotype to invade blue populations. The apparently stable co-existence of red and blue incipient species in many populations of Lake Victoria cichlids is discussed.Communicated by C. St. Mary     

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.     

Initiation and resolution of jumping spider contests: roles for size, proximity, and early detection of rivals

Animals are commonly expected to assess each other during contests in order to economically identify relative status. Escalated or long contests are expected to arise mainly when rivals have difficulty discriminating small differences. Results of the present study of male-male contests in Plexippus paykulli, a jumping spider (Salticidae) with acute vision, are not in accord with this widely held view. Despite the typical finding that size-advantaged rivals are more likely to win contests and that this tendency increases with size disparity, contest dynamics suggest that these tendencies are achieved in the absence of direct size assessment. In contests between different-sized spiders, maximum escalation and overall duration were predicted by the absolute size of the size-disadvantaged spider (usually the loser) rather than the size difference between the rivals. This result suggests that spiders base decisions of persistence on their own size, such that size-disadvantaged rivals usually reach their limits first, and then retreat. This interpretation is further supported by findings that maximum escalation and total duration were both positively related to size in contests between size-matched spiders. Spiders were more likely to win if they oriented and displayed first, and longer, more escalated, contests ensued if the size-disadvantaged spider was the first to orient and display. Proximity of rivals at contest outset also influenced contest dynamics, but not outcome.     

Rival recognition affects male contest behavior in sand lizards (Lacerta agilis)

Individual male sand lizards meet repeatedly during mate search and engage in costly interactions. If males can recognize rivals, the number of costly fights with a predictable outcome may be reduced. In staged contests between males, second interactions are on average significantly shorter than first ones, suggesting individual recognition. When aggressive behavior differs substantially between the sexes, the more aggressive sex may be predicted to have more variation in skin traits located on the parts of the body displayed during contests; this would facilitate individual recognition. Male (aggressive) sand lizards are more variable in two of three lateral skin traits displayed during males' contests, while females (non-aggressive) are more variable in one dorsal skin trait; in four other dorsal traits there was no difference in variability between the sexes.     

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.     

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.     

Symmetric fights as a measure of escalation potential in a symbiotic,territorial snapping shrimp

Summary The shrimp Alpheus armatus territorially defends the sea anemone it occupies, using as a weapon its large, specially modified snapping claw. This behavior was studied in experimental contests which were symmetric (matched individuals) with respect to sex, size, and residency. Characteristics of these contests were compared for two size-classes of male and female shrimp.There were no significant differences between these groups of shrimp in the number of bouts required to establish dominance or in the number of snaps exchanged. Large females had shorter contests than either small or large males, and losers of contests between large females were injured more frequently and more severely.This distinctiveness of large female contests could be interpreted as evidence that (i) controlling anemones is more important for large females, (ii) injuries are less important for large females, or (iii) large females lose the ability to assess one another because their contests are less frequent.If injuries are an accurate measure of the most important costs associated with fighting, then these results indicate that short contests are not necessarily the least costly, and that females can be more aggressive than males, as measured by escalation potential, in sexually selected species.     

Aggression and fighting ability are correlated in the swordtail fish Xiphophorus cortezi: the advantage of being barless

Males of the swordtail fish Xiphophorus cortezi are polymorphic for the pigment pattern vertical bars. Previous studies indicate that barred males exhibit higher levels of aggression towards males with bars than those without, while barless males fail to exhibit differential levels of aggression to either morph. In this study I matched barred and barless males for size and paired them in dyadic contests in order to determine if either morph was more dominant and if so, if dominance was the result of higher aggression levels. I found that barless males had higher bite frequencies and were able to win a majority of the contests while barred males consistently escalated to biting first, even though in most cases they were ultimately the losers. In order to determine whether the observed aggression levels and fighting abilities were inherent to being barless or a consequence of responses to the bars themselves, the dyads were re-paired once after barless males were given temporary bars and once after barred males had their bars removed. Thus, each morph encountered his opponent in both a barred and barless state. Regardless of bar state, naturally barless males continued to be more aggressive and more dominant than their barred counterparts. In addition, naturally barred males only won contests in which they bit more. These results indicate that for this species, aggression is an important component of winning contests when opponents are roughly the same size. As a result, naturally barless males as a whole appear to have higher resource holding potential (RHP) than naturally barred males of the same size because of their greater aggression levels.     

