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Natural cavities are monopolizable resources used as nest sites across many species exhibiting exclusive male care. However, our understanding of how the availability of this type of reproductive site influences male–male competition and the foraging costs associated with parental care is limited and based on studies of fish. The harvestman Magnispina neptunus offers the opportunity to explore these questions in invertebrates because males defend natural cavities used as oviposition sites and provide exclusive parental care. We hypothesized that low nest availability would increase male–male competition and the foraging costs of egg attendance. To test this hypothesis, we collected observational data in the field and conducted a laboratory experiment with two groups, low and high nest availability. Field data indicate natural cavity scarcity, as only 50% of the males were nest holders. Larger males were more likely to hold nests in the field, but body size did not predict nest ownership in the laboratory. In the field, nests with openings smaller than average were equally occupied by small and large males, while nests with openings larger than average were occupied almost exclusively by large males. The nest opening in the laboratory experiment was adjusted to be equal to the average found in the field, which may have increased nest defensibility, regardless of male size. Thus, prior residency may outweigh body size in determining nest possession. Because nest owners seem to have an advantage over intruders, the costs of abandoning a nest are likely high. This may explain why the frequency of nest takeovers and foraging outside the nests did not differ between experimental groups. In conclusion, although underexplored in empirical studies, external factors, such as the opening size of cavities defended by males of many species with resource‐defense mating systems, may play a key role in the defensibility of nest sites. [ABSTRACT FROM AUTHOR]

Understanding the maintenance and dynamics of phenotypic polymorphisms requires unpicking key ecological mechanisms shaping the fitness costs and benefits of expressing alternative phenotypes, generating selection. Seasonal migration versus year‐round residence expressed in partially migratory populations represents one common polymorphism that can experience strong selection through differential reproductive success. Yet, key hypothesised pathways that could generate such selection remain to be empirically tested.One hypothesis is that migratory tactics affect subsequent reproductive success through carry‐over effects on breeding site retention and resulting breeding dispersal. By remaining in breeding areas all year round, residents could retain their preferred breeding site between years, and consequently have higher reproductive success. Conversely, migrants that escape harsh non‐breeding season conditions could return in better condition, with high resource holding potential, and outcompete residents to retain their site. Such effects could further depend on migration timing and vary between years. Yet, such pathways have not been quantified, precluding empirical parameterisation of partial migration theory.We used 4 years of breeding and non‐breeding season data from partially migratory European shags (Gulosus aristotelis) to test whether the three most frequent migratory tactics in this population (full resident, early migrant departing soon after breeding, and late migrant departing in late autumn) differed in their breeding site retention; whether site retention predicted reproductive success; and hence whether effects of migratory tactic on reproductive success were explicable through site retention.Overall, residents were much more likely to retain their breeding site between years than both early and late migrants, and site retention was associated with increased reproductive success. Yet, these effects varied somewhat among years: late migrants were always least likely to retain their site but had variable relative reproductive success. Path analyses revealed that effects of migratory tactic on reproductive success were only partly attributable to breeding site retention.These results indicate that multiple mechanisms underlie reproductive selection on migratory tactics, potentially contributing to maintaining behavioural polymorphisms. Yet, the clear associations between migratory tactics and local breeding dispersal reveal that these movements can be strongly interlinked across seasons, shaping overall spatioseasonal dynamics in partially migratory systems. [ABSTRACT FROM AUTHOR]

When resources such as mates or mating sites are limited, selection drives the evolution of complex and frequently violent fighting behaviour. Contest outcome is determined by an individual's resource holding potential (RHP) in comparison to that of their opponent. During contests individuals may assess only their own RHP, or they may mutually assess both their own and their opponent's RHP to determine the effort they allocate to a contest. Male giraffe weevils (Lasiorhynchus barbicornis) bear an elongated rostrum used as a weapon during intense fights over access to females. We used sequential analysis to target the fine-scale structure of contests and phases of contest escalation to determine the assessment strategy used by rival male giraffe weevils in the wild. We found significant non-random temporal structure in behavioural transitions in all transition matrices, and winner and loser transition matrices were significantly different. Contest de-escalation was extremely rare compared to escalation and winners and losers used very different behaviours during contests overall. This study provides support for the sequential assessment model of mutual assessment by male giraffe weevils during competitive interactions. It also demonstrates the utility of sequential analysis in making sense of the often complex interactions occurring in wild insect populations. [ABSTRACT FROM AUTHOR]

Animals use colours, morphological structures and behaviour to advertise their dominance status and ability to obtain resources (Resource Holding Potential, RHP) in agonistic contexts with conspecifics. Dominance relationships are also known between species, although the interactions and the information used to assess the other species' RHP remain obscure and empirical evidence of interspecific recognition of status is hard to obtain. Using behavioural inference is particularly important when a species needs to assess the potential loss of fitness in terms of reproduction or survival (e.g., risk of damage or predation) that could be inflicted by another species acting as a predator, parasite or competitor. Our study shows for the first time the existence of interspecific recognition of status signals: Blue tits Cyanistes caeruleus (subordinate species) perceive Great tits Parus major (dominant species) as threats as hole‐nesting competitors, and respond differently based on the size of the great tit's black tie stripe. Blue tits seemed to recognize the meaning of great tits' black ties because they were bolder when defending their nests against small‐tie (less dominant) great tits than when repelling attacks by large‐tie (more dominant) great tit males. Our results stress that behavioural inference can potentially be used to assess the meaning of status signals that presumably evolved in an intraspecific signalling context. [ABSTRACT FROM AUTHOR]

