Aggressive Contests Research Articles

Influence of dietary nutrient balance on aggression and signalling in male field crickets

Indicator models of sexual selection predict that sexually selected trait elaboration should covary positively with condition. However, nutrition might influence the expression of multiple traits, where high-quality diets may result in positive trait correlations and low-quality diets in trade-offs. Although previous studies have examined how diet quantity or single nutrients, such as protein, carbohydrate and phosphorus, influence sexual traits, few have examined how dietary nutrient balance affects sexual trait expression. We therefore investigated how dietary protein:carbohydrate ratio and percentage of phosphorus influence the relationship between investment in mate attraction signalling and aggression. We fought groups of six adult male Gryllus veletis crickets, each consuming a unique diet, while recording their prefight and postfight signalling parameters. We found no evidence that diet influenced aggression or prefight signalling, with the exception that high-phosphorus diets had a negative influence on several signalling effort parameters. Body size was an important predictor of aggressive behaviour and most signalling parameters, suggesting that developmental diet may have a greater influence on these sexual traits. Several prefight signalling parameters were weakly related to aggression, suggesting that signalling may advertise competitive abilities. Males consuming high-carbohydrate and equal protein:carbohydrate diets experienced changes in signalling parameters that represented an overall increase in signalling effort following aggressive contests compared to males consuming high-protein diets, suggesting that dietary effects on signalling may only become apparent following periods of highly energetic activity. Changes in signalling following aggressive contests were also related to aggression levels modulated by dietary phosphorus content, such that only males consuming low-phosphorus diets were able to invest heavily in signalling after investing heavily in aggression. Our findings highlight the importance of research on the interplay between multiple sexually selected traits, and how dietary nutrient balance influences these relationships.

The outcomes of most aggressive interactions among closely related bird species are asymmetric.

Aggressive interactions among closely related species are common, and can play an important role as a selective pressure shaping species traits and assemblages. The nature of this selective pressure depends on whether the outcomes of aggressive contests are asymmetric between species (i.e., one species is consistently dominant), yet few studies have estimated the prevalence of asymmetric versus symmetric outcomes to aggressive contests. Here we use previously published data involving 26,212 interactions between 270 species pairs of birds from 26 taxonomic families to address the question: How often are aggressive interactions among closely related bird species asymmetric? We define asymmetry using (i) the proportion of contests won by one species, and (ii) statistical tests for asymmetric outcomes of aggressive contests. We calculate these asymmetries using data summed across different sites for each species pair, and compare results to asymmetries calculated using data separated by location. We find that 80% of species pairs had aggressive outcomes where one species won 80% or more of aggressive contests. We also find that the majority of aggressive interactions among closely related species show statistically significant asymmetries, and above a sample size of 52 interactions, all outcomes are asymmetric following binomial tests. Species pairs with dominance data from multiple sites showed the same dominance relationship across locations in 93% of the species pairs. Overall, our results suggest that the outcome of aggressive interactions among closely related species are usually consistent and asymmetric, and should thus favor ecological and evolutionary strategies specific to the position of a species within a dominance hierarchy.

A Game-Theoretical Winner and Loser Model of Dominance Hierarchy Formation.

Many animals spend large parts of their lives in groups. Within such groups, they need to find efficient ways of dividing available resources between them. This is often achieved by means of a dominance hierarchy, which in its most extreme linear form allocates a strict priority order to the individuals. Once a hierarchy is formed, it is often stable over long periods, but the formation of hierarchies among individuals with little or no knowledge of each other can involve aggressive contests. The outcome of such contests can have significant effects on later contests, with previous winners more likely to win (winner effects) and previous losers more likely to lose (loser effects). This scenario has been modelled by a number of authors, in particular by Dugatkin. In his model, individuals engage in aggressive contests if the assessment of their fighting ability relative to their opponent is above a threshold theta . Here we present a model where each individual can choose its own value theta . This enables us to address questions such as how aggressive should individuals be in order to take up one of the first places in the hierarchy? We find that a unique strategy evolves, as opposed to a mixture of strategies. Thus, in any scenario there exists a unique best level of aggression, and individuals should not switch between strategies. We find that for optimal strategy choice, the hierarchy forms quickly, after which there are no mutually aggressive contests.