Asymmetrical contests in defence of rock ptarmigan territories

Summary Contests involving territorial male rock ptarmigan (Lagopus mutus) occur frequently over the course of the breeding season. Fights are initiated when one male (Neighbour or Bachelor) intrudes on another's (Resident) territory. These contests involved two kinds of asymmetries: Neighbours were similar to Residents in mating status, size and experience, differing only in the gains accrued from winning a fight (payoff asymmetry) whereas Bachelors were smaller, less experienced males that had no mate or territory (fighting asymmetry, in addition to a relatively larger payoff asymmetry than in Resident x Neighbour contests). In this study, we describe and compare the contests resulting from these intrusions, and we assess how the asymmetries (fighting ability, payoffs) between opponents influenced contest intensity. Resident x Neighbour contests were brief, highly-ritualized affairs, while Resident x Bachelor interactions were long, intense battles, often ending in direct physical attack. These findings are in direct opposition to many theoretical and empirical studies on fighting asymmetries which suggest that contest intensity should be negatively correlated with the degree of fighting asymmetry between contestants. Since the potential losses associated with a contest differed between contestants and were highest for an intruding Neighbour, we conclude that payoff asymmetries were more important than fighting asymmetries in influencing a male's tendency to escalate.     

Male aggression varies with consortship rate and habitat in a dolphin social network

Behavioral Ecology and Sociobiology - In many species, outcomes of male duels determine access to females and, ultimately, male reproductive success. Ritualization of behavior in male contests can...     

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.     

Fluctuating asymmetry,interspecific aggression and male mating tactics in two species of Japanese scorpionflies

Summary Fluctuating asymmetry (FA) is defined as small, random deviations from bilateral symmetry in a generally perfect bilaterally symmetrical morphological trait. FA in forewing length affects the outcomes of interspecific contests for food items (dead arthropods) between two species of Japanese scorpionflies, Panorpa nipponensis and P. ochraceopennis, in nature. FA differences between same-sex contestants are more important than either body size or ownership of food in determining the outcomes of interspecific contests; for both sexes, winners statistically significantly more often have relatively low FA. Two condition-dependent mating tactics are used by the males of each species: (a) a male may defend a dead-arthropod nuptial gift, or (b) a male without such an arthropod may wait near a male with one. In both tactics, males release long-distance sex pheromones. Groups of pheromone-releasing males are made up of one male with a nuptial gift and his satellites; the males in a group may be conspecifics or heterospecifics. Males that lose contests for nuptial gifts often become satellites of the contest winners whether or not winners are conspecific. Satellite males have statistically significantly greater FA than males with nuptial gifts in heterospecific male display groups. Satellite males mate infrequently and briefly compared to resource-holding males. Satellites of heterospecific males copulate with conspecific females displaced from nuptial gifts by the resource-holding males of the other species. In both species, the largest and smallest individuals have the greatest FA, and intermediate-sized individuals have the least; this same pattern often occurs in other animals.     

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.     