Animals often engage in contests over limited resources. The probability of winning a contest is primarily determined by the individual's fighting ability relative to its opponent and the perceived value of the disputed resource. Individuals of the hermit crab Calcinus californiensis frequently fight over gastropod shells. We conducted a free-choice experiment to assess the factors that determine individuals’ choice of opponent, contest initiation, and contest resolution. We placed hermit crabs occupying two shell species into a large container and monitored agonistic interactions between crabs. We assessed the asymmetries in fighting ability based on the differences in body mass between the opponents. The shell species and fit (i.e., shell size relative to crab body size) were used as measures of the objective and subjective resource value, respectively. Motivation influenced contest initiation; the crabs occupying too-tight shells were more likely to initiate a contest than the ones in looser-fitting shells. In most cases, the attackers fought for a shell with a looser fit, even if that meant losing a tighter-fitting shell of the preferred shell species. The fighting success for attackers was positively associated with the number of bouts of rapping and the shell size improvement. However, success was negatively correlated with body size asymmetry; attackers that chose opponents larger than themselves were more likely to evict their opponent than the attackers that chose opponents that were smaller. Experimental designs that allow animals to select their own opponents, rather than assigning specific opponents, can strongly contribute to knowledge of agonistic interactions. Significance statement: Fighting dynamics are commonly assessed by estimating the influence of the resource-holding potential and resource value in pair-matched opponents. Here, however, we examined the influence of asymmetries in resource-holding potential and the objective and subjective value of gastropod shells on the choice of a contender and contest resolution through a free-choice opponent experiment in hermit crabs. The contest initiation was driven by the motivation to obtain a better gastropod shell species or one with a better size fit—factors which are known to improve individual fitness. Furthermore, fighting success was associated with an individual's persistence in displaying aggressive behaviors and its motivation to obtain a better resource. However, contrary to the predictions of game theoretical models, fighting success was higher when attackers chose opponents larger than themselves. Our results highlight the relevance of assessing fighting under more natural conditions by allowing animals to select their opponents. [ABSTRACT FROM AUTHOR]

Skill, the ability to perform a challenging behaviour well, has been shown to be an important determinant of success in a variety of contexts, including human sports, animal courtship and, most recently, animal contests. Because skilful movement requires precise motor control, skill is assumed to be underpinned by traits that determine these abilities. However, while these traits determine an individual's potential to perform effective movements (known as technique), this potential may not translate into skilful fighting due to interference from the opponent. Here, we investigated the relationship between technique and skill using the European hermit crab, Pagurus bernhardus. By examining the spatial distribution of shell raps, we found that, on average, technique exhibited during a 'training' fight predicted the level of skill displayed in a real contest. However, our results also demonstrated substantial among-individual variation in the direction of change across the two fights, with some individuals exhibiting better technique than skill and others showing the opposite pattern. Finally, we found that winners, but not losers, progressively adapted their targeting of strikes when faced with a fully functional opponent. Our results indicate that skill is a combination of innate technique and the ability to adapt to an opponent's behaviour. • Skill has recently been shown to be a key determinant of fighting success. • Skill is assumed to rely on innate capacities for motor control (technique). • But opponent interference may impede an individual's ability to perform skilfully. • Here we show that innate technique is indicative of the ability to fight skilfully. • We also find that skilful fighting relies on the ability to adapt to your opponent. [ABSTRACT FROM AUTHOR]

Animal groups are often organized hierarchically, with dominant individuals gaining priority access to resources and reproduction over subordinate individuals. Initial dominance hierarchy formation may be influenced by multiple interacting factors, including an animal’s individual attributes, conventions and self-organizing social dynamics. After establishment, hierarchies are typically maintained over the long-term because individuals save time, energy and reduce the risk of injury by recognizing and abiding by established dominance relationships. A separate set of behaviours are used to maintain dominance relationships within groups, including behaviours that stabilize ranks (punishment, threats, behavioural asymmetry), as well as signals that provide information about dominance rank (individual identity signals, signals of dominance). In this review, we describe the behaviours used to establish and maintain dominance hierarchies across different taxa and types of societies. We also review opportunities for future research including: testing how self-organizing behavioural dynamics interact with other factors to mediate dominance hierarchy formation, measuring the long-term stability of social hierarchies and the factors that disrupt hierarchy stability, incorporating phenotypic plasticity into our understanding of the behavioural dynamics of hierarchies and considering how cognition coevolves with the behaviours used to establish and maintain hierarchies. This article is part of the theme issue ‘The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies’. [ABSTRACT FROM AUTHOR]

The evolution of territoriality reflects the balance between the benefit and cost of monopolising a resource. While the benefit of territoriality is generally intuitive (improved access to resources), our understanding of its cost is less clear. This paper combines: 1. a review of hypotheses and meta-analytic benchmarking of costs across diverse taxa; and 2. a new empirical test of hypotheses using a longitudinal study of free-living male territorial lizards. The cost of territoriality was best described as a culmination of multiple factors, but especially costs resulting from the time required to maintain a territory (identified by the meta-analysis) or those exacerbated by a territory that is large in size (identified by the empirical test). The meta-analysis showed that physiological costs such as energetic expenditure or stress were largely negligible in impact on territory holders. Species that used territories to monopolise access to mates appeared to incur the greatest costs, whereas those defending food resources experienced the least. The single largest gap in our current understanding revealed by the literature review is the potential cost associated with increased predation. There is also a clear need for multiple costs to be evaluated concurrently in a single species. The empirical component of this study showcases a powerful analytical framework for evaluating a range of hypotheses using correlational data obtained in the field. More broadly, this paper highlights key factors that should be considered in any investigation that attempts to account for the evolutionary origin or ecological variation in territorial behaviour within and between species. [ABSTRACT FROM AUTHOR]