Motivation but not body size influences territorial contest dynamics in a wild cichlid fish

Contests over resources are a key facet of social behaviour and have received extensive theoretical attention. However, the assumptions and predictions of the theoretical models have rarely been tested experimentally in wild free-living vertebrates. Here, we look at resource contests in wild Neolamprologus pulcher, a cooperatively breeding cichlid fish that lives in permanent territories in Lake Tanganyika, Africa. To elicit a contest, we removed a dominant breeding male from his territory and held him for either a short (4.5 h) or a long (20 h) period. The original resident male was then returned to his territory, which typically resulted in an aggressive contest with a usurping male that had taken over in the original resident's absence. We found that contests were shorter than those previously observed in a laboratory setting, with more physically aggressive attacks. Contrary to our predictions, the relative size difference between the males had no effect on contest dynamics or outcome, probably because all of the males involved in these contests were similar in body size. Instead, motivational factors influenced contest dynamics. Longer original resident removal times increased usurper male aggression, as well as the duration and intensity of the contests. Original residents were more aggressive when contesting over their sole territory (socially monogamous) than when contesting over one of their multiple territories (socially polygynous). Usurpers won more contests overall, and more aggressive fish were more likely to win contests. To our knowledge, this is the first study to examine resource contests in wild free-swimming cichlids, and our results reveal that subjective resource value is a primary driver of the dynamics and outcome of territorial conflicts among dominant N. pulcher males. Our results further suggest that respect for ownership may reduce conflict in N. pulcher and be important in governing colony structure.

Quiet threats: soft song as an aggressive signal in birds

Theory suggests that aggressive signals must be costly if they are to be reliable. Recent research in birds has shown, however, that in many species the best predictors of impending attack are low-amplitude vocal signals, soft songs or soft calls, that seem cheap to produce and easy to cheat. This observation leads to two related but separate questions: (1) why use low-amplitude signals to communicate aggressiveness and (2) what maintains the reliability of soft signals of aggression? We review potential answers to both questions and present evidence relevant to each. While some hypotheses are logically sound, others have logical flaws, and most of the hypotheses have yet to be critically tested. One exception is the hypothesis that the reliability of soft signals of aggressiveness is maintained by receiver retaliation, which has been supported by experimental evidence in multiple species. We emphasize the need for further research, particularly to answer the question of why soft song is soft, and outline future research directions. • Low-amplitude vocalizations are honest signals used in aggressive contests by animals. • Here we address two questions: why are they soft, and why are they reliable? • We review several hypotheses and pertaining evidence, as well as assumptions. • The reliability of soft vocalizations is due to receiver retaliation. • More research is needed to understand why soft vocalizations are soft.

Analysis of the phenotypic link between behavioural traits at mixing and increased long-term social stability in group-housed pigs

Mixing of growing pigs results in aggressive contests between group members. As aggression serves to establish dominance relationships, it is possible that increased initial aggression may facilitate the formation of social hierarchies. The objective of the study was to investigate whether there is a phenotypic link between behavioural traits of aggression at mixing and increased long-term group social stability. Aggressive behavioural traits were recorded for 24h after mixing, whereas the numbers of skin lesions (anterior, central and posterior) were obtained 24h (SL24h) and 3 weeks post-mixing (SL3wk) for 1,166 pigs. At the group level, aggressive behavioural traits were positively correlated with anterior SL24h (0.34 to 0.67; P<0.01) at mixing, and negatively with central SL3wk (−0.28 to −0.38; P<0.01) in the stable group. At the individual animal level, most behavioural traits of aggressiveness correlated positively with SL24h (0.09 to 0.53; P<0.001), whereas the opposite associations were found for SL3wk (−0.06 to −0.14; P<0.05). Within aggressive cohorts, animals with a high fight success rate received slightly fewer SL24h than equally aggressive, but unsuccessful pen mates, while animals that avoided aggression received the fewest SL24h. Corresponding associations were reversed in the stable group. These results provide evidence that increased aggression at mixing may aid stable hierarchy formation. This raises an ethical dilemma in pigs production, but potentially also in other species, that increased acute aggression during mixing may actually decrease chronic aggression in groups and thus benefit the long term welfare of the group.

Building a dishonest signal: the functional basis of unreliable signals of strength in males of the two-toned fiddler crab, Uca vomeris.