Chelipeds are the real weapon: cheliped size is a more effective determinant than body size in male–male competition for mates in a hermit crab

To understand the evolution of weapons, we must understand both their functions and relative importance compared to body size in determining fighting success. Many decapod crustaceans develop disproportionately large chelipeds for their body size and use them as a weapon in agonistic interaction. There are, however, examples where weapons are merely signals of resource holding potential (RHP) and the RHP is actually determined by body size. We investigated the function and relative efficacy of body size and major cheliped size in male–male contests for females in the hermit crab Diogenes nitidimanus. Contests over females took two forms: (1) males preemptively guarded females and opponents did not fight with the guarding male. Cheliped size contributed significantly to the settlement of these contests and probably functioned as a visual signal for the opponents. (2) Guarding males engaged in physical combat with an opponent. In these cases, both body and cheliped sizes affected contest outcomes. The effect size for cheliped size was as strong, or stronger, than that for body size. These results suggest that large chelipeds have evolved as a true weapon and are effective in escalated fights for resources. Therefore they are also efficient visual signals for settling contests with only display. Our results are a rare example that clearly demonstrate that weapons are a more important determinant of fights than body size when both body and weapon size affect resource acquisition.     

When to be neighbourly: differential agonistic responses in the lizard Platysaurus broadleyi

Differential treatment of neighbours and strangers (the dear enemy phenomenon) can reduce the costly effects of territory defence. Individual recognition, and by extension the dear enemy phenomenon, was examined in the cordylid lizard Platysaurus broadleyi. I had no knowledge of familiarity between individuals and therefore classed intruders as neighbours or non-neighbours and tested for differences in agonistic response consistent with the dear enemy phenomenon. In the dear enemy phenomenon, levels of aggression are inversely related to degree of familiarity such that residents are more tolerant of immediate neighbours than distant territory holders. A manipulative field experiment in conjunction with field observations of known territory holders revealed that resident males allowed neighbours to approach more closely than non-neighbours before challenging them, residents were more aggressive towards non-neighbours than neighbours, contests between neighbours were significantly longer than between neighbours and non-neighbours and contests between residents and non-neighbours were likely to result in a win for the resident, while neighbours frequently drew contests. These results suggest a level of recognition consistent with the dear enemy phenomenon. I also manipulated front leg colour to test the hypothesis that it alone could serve as a cue for individual recognition. I found no support for this hypothesis and suggest that multiple cues operate in individual recognition. Received: 4 March 1999 / Received in revised form: 11 April 1999 / Accepted: 19 April 1999     

Social control of deception among status signalling house sparrows Passer domesticus

Summary Motivation to fight and willingness to risk the use of dangerous weapons, two factors which can be decisive in predicting the winners of asymmetric animal contests, are readily bluffed in display. When contests are decided by motivation to fight or daring, more reliable information about contest asymmetries may be gained by watching actual fights than by watching displays. By presenting mounts to territorial red-winged blackbirds and scoring how aggressively a territory holder attacks the mount, then recording any subsequent intrusions over established boundaries, I showed that neighbors are likely to intrude upon focal males that fail to attack the mount vigorously. This result suggests that red-winged blackbirds watch contests to assess the RHP of neighbors.     

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.     

Assessment and individual recognition of opponents in the pygmy swordtails Xiphophorus nigrensis and X. multilineatus

We examined factors that determine the outcome of agonistic encounters between male pygmy swordtail fish. Xiphophorus nigrensis and X. multilineatus males formed dominance relationships based on body size in staged laboratory encounters. There was a significant negative correlation between size asymmetry and fight intensity, suggesting that males assessed size in the encounters. However, a significant proportion of the variation in fight intensity in contests that escalated to bites could not be explained by size asymmetry. Aggressive motivation may also influence the outcome of contests and could be assessed in agonistic encounters. Theory suggests that signals of aggressive intention will be evolutionarily stable if individuals can recognize opponents and encounter one another repeatedly. In addition, individual recognition is one way that dominance hierarchies can be maintained. Here we demonstrate that males from both species can recognize individuals. In addition, at least some X. nigrensis males were site-faithful in the field, suggesting males encounter the same opponents repeatedly.Communicated by G.M. Klump     

Body morphologies shape territorial dominance in the satyrine butterfly <Emphasis Type="Italic">Lethe diana</Emphasis>

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.