Intraspecific conflict can be costly; therefore, many species engage in ritualized contests composed of several stages. Each stage is typically characterized by different levels of aggression, arousal, and physical conflict. During these different levels of “intensity,” animals benefit from communicating potential information related to features such as resource holding potential, relative fighting ability, level of aggression, intent (i.e., fight or flight), and whether or not the competitor currently holds the resource (e.g., a receptive female). This information may be conveyed using both visual displays and a complex acoustic repertoire containing fixed (e.g., age, sex, and body size) and flexible information (e.g., motivation or arousal). Calls that contain fixed information are generally considered “discrete” or stereotyped, while calls that convey flexible information are more “graded,” existing along an acoustic continuum. The use of displays and calls, and the potential information they convey, is likely dependent on factors like intensity level. The breeding system of humpback whales (Megaptera novaeangliae) involves intense male competition for access to a relatively limited number of breeding females (the resource). Here, we investigated the behavior and acoustic repertoire of competitive groups of humpback whales to determine if an increase in intensity level of the group was correlated with an increase in the complexity of the vocal repertoire. We categorized the behavior of humpback whales in competitive groups into three mutually exclusive stages from low to high intensity. While discrete calls were infrequent compared to graded calls overall, their use was highest in “low” and “moderate” intensity groups, which may indicate that this stage of contest is important for assessing the relative resource holding potential of competitors. In contrast, visual displays, call rates, and the use of graded call types, were highest during “high intensity” competitive groups. This suggests that flexible information may be more important in “high intensity” levels as males continue to assess the motivation and intent of competitors while actively engaged in costly conflict. We have shown that the relatively complex social call repertoire and visual displays of humpback whales in competitive groups likely functions to mediate frequently changing within-group relationships.

Aggressive behaviour is common in animals and typically has lifetime consequences. As younger males have higher residual reproductive value than older males and lose more from injuries than older males do, the propensity for fighting tends to increase with age in many empirical reports and species. However, fighting patterns in those empirical reports cannot confirm the hypothesis that individuals cannot readily inflict injuries on their opponents. To address this shortcoming, a parasitoid wasp species, Anastatus disparis (Hymenoptera: Eupelmidae), was used as an experimental model to explore the characteristics of aggression from a life-history perspective; this wasp exhibits extreme fighting, resulting in contestants experiencing injury and death. Results showed that the energetic costs of fighting to injury significantly shortened life and caused the loss of most mating ability. Inconsistent with general predictions, the frequency and intensity of fighting in A. disparis significantly decreased with male age. Further study results showed significantly more young males were received by and successfully mated with virgin females, and most genes related to energy metabolism were downregulated in aged males. Our study provided supporting evidence that young A. disparis males show more aggression likely because of their resource holding potential and sexual attractiveness decline with age. [ABSTRACT FROM AUTHOR]

We consider a game theoretical model of kleptoparasitic interaction between two individuals, the Owner and the Intruder. The Owner is in possession of a resource and must decide whether to defend the resource against the Intruder or flee. If the Owner defends, the Intruder must decide whether to fight with the Owner or flee. The outcome of the fight depends on the hunger of the individuals, the hungrier the individual is, the more likely they are to win the fight. We consider three scenarios: (a) both individuals know their own and their opponent's hunger, (b) individuals only know their own hunger but not that of their opponent, and (c) individuals do not know their own nor the opponent's hunger levels. We determine Nash equilibrium strategies in each scenario. We conclude that Owner is generally willing to defend more often than the Intruder is willing to attack. Also, the Intruder's payoff is largest in the full information case; but the Owner may benefit in the no information or partial information cases when the cost of the fight is neither too large nor too small. [ABSTRACT FROM AUTHOR]

During aggression, animals signal their physical abilities to deter opponents from fighting. Competitors that have a coevolved system of communication can resolve the outcome of a fight without paying the cost of engaging in potentially injurious fighting. Invasive species complicate the dynamics of competition, because such species may not have a coevolved system of communication. For example, many species of crayfish have invaded freshwater ecosystems, where they compete with species never encountered before. We studied the traits used to escalate aggression within and between two species of invasive crayfish in Arizona, Procambarus clarkii and Faxonius virilis. Through staged encounters, we found that individuals of both species escalated aggression according to their relative claw size. Furthermore, claw size (instead of claw strength) determined the outcome of interactions that escalated to physical combat. Despite P. clarkii and F. virilis presumably not having a coevolved system of communication, claw size predicted the outcome of interspecific aggression. In crayfish and other species that have invasive potential, future investigations into intraspecific versus interspecific communication dynamics are warranted as they may shed light on the successes and failures of past and present species introductions. • We studied traits used to escalate aggression in two species of invasive crayfish. • Both species escalated and resolved disputes using their claws as signals. • Claw size determined the outcome of interactions that escalated to physical combat. • Species' ability to communicate with competitors may affect invasive potential. [ABSTRACT FROM AUTHOR]