Males of many species use signals during aggressive contests to communicate their fighting capacity. These signals are usually reliable indicators of an individual's underlying quality; however, in several crustacean species, displays of weapons do not always accurately reflect the attribute being advertised. Male fiddler crabs possess one enlarged claw that is used to attract females and to intimidate opponents during territorial contests. After the loss of their major claw, males can regenerate a replacement claw that is similar in size but considerably weaker. As this inferior weapon can still be used to successfully intimidate rivals, it represents one of the clearest cases of unreliable signalling of strength during territorial contests. We investigated the functional mechanisms that govern signal reliability in the two-toned fiddler crab, Uca vomeris. Male U. vomeris exhibit both reliable and unreliable signals of strength via the expression of original and regenerated claw morphs. We examined the morphological, biomechanical and biochemical characteristics of original and regenerated claws to establish the best predictors of variation in claw strength. For a given claw size, regenerated claws have less muscle mass than original claws, and for a given muscle mass, regenerated claws were significantly weaker than original claws. The mechanical advantage was also lower in regenerated claws compared with original claws. However, the activity of three catabolic enzymes did not differ between claw types. We conclude that the structural and physiological predictors of force production influence the frequencies of reliable and unreliable signals of strength in U. vomeris. This study furthers our understanding of the proliferation of unreliable signals in natural populations.

Mortal combat and competition for oviposition sites in female pollinating fig wasps

Aggressive contests between animals are common but rarely result in death because the benefits of winning a fight rarely exceed the cost of losing. Lethal combat can evolve, however, when the contested resource translates to much of the future reproductive success of each combatant. Female agaonid fig wasps pollinate and lay their eggs in the flowers within the enclosed inflorescences (“figs”) of fig trees (Ficus spp.). Wasps rarely leave the first fig entered so the reproductive success of each “foundress” usually depends on the availability of flowers within a single fig. We report for the first time lethal combat between female agaonids, in the undescribed Pegoscapus sp. that pollinates Ficus citrifolia in southeastern Brazil. In staged dyadic contests, wasps showed no aggression outside or inside the fig until one foundress oviposited. The first wasp to oviposit then became aggressive, which usually resulted in the death of its competitor. Examination of dead foundresses in naturally occurring figs showed that injuring competitors, particularly through decapitation, was effective at reducing their oviposition rates. In a Panamanian F. citrifolia population, pollinated by another wasp species, Pegoscapus tonduzi , there was little aggression between foundresses in similar contests. We suggest that reduced aggression in P. tonduzi reflects less competition for resources essential for successful reproduction due to on average fewer foundresses per flower in Panama.

Dependence of aggression and hostility level on the type of temperament and properties of the central nervous system

Across the animal kingdom, social interactions, ranging from affiliative cooperative exchanges to intensely aggressive contests, have a fundamental impact upon the health and wellbeing of an individual. Social life is often quite intricate, full of rewards, but also the cause of serious conflict and intense stresses. In this chapter, the sociobiology of feral rodents and their natural defense mechanisms are described as a starting point in the evaluation of the causes and consequences of aggressive behaviors and social stress in laboratory animals. The use of ecologically relevant conditions provides animal models with a high degree of face and construct validity. Considering the ecological significance of the dynamic changes in behavior and physiology of animals living in a social structure allows an answer to the fundamental question of the adaptive or maladaptive nature of the changes observed. It is important to notice that societies are by definition based on individuals, each with its own individual trait characteristics. Hence, a focus on individual differences may provide answers to the question of individual vulnerability and resilience.

When David beats Goliath: the advantage of large size in interspecific aggressive contests declines over evolutionary time.

