Assessment of fighting ability in animal contests

Aggressiveness overcomes body-size effects in fights staged between invasive and native fish species with overlapping niches

Approximately 50 years ago, Nile tilapia were accidentally introduced to Brazil, and the decline of pearl cichlid populations, which has been intensified by habitat degradation, in some locations has been associated with the presence of Nile tilapia. There is, however, little strong empirical evidence for the negative interaction of non-native fish populations with native fish populations; such evidence would indicate a potential behavioural mechanism that could cause the population of the native fish to decline. In this study, we show that in fights staged between pairs of Nile tilapia and pearl cichlids of differing body size, the Nile tilapia were more aggressive than the pearl cichlid. Because this effect prevailed over body-size effects, the pearl cichlids were at a disadvantage. The niche overlap between the Nile tilapia and the pearl cichlid in nature, and the competitive advantage shown by the Nile tilapia in this study potentially represent one of several possible results of the negative interactions imposed by an invasive species. These negative effects may reduce population viability of the native species and cause competitive exclusion.

Is boldness a resource-holding potential trait? Fighting prowess and changes in startle response in the sea anemone, Actinia equina

Contest theory predicts the evolution of a stable mixture of different strategies for fighting. Here, we investigate the possibility that stable between-individual differences in startle-response durations influence fighting ability or 'resource-holding potential' (RHP) in the beadlet sea anemone, Actinia equina. Both winners and losers showed significant repeatability of pre-fight startle-response durations but mean pre-fight startle-response durations were greater for eventual losers than for eventual winners, indicating that RHP varies with boldness. In particular, individuals with short startle responses inflicted more attacks on their opponent. Both repeatability and mean-level responses were changed by the experience of fighting, and these changes varied with outcome. In losers, repeatability was disrupted to a greater extent and the mean startle-response durations were subject to a greater increase than in winners. Thus, following a fight, this behavioural correlate of RHP behaves in a way similar to post-fight changes in physiological status, which can also vary between winners and losers. Understanding the links between aggression and boldness therefore has the potential to enhance our understanding of both the evolution of animal personality and the 'winner and loser effects' of post-fight changes in RHP.

Integrating personality research and animal contest theory: aggressiveness in the green swordtail Xiphophorus helleri

Aggression occurs when individuals compete over limiting resources. While theoretical studies have long placed a strong emphasis on context-specificity of aggression, there is increasing recognition that consistent behavioural differences exist among individuals, and that aggressiveness may be an important component of individual personality. Though empirical studies tend to focus on one aspect or the other, we suggest there is merit in modelling both within- and among-individual variation in agonistic behaviour simultaneously. Here, we demonstrate how this can be achieved using multivariate linear mixed effect models. Using data from repeated mirror trials and dyadic interactions of male green swordtails, Xiphophorus helleri, we show repeatable components of (co)variation in a suite of agonistic behaviour that is broadly consistent with a major axis of variation in aggressiveness. We also show that observed focal behaviour is dependent on opponent effects, which can themselves be repeatable but were more generally found to be context specific. In particular, our models show that within-individual variation in agonistic behaviour is explained, at least in part, by the relative size of a live opponent as predicted by contest theory. Finally, we suggest several additional applications of the multivariate models demonstrated here. These include testing the recently queried functional equivalence of alternative experimental approaches, (e.g., mirror trials, dyadic interaction tests) for assaying individual aggressiveness.

Lateralization of lateral displays in convict cichlids

We examine lateralization of lateral displays in convict cichlids, Amatitlania nigrofasciata, and show a population level preference for showing the right side. This enables contesting pairs of fish to align in a head-to-tail posture, facilitating other activities. We found individuals spent a shorter mean time in each left compared with each right lateral display. This lateralization could lead to contesting pairs using a convention to align in a predictable head-to-tail arrangement to facilitate the assessment of fighting ability. It has major implications for the common use of mirror images to study fish aggression, because the 'opponent' would never cooperate and would consistently show the incorrect side when the real fish shows the correct side. With the mirror, the 'normal' head-to-tail orientation cannot be achieved.