Residency is an important predictor of success in contests with ownership asymmetries. Residency often can interact with a winning experience. However, given that some residents lose a contest even when showing an ownership advantage and that the process leading to loss often determines the loser's subsequent success, prior ownership might also interact with a loss. Here, we staged experimental contests between males of the hermit crab Pagurus minutus with a similar‐sized weapon (i.e., cheliped) to examine this possibility. Male–male contests in this species occur between a solitary intruder and an owner guarding a mature female. We evaluated (a) whether resource ownership and female value affect the contest outcome and (b) whether the probability of winning after losing differs depending on the initial role of the loser (i.e., owner or intruder) by using precopulatory guarding pairs of P. minutus collected from the field. In the first fighting trial, we found an ownership advantage and increasing owner success as the body size of his partner increased. Although some owners lost the fight, in contrast to our prediction, the frequency of losing in the second fighting trial did not differ between prior owners and prior intruders. Because losers from the first fighting trial of male–male contests have no female regardless of their initial role, this shared solitary status might be related to the lack of difference in success in the second fighting trial. Moreover, unlike in other animals, resident status might not always assure greater fighting ability in P. minutus males because guarding Pagurus males can avoid male–male contests by climbing up objects in the field. Losers in the first trial, therefore, may have been weaker contestants based on traits other than size, regardless of whether they were owners or intruders. [ABSTRACT FROM AUTHOR]

When a species moves into a new area, direct contests with functionally similar residents may determine whether they can establish themselves. In recent years, the web-building cellar spider, Pholcus manueli, has largely displaced its larger well-established congener, Pholcus phalangioides, in SW Ohio. The relationship between these two species is interesting both because they are congeners and because both are invasive; the resident, P. phalangioides, had robust populations in the area for more than 100 years. We hypothesized that this example of over-invasion was related to interactions over web sites or webs. We first staged interspecific contests between adult females over empty web sites and existing webs in the laboratory. The smaller P. manueli never won but many encounters ended as ties with little active engagement between the spiders. We then staged contests with juvenile spiders that were close in size. We introduced well-fed or food-limited P. manueli into webs of P. phalangioides. In these matches, the relative size difference, a proxy for resource holding potential, determined the outcome. However, fights involving well-fed P. manueli were shorter and more aggressive than contests with hungry individuals. Additionally, well-fed invaders were more likely to take over webs whilst hungry P. manueli were less likely to engage in contests, thus allowing coexistence. We posited that newly introduced individuals, immersed in a population of P. phalangioides, persisted on the sidelines of the resident's webs, and engaged in interspecific aggression only when the dynamics of the interaction were in their favor. [ABSTRACT FROM AUTHOR]

More is known about aggression initiation and persistence in other animals, and anger in people, than about their cessation. This review summarizes knowledge of relevant factors in aggression, mostly in vertebrates, and anger termination in people. The latency, probability and intensity of offensive aggression in mice is controlled by activity in a neuronal subpopulation in ventromedial hypothalamus [VMH]. This activity instantiates an aggressive state termed angriffsbereitschaft ["attack-readiness"]. Fighting in many species is broken into bouts with interbout breaks due to fatigue and/or signals from dorsal raphe to VMH. Eventually, losers decide durations and outcomes of fighting by transitioning to submission or flight. Factors reducing angriffsbereitschaft and triggering these defeat behaviors could include metabolic costs, e.g., lactate accumulation and glucose depletion detected by the hypothalamus, central fatigue perhaps sensed by the Salience Network [insula and anterior cingulate gyrus] and pain of injuries, the latter insufficiently blunted by opioid and non-opioid stress analgesia and transduced by anterior VMH neurons. Winners' angriffsbereitschaft continue for awhile, as indicated by post-victory attacks and, perhaps, triumph displays of some species, including humans. In longer term situations, sensory and/or response habituation of aggression may explain the "Dear enemy" tolerance of competitive neighbors. Prolonged satiation of predatory behavior could involve habenula-regulated reduction of dopaminergic reward in nucleus accumbens. Termination of human anger involves at least three processes, metaphorically termed decay, quenching and catharsis. Hypothesized neural mechanisms include anger diminution by negative feedback from accumbens to anterior cingulate and/or activity in the Salience Network that controls anger's "accumulation/offset" phase. [ABSTRACT FROM AUTHOR]

Floaters are sexually mature individuals that are not able to reproduce by defending breeding resources. Floaters often visit active nests, probably to gather public information or to compete for a nesting site. We tested the hypothesis that floaters preferentially prospect nests in which they have a better chance of taking over, and that they do so by assessing the owners' resource holding potential (RHP). We manipulated the flight capacity of male and female breeders in a population of spotless starlings ( Sturnus unicolor ) by clipping two flight feathers per wing before egg laying, thus increasing their wing-load and likely impairing their condition. We subsequently monitored breeder and floater activity by means of transponder readers during the nestling period. We found that nests owned by wing-clipped males were visited by a greater number of male floaters than control nests. This effect was absent in the case of wing-clipped females. The number of male floaters also increased with increasing nestling age and number of parental visits. The experiment shows that male floaters preferentially prospect nests in which the owner shows a reduced RHP, a strategy that likely allows them to evict weak owners and take over their nests for future reproductive attempts.