Body size has long been recognized to play a key role in shaping species interactions. For example, while small species thrive in a diversity of environments, they typically lose aggressive contests for resources with larger species. However, numerous examples exist of smaller species dominating larger species during aggressive interactions, suggesting that the evolution of traits can allow species to overcome the competitive disadvantage of small size. If these traits accumulate as lineages diverge, then the advantage of large size in interspecific aggressive interactions should decline with increased evolutionary distance. We tested this hypothesis using data on the outcomes of 23,362 aggressive interactions among 246 bird species pairs involving vultures at carcasses, hummingbirds at nectar sources, and antbirds and woodcreepers at army ant swarms. We found the advantage of large size declined as species became more evolutionarily divergent, and smaller species were more likely to dominate aggressive contests when interacting with more distantly-related species. These results appear to be caused by both the evolution of traits in smaller species that enhanced their abilities in aggressive contests, and the evolution of traits in larger species that were adaptive for other functions, but compromised their abilities to compete aggressively. Specific traits that may provide advantages to small species in aggressive interactions included well-developed leg musculature and talons, enhanced flight acceleration and maneuverability, novel fighting behaviors, and traits associated with aggression, such as testosterone and muscle development. Traits that may have hindered larger species in aggressive interactions included the evolution of morphologies for tree trunk foraging that compromised performance in aggressive contests away from trunks, and the evolution of migration. Overall, our results suggest that fundamental trade-offs, such as those associated with body size, are more likely to break down over evolutionary time, changing the rules that govern species interactions and structure ecological communities.

An experimental test of the role of structural blue and melanin-based chestnut coloration in aggressive contests in male eastern bluebirds

Male eastern bluebirds (Sialia sialis) have feathers with either structurally based blue coloration or melanin-based chestnut coloration, and they hold territories during the breeding season that they defend vigorously. We tested whether the melanin pigmentation or structural coloration of feathers serve as signals during intrasexual aggressive encounters by placing color-modified stuffed bluebirds in male territories. We recorded the time to attack and the intensity of attacks on each model, and we then compared the response to color-enhanced versus color-reduced models. Male bluebirds attacked models with brighter and more chromatic blue coloration significantly more often than they attacked models with darker and less chromatic blue coloration. In contrast, the darkness of the chestnut breast coloration did not have a significant effect on the rate at which models were attacked. We conclude that territorial male bluebirds perceive intruding males with brighter blue coloration as a greater threat than males with drabber blue coloration, presumably because blue coloration is a signal of fighting ability. In contrast, the chestnut coloration of breast feathers appears to be a signal of gender and sexual maturity and not a graded signal of social status.

Contested Virilities: Constructing Masculinity in Dante’s Tenzone with Forese

This study explores the construction of Forese Donati’s gendered identity in Dante’s tenzone with Forese and, to a lesser extent, in the Purgatorio. The contradictory Forese episodes are analyzed in terms of gender practice and performance, and shown to be complementary representations of masculinity. I argue that as an aggressive contest for verbal dominance, the tenzone with Forese captures the dynamic struggle to assert hegemonic virility within Florence’s urban aristocracy. Viewed through the lens of a society that placed specific demands upon the men entrusted with the care of their households, Forese’s maleness is measured against his (in)ability to physically, economically, and affectively satisfy the needs of his wife. A gendered approach provides new insights into the nature of Dante’s comic insults, and casts doubt on some of the basic assumptions behind palinodic readings that connect the two encounters with Forese.

Information gathering during contests: The relationship between lateralisation and contestant behaviour during fallow deer fights

One class of model relating to animal contest behaviour assumes that individuals gather information concerning their opponents’ competitive ability; these models argue that such a process allows contestants to avoid engaging in dangerous fighting behaviour with a superior opponent. The brain hemispheres of vertebrates are lateralised in that they are specialised for processing different type of information. Within the context of the current study, we might expect that lateralisation would play a role in facilitating the assessment of opponent quality; nevertheless, the degree of lateralisation shown by individuals can vary suggesting that contest behaviour might also vary based on the ability to process information about competitor quality. The current study tests this hypothesis by predicting that the duration that individuals engage in fighting and the rate of aggressive contest actions should decrease as lateralisation increases. There was a positive relationship between two laterality indices and the duration spent in antler contact; thus lateralised individuals experienced greater time costs. Further, lateralised individuals also experienced a greater disparity in contest actions: there was a negative relationship between lateralisation and the difference in the mean number of backward pushes achieved during fights. When only opponent signal rate was considered there was no effect of lateralisation, therefore, there is support for a mutual assessment process. These results suggest that information gathering via lateral displays may be disadvantageous to lateralised individuals during escalated fighting.