Fight tactics in wood ants: individuals in smaller groups fight harder but die faster

When social animals engage in inter-group contests, the outcome is determined by group sizes and individual masses, which together determine group resource-holding potential ('group RHP'). Individuals that perceive themselves as being in a group with high RHP may receive a motivational increase and increase their aggression levels. Alternatively, individuals in lower RHP groups may increase their aggression levels in an attempt to overcome the RHP deficit. We investigate how 'group RHP' influences agonistic tactics in red wood ants Formica rufa. Larger groups had higher total agonistic indices, but per capita agonistic indices were highest in the smallest groups, indicating that individuals in smaller groups fought harder. Agonistic indices were influenced by relative mean mass, focal group size, opponent group size and opponent group agonistic index. Focal group attrition rates decreased as focal group relative agonistic indices increased and there was a strong negative influence of relative mean mass. The highest focal attrition rates were received when opponent groups were numerically large and composed of large individuals. Thus, fight tactics in F. rufa seem to vary with both aspects of group RHP, group size and the individual attributes of group members, indicating that information on these are available to fighting ants.

Syrian hamster males below an age threshold do not elicit aggression from unfamiliar adult males

In many species, young males are the dispersers, leaving their natal area after weaning to establish a breeding area of their own. As young males disperse, however, they are bound to encounter unfamiliar adult males with established territories. Such interactions between an adult male and a young male may always be agonistic. Alternatively, there may be an age threshold below which aggression is not elicited and above which the adult male is aggressive toward the juvenile male. To test these two alternative hypotheses, we paired 47 young Syrian hamster (Mesocricetus auratus) males ranging from 24 to 65 days of age with 47 adult male hamsters and measured aggressive and investigatory behavior for 5 min. We observed no aggression by the adult toward young males between 24 and 47 days of age or toward the single male that was 49 days of age. Young males that were 50 days of age or older, however, elicited significant levels of aggression from the adults. These results indicate that in Syrian hamsters, young males are less vulnerable to adult aggression up to an age threshold and are more vulnerable to adult aggression beyond that threshold. This pattern may facilitate the establishment of territories by dispersing young males below that age threshold.

The aggressive behavior of Nile tilapia introduced into novel environments with variation in enrichment

Many studies show environmental enrichment is correlated with benefits to captive animals; however, one should not always assume this positive relationship given that enrichment increases the amount of resources that a territorial animal must defend and possibly affects its aggressive dynamics. In this study, we tested if environmental enrichment affects aggressive interactions in the aggressive fish Nile tilapia (Oreochromis niloticus). We compared fights staged between pairs of male tilapia of similar size (= matched in resource holding potential) in a novel arena that was either barren or enriched, to examine whether enrichment enhances territory value in line with theoretical predictions, with the potential for compromised welfare. We evaluated time elapsed until the first attack (latency), frequency of aggressive interactions and fight duration. We detected fight dynamic differences at the pair level. Higher resource value generated increased aggression but had no effect on fight duration or latency. This conclusion is in line with game theory predictions concerning resource value and contradicts the theory that enrichment of the environment will serve welfare purposes.

Dogs' expectation about signalers' body size by virtue of their growls

Several studies suggest that dogs, as well as primates, utilize a mental representation of the signaler after hearing its vocalization and can match this representation with other features provided by the visual modality. Recently it was found that a dogs' growl is context specific and contains information about the caller's body size. Whether dogs can use the encoded information is as yet unclear. In this experiment, we tested whether dogs can assess the size of another dog if they hear an agonistic growl paired with simultaneous video projection of two dog pictures. One of them matched the size of the growling dog, while the other one was either 30% larger or smaller. In control groups, noise, cat pictures or projections of geometric shapes (triangles) were used. The results showed that dogs look sooner and longer at the dog picture matching the size of the caller. No such preference was found with any of the control stimuli, suggesting that dogs have a mental representation of the caller when hearing its vocalization.