The Chinese mitten crab Eriocheir sinensis is an invasive species outside native range and represents a substantial threat to recipient ecosystems. It is regarded as an invasive species in Japan and has recently been found in Tokyo Bay, but little is known about its possible impacts on native species, especially on the congener Japanese mitten crab Eriocheir japonica. This study examined shelter competition between E. sinensis and E. japonica under laboratory conditions. Shelter competition trials were conducted in pairs of native and invasive males in different size combinations. Our results indicated that competition between two species was size-dependent: E. japonica always successfully defended shelter when competing with size-matched or slightly larger E. sinensis; E. sinensis always won shelter when its carapace or chela were at least 10 or 7% larger, respectively, than those of E. japonica. The two species exhibited different behavioural patterns: compared with E. sinensis, E. japonica frequently displayed positive behaviour (approach, walking leg contact, chela contact) and seldomly displayed negative behaviour (retreat). Our results highlight the important role of strong native competitors in controlling invasion success and suggest E. sinensis may be unlikely to exclude native E. japonica from its habitat by direct competition for shelter. [ABSTRACT FROM AUTHOR]

Highlights • Dominance and dominance hierarchies have consequences for animal fitness. • Size, age, or sex may influence dominance and dominance relationships. • Larger round goby were socially dominant to smaller goby. • Males and females did not differ in patterns of dominance establishment. Abstract Organisms living at high densities may be forced to engage in conflict for access to resources such as food or shelter. When these resources are limited, the outcome of interactions may have important fitness implications. We investigated the behavioural interactions of the invasive Round Goby (Neogobius melanostomus) in a shelter-limited environment. Round Goby are benthic fish that utilize rocky shelters for predator avoidance and as reproductive sites in which territorial males defend clutches of eggs. Previous work on this and other species has shown that larger individuals have greater resource holding potential in dyadic interactions. In order to understand the outcome of agonistic interactions in more complex social environments, we observed groups of three goby of the same sex which varied in relative size in an aquarium in which individuals had the opportunity to compete for access to shelters. We predicted that larger goby would behave aggressively towards smaller goby, and outcompete smaller goby for access to shelters. Because males defend shelters while breeding, we also predicted that male goby would compete more aggressively than females over dominance status. We found that larger goby in groups were socially dominant to smaller goby, regardless of sex. Additionally, we found that the largest goby in each group was involved in more aggressive interactions than the second or third largest goby in each group. We found no effect of relative size or sex on aggressive interaction or the emergent dominance relationships. Our findings highlight that aspects of the social environment may limit the opportunity for individuals to establish dominance or establish ownership of resources. [ABSTRACT FROM AUTHOR]

To test how body size might influence lake sturgeon Acipenser fulvescens anti‐predator behaviour, asymmetrically body‐size‐matched individuals were exposed to conspecific chemical alarm cues with the presence or absence of food. Additionally, to test resource holding potential (RHP), hatchery‐reared juvenile A. fulvescens were asymmetrically (c. 60% mass difference) and symmetrically (c. 3% mass difference) size matched in individual tanks. Results suggest that A. fulvescens of higher body condition, rather than body length or mass, may take greater risks when presented with foraging opportunities and realize a higher RHP. As hatcheries are likely to select individuals of higher body condition, more research is necessary to understand the role body condition may have on behavioural responses and ensuing fitness post‐release. [ABSTRACT FROM AUTHOR]

An individual's performance during a fight is influenced by a combination of their capacity and willingness to compete. While willingness to fight is known to be determined by both intrinsic and extrinsic drivers, an individual's capacity to fight is generally thought of as solely intrinsic, being driven by a host of physiological factors. However, evidence indicates that variation in fighting ability can also be generated through exposure to different environmental conditions. Environmental contributions to fighting ability may be particularly important for animals living in spatially and temporally heterogeneous habitats, in which fights can occur between rivals recently exposed to different environmental conditions. The rapidly changing environment experienced within intertidal zones, for example, means that seawater parameters, including dissolved oxygen content and temperature, can vary across small spatial and temporal scales. Here, we investigated the relative importance of these extrinsic contributions to fighting ability and resource value on contest dynamics in the beadlet sea anemone Actinia equina. We manipulated the extrinsic fighting ability of both opponents (through dissolved oxygen concentration prior to fights) and resource value (through seawater flow rate during the fight). Our results indicate that the extrinsic fighting ability of both opponents can interact with resource value to drive escalation patterns and that extrinsic drivers can be more important in determining contest dynamics than the intrinsic traits commonly studied. Our study highlights the need to combine data on intrinsic state and extrinsic conditions in order to gain a more holistic view of the factors driving contest behaviour. [ABSTRACT FROM AUTHOR]