Kleptoparasitic Interactions under Asymmetric Resource Valuation

We introduce a game theoretical model of stealing interactions. We model the situation as an extensive form game when one individual may attempt to steal a valuable item from another who may in turn defend it. The population is not homogeneous, but rather each individual has a different Resource Holding Potential (RHP). We assume that RHP not only influences the outcome of the potential aggressive contest (the individual with the larger RHP is more likely to win), but that it also influences how an individual values a particular resource. We investigate several valuation scenarios and study the prevalence of aggressive behaviour. We conclude that the relationship between RHP and resource value is crucial, where some cases lead to fights predominantly between pairs of strong individuals, and some between pairs of weak individuals. Other cases lead to no fights with one individual conceding, and the order of strategy selection is crucial, where the individual which picks its strategy first often has an advantage.

Heightened serotonin influences contest outcome and enhances expression of high-intensity aggressive behaviors

The outcome of behavioral interactions between organisms can have significant fitness implications. Therefore, it is of great theoretical and practical importance to understand the mechanisms that modify different agonistic behaviors. Changes in central monoamines, such as serotonin (5-HT), contribute to modifying the expression of aggressive encounters in both vertebrates and invertebrates. In several invertebrate groups, neural 5-HT has been linked to heightened aggression and conflict escalation. The male stalk-eyed fly (Teleopsis dalmanni) competes with conspecifics daily over access to resources such as food and mates. Because encounters escalate in a stereotypical manner, stalk-eyed flies provide an excellent model system to study behavioral syndromes. We hypothesized that noninvasive, pharmacological augmentation of brain 5-HT by administration of the precursor, 5-hydroxytryptophan (5-HTP), would increase stereotypic behavioral escalation and the probability of winning a conflict over food. Size-matched male 5-HTP-treated and untreated flies were placed in a forced-fight paradigm and their aggressive behaviors scored. Individuals with higher brain 5-HT levels had a markedly higher probability of winning the contests, displayed greater levels of high-intensity aggressive behaviors and fewer retreats. Pretreatment with 5-HTP did not significantly alter octopamine or tyramine, suggesting that central 5-HT may modulate aggression in these organisms and play a role in determining reproductive success and resource attainment.

Male collared lizards,Crotaphytus collaris(Sauria: Crotaphytidae), signal females by broadcasting visual displays

The signalling function of displays broadcast when animals are distant from conspecifics can be difficult to determine. I tested the extent to which visually transmitted broadcast displays given by free-ranging territorial male collared lizards signalled same-sex rivals or females. One test involved recording the frequency of broadcast displays, aggressive contests with rivals, and courtship encounters with females during ten reproductive seasons when local sex ratios varied markedly. The frequency of broadcast displays decreased as the ratio of male competitors to females increased. The frequency with which males initiated contests with rivals was not related to the ratio of competitors to females, whereas the frequency of courtship interactions decreased with sex ratio because there were fewer females to court. The behaviour of males that defended territories during two successive seasons showed a similar pattern. Broadcast display frequency was positively correlated with courtship frequency, but not with the frequency of contests with rivals. Lastly, individual males gave more broadcast displays during focal observations when they also engaged in courtship encounters with females than other observations when they engaged in aggressive conflicts with rival males. Although these results do not reject the possibility that broadcast displays may also signal male rivals, they support a major role of these displays in advertisement to females.

Steroid hormone levels in calling males and males practicing alternative non-calling mating tactics in the green treefrog, Hyla cinerea

Circulating glucocorticoids and androgens often figure prominently in mating tactic expression in vertebrates. In anuran amphibians (frogs and toads), for example, recent models predict that the depletion of energy reserves during vocalization will result in increased glucocorticoid levels; high glucocorticoids are expected to negatively affect androgen level to mediate transitions from calling to non-calling behavior. Consistent with these predictions, we show that male green treefrogs (Hyla cinerea) adopting an alternative non-calling “satellite” mating tactic were in poorer condition and had higher circulating corticosterone levels and lower androgen levels than calling males. Body condition was inversely related to corticosterone level and positively related to testosterone, but not dihydrotestosterone, level. Corticosterone level was inversely related to testosterone level but not dihydrotestosterone level. Lastly, we show that calling males that were involved in aggressive bouts had higher corticosterone levels than calling males that were not involved in aggressive bouts. Our results are thus consistent with the prediction that aggressive interactions with conspecific males contribute to high corticosterone levels in satellite males that were observed to lose aggressive contests with larger calling males.