Social eavesdropping and the evolution of conditional cooperation and cheating strategies

The response of bystanders to information available in their social environment can have a potent influence on the evolution of cooperation and signalling systems. In the presence of bystanders, individuals might be able to increase their payoff by exaggerating signals beyond their means (cheating) or investing to help others despite considerable costs. In doing so, animals can accrue immediate benefits by manipulating (or helping) individuals with whom they are currently interacting and delayed benefits by convincing bystanders that they are more fit or cooperative than perhaps is warranted. In this paper, I provide some illustrative examples of how bystanders could apply added positive selection pressure on both cooperative behaviour and dishonest signalling during courtship or conflict. I also discuss how the presence of bystanders might select for greater flexibility in behavioural strategies (e.g. conditional or condition dependence), which could maintain dishonesty at evolutionarily stable frequencies under some ecological conditions. By recognizing bystanders as a significant selection pressure, we might gain a more realistic approximation of what drives signalling and/or interaction dynamics in social animals.

Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp's telson

Resisting impact and avoiding injury are central to survival in situations ranging from the abiotic forces of crashing waves to biotic collisions with aggressive conspecifics. Although impacts and collisions in biology are ubiquitous, most studies focus on the material properties of biological structures under static loading. Here, we examine the mechanical impact properties of the mantis shrimp's telson, a piece of abdominal armor that withstands repeated, intense impacts from the potent hammer-like appendages used by conspecifics during ritualized fighting. We measured the coefficient of restitution, an index of elasticity, of the telson and compared it with that of an adjacent abdominal segment that is not impacted. We found that the telson behaves more like an inelastic punching bag than an elastic trampoline, dissipating 69% of the impact energy. Furthermore, although the abdominal segment provides no mechanical correlates with size, the telson's coefficient of restitution, displacement and impact duration all correlate with body size. The telson's mineralization patterns were determined through micro-CT (Computed Tomography) and correspond to the mechanical behavior of the telson during impact. The mineralized central region of the telson ‘punched’ inward during an impact whereas the surrounding areas provided elasticity owing to their reduced mineralization. Thus, the telson effectively dissipates impact energy while potentially providing the size-related information crucial to its role in conspecific assessment. This study reveals the mechanical infrastructure of impact resistance in biological armor and opens a new window to the biomechanical underpinnings of animal behavior and assessment.

Individual aggressiveness in the crab Chasmagnathus: Influence in fight outcome and modulation by serotonin and octopamine

In a previous work we found that size-matched Chasmagnathus crabs establish winner-loser relationships that were stable over successive encounters but no evidence of escalation was revealed through fights. Here, we evaluated the hypothesis that size-matched fights between these crabs would be resolved according to the contestants' level of aggressiveness. Moreover, we aim at analysing the proximate roots of aggression, addressing the influence of the biogenic amines serotonin (5HT) and octopamine (OA) in crab's agonistic behaviour. To achieve these purposes, the following experiments were carried out. First, we performed successive fight encounters between the same opponents, varying the number of encounters and the interval between them, to assess the stability and progression of the winner-loser relationship. Then, we analysed dominance relationships in groups of three crabs, evaluating the emergence of linearity. Thirdly, we examined the effects of 5HT and OA injections over the fight dynamics and its result. Our findings show that contest outcome is persistent even through four encounters separated by 24h, but a comparison between encounters does not reveal any saving in fight time or increase in the opponent disparity. Within a group of crabs, a rank-order of dominance is revealed which is reflected in their fight dynamics. Interestingly, these results would not be due to winner or loser effects, suggesting that fight outcome could be mainly explained as resulting from differences in the level of aggressiveness of each opponent. Moreover, this individual aggressiveness can be modulated in opposite directions by the biogenic amines 5HT and OA, being increased by 5HT and decreased by OA.