Increasing concentrations of atmospheric carbon dioxide are causing oceanic pH to decline worldwide, a phenomenon termed ocean acidification. Mounting experimental evidence indicates that near-future levels of CO2 will affect calcareous invertebrates such as corals, molluscs and gastropods, by reducing their scope for calcification. Despite extensive research into ocean acidification in recent years, the effects on non-calcifying anthozoans, such as sea anemones, remain little explored. In Western Europe, intertidal anemones such as Actinia equina are abundant, lower trophic-level organisms that function as important ecosystem engineers. Changes to behaviours of these simple predators could have implications for intertidal assemblages. This investigation identified the effects of reduced seawater pH on feeding and contest behaviour by A. equina. Video footage was recorded for A. equina feeding at current-day seawater (pH 8.1), and the least (pH 7.9) and most (pH 7.6) severe end-of-century predictions. Footage was also taken of contests over ownership of space between anemones exposed to reduced pH and those that were not. No statistically significant differences were identified in feeding duration or various aspects of contest behaviour including initiating, winning, inflating acrorhagi, inflicting acrorhagial peels and contest duration. Multivariate analyses showed no effect of pH on a combination of these variables. This provides contrast with other studies where anemones with symbiotic algae thrive in areas of natural increased acidity. Thus, novel experiments using intraspecific contests and resource-holding potential may prove an effective approach to understand sub-lethal consequences of ocean acidification for A. equina, other sea anemones and more broadly for marine ecosystems. [ABSTRACT FROM AUTHOR]

Safe and effective conflict resolution is critical for survival and reproduction. Theoretical models describe how animals resolve conflict by assessing their own and/or their opponent's ability (resource holding potential, RHP), yet experimental tests of these models are often inconclusive. Recent reviews have suggested this uncertainty could be alleviated by using multiple approaches to test assessment models. The mantis shrimp Neogonodactylus bredini presents visual displays and ritualistically exchanges high-force strikes during territorial contests. We tested how N. bredini contest dynamics were explained by any of three assessment models--pure self-assessment, cumulative assessment and mutual assessment--using correlations and a novel, network analysis-based sequential behavioural analysis. We staged dyadic contests over burrow access between competitors matched either randomly or based on body size. In both randomly and size-matched contests, the best metric of RHP was body mass. Burrow residency interacted with mass to predict outcome. Correlations between contest costs and RHP rejected pure self-assessment, but could not fully differentiate between cumulative and mutual assessment. The sequential behavioural analysis ruled out cumulative assessment and supported mutual assessment. Our results demonstrate how multiple analyses provide strong inference to tests of assessment models and illuminate how individual behaviours constitute an assessment strategy. [ABSTRACT FROM AUTHOR]

Oviposition sites may be challenging and energetically costly to access for females in the presence of competitors contesting that resource. Additionally, oviposition sites may be difficult to reach, and penetrating a hard substrate can raise energy costs. In the seeddrilling parasitic wasp Eupelmus vuilleti, females actively fight with conspecific competitors over access to hosts. They are often observed laying eggs on already parasitized hosts (superparasitism) living inside cowpea seeds despite the resulting larval competition. Using flow-through respirometry, we quantified the metabolic costs of fighting and of drilling through the seed to access the host, to understand the wasp's fighting strategies and the occurrence of superparasitism. Agonistic interactions such as kicks or pushes generated very small instantaneous costs, but the females that won their contests had higher pre-contest metabolic rates, suggesting a potential long-term cost associated with dominance. We also found that drilling holes through the seed accounted for approximately 15% of a wasp's estimated daily energy budget, and that females can reduce these drilling costs by reusing existing holes. Because exploiting new seeds incurs both drilling costs and the risk of fights, it appears cost effective in some situations for females to avoid confrontations and lay eggs in existing holes, on already parasitized hosts. Our study helps explain the evolution of superparasitism in this system. [ABSTRACT FROM AUTHOR]

Cannibalism can have important demographic and ecological effects on populations. Typically, cannibalism is size-structured, where larger individuals eat smaller conspecifics. Initial cursory observations of the whip spider, Phrynus longipes, however, suggested that cannibalism might not be size-structured in this species, perhaps because cannibalism is often a by-product of territory contests. We staged paired interactions and recorded latency to escalate to physical aggression or cannibalize to understand the dynamics of cannibalism and resource contests. We employed a multimodel comparative approach to tease apart the contest characteristics that best predicted cannibalism during behavioral trials between P. longipes opponents. We found that, while armament size symmetry predicted escalation of contests, cannibalism was best predicted by body mass asymmetry. Further, cannibalism was most likely to occur among individuals similar in armament, but dissimilar in body mass. This suggests a discrepancy between phenotypes that may have evolved to communicate resource holding potential (e.g., armaments which benefit individuals if dishonest), and body mass as a cue of resource holding potential. [ABSTRACT FROM AUTHOR]

What attributes make some individuals more likely to win a fight than others? A range of morphological and physiological traits have been studied intensely but far less focus has been placed on the actual agonistic behaviours used. Current studies of agonistic behaviour focus on contest duration and the vigour of fighting. It also seems obvious that individuals that fight more skilfully should have a greater chance of winning a fight. Here, we discuss the meaning of skill in animal fights. As the activities of each opponent can be disrupted by the behaviour of their rival, we differentiate among ability, technique and skill itself. In addition to efficient, accurate and sometimes precise movement, skilful fighting also requires rapid decision-making, so that appropriate tactics and strategies are selected. We consider how these different components of skill could be acquired, through genes, experiences of play-fighting and of real fights. Skilful fighting can enhance resource holding potential (RHP) by allowing for sustained vigour, by inflicting greater costs on opponents and by minimizing the chance of damage. Therefore, we argue that skill is a neglected but important component of RHP that could be readily studied to provide new insights into the evolution of agonistic behaviour. [ABSTRACT FROM AUTHOR]