Signaling effort does not predict aggressiveness in male spring field crickets

The relationship between signaling and aggression is still unclear despite several decades of research. However, there is a growing interest in studying signals that predict aggressive behavior or fighting ability. The goals of our study were threefold: we investigated the relationship between signaling effort and aggression, the relationship between body condition and aggression, and the effect of fighting experience on subsequent signaling behavior in wild-caught and laboratory-reared male spring field crickets (Gryllus veletis). We found that aggressive behavior was not related to signaling effort, body size, or body condition. For contest winners, wild-captured males were more aggressive than laboratory-reared males. Signaling effort was highly repeatable within individuals, but aggressive behavior had low repeatability. We found no evidence for a winner or loser effect on signaling; there was no change in signaling effort when we compared contest winners and losers before and after they participated in aggressive contests. Long-distance acoustic signaling and aggressive behavior appear to be independent of one another in spring field crickets, perhaps serving different functions in female attraction and male–male competition, respectively.

Genetic associations between behavioral traits and direct-social effects of growth rate in pigs.

This study examined the behavioral consequences of selecting pigs using a social genetic model for growth. Calculations enable each member of a group of pigs to be given a direct breeding value (DBV) and a social breeding value (SBV), which can be summarized into a total breeding value (TBV) for growth. Selection for growth TBV could affect animal behavior because social effects account for within-group interactions. Data were recorded from 96 groups of Yorkshire and Yorkshire × Landrace pigs in a nucleus herd. Each group contained 15 pigs fed ad libitum from 2 feeders; the space allowance was 0.85 m2/pig. Average daily gain was quantified from 35 to 100 kg of BW. Fighting and bullying activity at mixing (period 1), lying frequency 3 wk after mixing (period 2), and counts of skin lesions in periods 1 and 2 were recorded. The DBV for these traits were estimated with a classic animal model. We simulated different correlations between the direct genetic effect and the social genetic effect on growth rate (r(DS)), 2 components that respectively determine a pig's genetic capacity to grow and its genetic influence on growth of group mates: r(DS) was successively assumed to be 0 and ±0.12, ±0.20, ±0.29, and ±0.58. Finally, the correlations between DBV, SBV, and TBV for ADG, as well as the DBV for behavior and skin lesions, were calculated and tested for a level of significance at P < 0.05. The gradient from negative to positive values of r(DS) refers to a progressive path running from genetic antagonism to genetic mutualism for growth. If rDS in the population truly ranged between -0.58 and -0.20, correlations for TBV for ADG with DBV for fighting and bullying progressively increased with rDS. Consequently, if rDS was low (between -0.12 and +0.12) or positive (>+0.12), pigs with high TBV for ADG had higher DBV for bullying other pigs in the group and for fighting than pigs with lower TBV for ADG. Pigs with high TBV for ADG did not differ from other pigs in their DBV for lesions to the anterior part of the body, but they had a lower DBV for posterior lesions, whereas in period 2, they had higher DBV for posterior lesions and lower DBV for lying. Under genetic mutualism for growth and in housing conditions similar to those in the present study, selection for growth TBV would promote the rapid establishment of the dominance relationships, with more aggressive contests among group mates at mixing. Pigs would subsequently be more active but, judging by skin lesions, less willing to fight in a more stable social situation.

Changes in reproductive behavior in adult damselfly Calopteryx splendens (Odonata: Calopterygidae) in response to flood

AbstractThe reproductive behavior of adult Calopteryx splendens males and females inhabiting the Nida River, south Poland, was studied and compared during a pre‐flood and a post‐flood year. The flood disturbance in 2010 caused a decrease in aquatic macrophytes, thus reducing availability of potential territories and consequently, significantly influencing male behavior towards a frequent non‐territorial strategy. Many males in the post‐flood population had damaged wings due to extremely aggressive contests. Male–male tandems were commonly observed; this is an uncommon behavior in C. splendens. Although the sex ratio was male‐biased throughout the whole study, we observed more males in the post‐flood year. We also observed less‐frequent copulations and ovipositions during the post‐flood year. The only unchanged characteristic was population density, which did not differ before and after the flood disturbance. Floods have significant impact on damselfly reproductive sites and this, due to changes in behavior and sex ratio, may result in further consequences on population dynamics.