Costly punishment is thought to have evolved because it promotes cooperation and the equitable sharing of resources, but the costs associated with punishment – for both the punisher and the punished – limit the efficiency of this enforcement system in economic interactions. Reputation may also guide decision-making, but this information is not always available (e.g., in interactions involving strangers). Across several bargaining studies, we provide evidence of an efficient and flexible “threat-based” bargaining system that can influence the division of resources without the need for costly punishment and reputational information. We found that participants, without prompting, dynamically adjusted bargaining based on the perceived threat-potential (resource holding power and aggressiveness) of the bargaining partner, giving larger offers to individuals who appeared more threatening. These effects of perceived threat-potential were strongest among participants who were most vulnerable to harm in physical contests (women vs men and weaker men vs stronger men), despite that offers were made on-line and anonymously to photographs of the individuals rather than in face-to-face interactions. These results may reflect an overgeneralization of a real-world threat heuristic that allows low threat individuals to extract resources when possible, while avoiding physical retaliation and harm, and high threat individuals to appropriate larger shares of a resource through static facial cues of threat rather than by physically expressing their propensity to punish. Previously, researchers have highlighted the monetary advantages of attractiveness (the “beauty premium”), but the effects of threat either trumped, devalued, or were equivalent to those of attractiveness. [ABSTRACT FROM AUTHOR]

An individual's metabolic competence is important when escalating to costly behaviours in agonistic encounters. The use of broken shells in the wild induces hermit crabs to reduce their overall metabolic rate; however, these crabs perform better in size-symmetric encounters than crabs that used intact shells in the wild. Given this surprising finding, we explored whether carrying a broken shell increases the resource holding potential (RHP) of hermit crabs by building up their muscle strength. These shells are very heavy, with an unusable portion due to being broken; however, the crabs have to carry the complete structure. Additionally, we explored whether the use of broken shells induces long-lasting consequences in motivation (aggressiveness). We conducted body size asymmetric contests to decrease the relative effect of aggressiveness by increasing the effect of RHP. We predicted a decrease of the agonistic behaviours of crab that occupied broken shells in the wild (CBS) relative to those that occupied intact shells (CIS). Symmetric and asymmetric contests were conducted in pairs formed of a CBS and a CIS, both forced to abandon them and occupy a Nerita shell. The CBS had lower muscle strength, but fought at higher intensity in symmetric contests than CIS. The differences in body mass decreased the expression of agonistic behaviours of CBS relative to CIS. Prolonged poor conditions following the use of a broken shell seem to increase aggressiveness by affecting the long-term subjective resource value. Our findings highlight the role of the recent past associated with shell used on fighting performance. Significance statement: Hermit crabs commonly fight to exchange their inadequate shell for a new one. Although high metabolic rates favour the fighting ability, crabs that occupied broken shells in the wild have low metabolism and perform better in shell fighting than crabs that used intact shells. Broken shells are heavy and larger due to having an unusable portion, but the crab has to carry the complete structure. We explored whether carrying these shells increases muscle strength and/or the aggressiveness of crabs, which might explain their fighting dynamic. Unexpectedly, crabs with broken shells were found to be weaker than those with intact ones. Instead, the use of a broken shell in the wild enhances individual aggressiveness. The results highlight that a previous history under adverse conditions can increase the likelihood of animals with functional disadvantages acquiring better resources. [ABSTRACT FROM AUTHOR]

Many factors can affect the probability for an individual to obtain a high social rank, including size, weaponry, and behavioral attributes such as aggression. Recent experiences of winning or losing can also affect the chances of winning future contests, commonly referred to as "winner-loser effects". Individuals often differ in behavior in a consistent way, including in aggression, thereby showing differences in personality. However, the relative importance of recent experience and aspects of personality in determining rank, as well as the extent to which winning or losing affects aggression, has rarely been studied. Here, we investigate these questions using male domestic fowl. We matched males for body size, comb size, and aggression in pair-wise duels to: 1) study the effect of contest outcome on aggression and 2) compare the effect of individual aggression and contest experience on future social status in small groups. We found that aggression was a highly repeatable personality trait and that aggression increased after winning and decreased after losing. Nevertheless, such winner-loser effects were not enough to increase the odds of becoming dominant in a small group. Instead, aggressiveness measured prior to a contest experience best predicted future rank. Boldness and exploration did not predict rank and of the 2, only boldness was positively correlated with aggressiveness. We conclude that for male domestic fowl in contests among phenotypically matched contestants, aggressiveness is more important for obtaining high rank than winner-loser effects, or other aspects of personality. [ABSTRACT FROM AUTHOR]

Avian plumage represents some of the greatest diversity in integument coloration of all animals. Plumage signals are diverse in function, including those that allow for assessing potential mates or the mitigation of agonistic interactions between rivals. Many bird species possess multiple ornamental traits that have the potential to serve as multiple or redundant signals. For example, male golden-winged warblers ( Vermivora chrysoptera) have brilliant carotenoid-based yellow crowns, melanin-based black throats, and structurally based white patches on their outer tail feathers. Using a correlative approach, we investigated whether plumage ornaments have the potential to reliably signal ability to acquire higher quality territory, aggressive response to simulated territorial intrusions, and reproductive success. We found that both crown chroma and tail brightness were significantly related to habitat quality and aggression; more ornamented birds held territories with higher quality habitat and were less aggressive toward simulated conspecific stimuli. Older birds sang less threatening songs than younger birds and were more likely to sing their mate attraction song type (type 1) rather than songs typically reserved for agonistic interactions (type 2). Finally, despite our previous research demonstrating that habitat strongly predicts reproductive success in this warbler population, we found no evidence of a direct link between ornamentation and reproductive success. Overall, these data suggest that younger males, and those with lower quality ornaments, compensate with more aggressive behaviors. Additional research is needed to investigate the dynamics between behavioral traits and ornaments to better understand complex signaling and how golden wing signals function in conspecific interactions (male-male interactions and mate-choice). [ABSTRACT FROM AUTHOR]

Theoretical models have been developed to understand how animals decide to withdraw from a contest. They provide testable predictions regarding the relationship between resource holding potential (RHP) and contest duration that assume linear relationships among RHP traits. However, RHP traits might scale with body size according to power laws. Furthermore, investment across different RHP traits may vary. Herein, we provide a model that encompasses the allometric relationship between body size and other RHP traits. First, we partition RHP traits into "offensive" traits (i.e., the ability to inflict damage) and "defensive" traits (i.e., persistence in a contest). Defensive traits may in turn be subdivided into "damage endurance" (DE) or the ability to absorb damage and "stamina." We then model scenarios where: 1) there are power relationships among RHP traits; 2) individuals invest differently in defensive and offensive traits; 3) offensive traits and DE have a positive/negative relationship with body size. We modeled sized-matched injurious contests where 1) offensive capacity (OC) increases superlinearly with body size, 2) DE increases superlinearly, and 3) OC increases superlinearly but DE increases sublinearly. Our analyses indicate that if RHP traits scale linearly current predictions are upheld for injurious contests-contest duration increases with body size. However, with power relationships we can expect nonlinear relationships. Here, contest duration increased with body size until a maximum, decreasing afterwards. Thus, considering allometric relationships between body size and RHP traits may lead to new insights in animal contest theory and may help to solve discrepancies between current theory and empirical data. [ABSTRACT FROM AUTHOR]

Males of the damselfly Hetaerina rosea may defend mating sites along river margins (resident males) or, alternatively, wander among different areas presumably searching for mates (nonterritorial males). Although the occurrence of territorial and nonterritorial males of H. rosea is very common in Brazil, studies examining which factors may be responsible for the adoption of alternative mate-locating tactics in this species are inexistent. We investigated the relationship between the adoption of these alternative mate-locating tactics by males of H. rosea and two possible causes: body weight and male abundance. We carried the study in three areas: sites 1, 2 and 3. Samples were monthly undertaken in sites 1 and 2 between September/2001 and August/2002 and in site 3 between May/1999 and January/2001. Using the scan method with fixed areas and mark-resighting techniques, we did not find any relationship between the proportion of nonterritorial males and male abundance per month on sites 2 (n=6) and 3 (n=7), indicating that the adoption of alternative mate-locating tactics is not affected by competition for territories. In the same way, nonterritorial and resident males showed similar body and thoracic weight measures (n=30 and n=27 for sites 2 and 3 respectively). Maybe the nonterritorial tactic is adopted by individuals searching for better territories or males that were evicted from their defended sites. The absence of relationship between weight and male territorial status is in accordance with other Hetaerina species. However, other traits not investigated here such as parasitic load, fat content and age may influence the adoption of different mate-acquisition tactics in H. rosea males. Rev. Biol. Trop. 57 (1-2): 361-370. Epub 2009 June 30.Los machos de la libélula Hetaerina rosea pueden defender sus sitios de apareamiento a lo largo de márgenes de ríos (machos residentes) o, alternativamente, deambulan entre diferentes áreas presumiblemente buscando pareja (machos no territoriales). Aunque la existencia de machos territoriales y no territoriales de H. rosea es muy común en Brasil, no hay estudios que examinen cuáles factores pueden ser responsables para la adopción de tácticas alternativas para localizar pareja en esta especie. Investigamos la relación entre la adopción de estas tácticas alternativas para localizar pareja por machos de H. rosea, y dos posibles causas: peso del cuerpo y abundancia de machos. Realizamos el estudio en tres áreas: sitios 1, 2 y 3. Las muestras se tomaron mensualmente en sitios 1 y 2 entre setiembre 2001 y agosto 2002, y en sitio 3 entre mayo 1999 y enero 2001. La adopción de tácticas alternativas de localización de machos no se ve afectada por competencia por territorios. La ausencia de relación entre peso y estatus del macho territorial coincide con otras especies de Hetaerina. No obstante, otros rasgos que no fueron estudiados aquí, como la carga de parásitos, contenido de grasa y edad, pueden influenciar la adopción de diferentes tácticas de adquisición de pareja en machos de H. rosea.

We present a general theoretical model for the spatio-temporal dynamics of animal contests. Inspired by interactions between physical particles, the model is formulated in terms of effective interaction potentials, which map typical elements of contest behaviour into empirically verifiable rules of contestant motion. This allows us to simulate the observable dynamics of contests in various realistic scenarios, notably in dyadic contests over a localized resource. Assessment strategies previously formulated in game-theoretic models, as well as the effects of fighting costs, can be described as variations in our model's parameters. Furthermore, the trends of contest duration associated with these assessment strategies can be derived and understood within the model. Detailed description of the contestants' motion enables the exploration of spatio-temporal properties of asymmetric contests, such as the emergence of chase dynamics. Overall, our framework aims to bridge the growing gap between empirical capabilities and theory in this widespread aspect of animal behaviour.