Fighting behaviour, vigilance and predation risk in the cichlid fish Nannacara anomala

Behavioral responses of a clonal fish to perceived predation risk

Predation threat is a major driver of behavior in many prey species. Animals can recognize their relative risk of predation based on cues in the environment, including visual and/or chemical cues released by a predator or from its prey. When threat of predation is high, prey often respond by altering their behavior to reduce their probability of detection and/or capture. Here, we test how a clonal fish, the Amazon molly (Poecilia formosa), behaviorally responds to predation cues. We measured aggressive and social behaviors both under 'risk', where chemical cues from predatory fish and injured conspecifics were present, and control contexts (no risk cues present). We predicted that mollies would exhibit reduced aggression towards a simulated intruder and increased sociability under risk contexts as aggression might increase their visibility to a predator and shoaling should decrease their chance of capture through the dilution effect. As predicted, we found that Amazon mollies spent more time with a conspecific when risk cues were present, however they did not reduce their aggression. This highlights the general result of the 'safety in numbers' behavioral response that many small shoaling species exhibit, including these clonal fish, which suggests that mollies may view this response as a more effective anti-predator response compared to limiting their detectability by reducing aggressive conspecific interactions.

Disentangling the "many-eyes", "dilution effect", "selfish herd", and "distracted prey" hypotheses in shaping alert and flight initiation distance in a colonial seabird

Group living is thought to have important antipredator benefits for animals, owing to the mechanisms of shared vigilance ("many-eyes" hypothesis), risk dilution ("dilution effect" hypothesis), and relative safety in the center of the group ("selfish herd" hypothesis). However, it can also incur costs since social stimuli, such as conspecific aggression, may distract individuals from anti-predator behavior ("distracted prey" hypothesis). We simultaneously evaluated how these four different hypotheses shape anti-predator behaviors of breeding king penguins (Aptenodytes patagonicus), which aggregate into large colonies, experience frequent aggressive social interactions, and are regularly exposed to predation by giant petrels (Macronectes sp.) and brown skuas (Catharacta loonbergi) when breeding on land. We approached 200 incubating penguins at four different periods of the breeding season across a range of overall increasing colony densities. We measured the distance at which focal birds detected the approaching threat (alert distance: AD), whether birds decided to flee or not, and the distance of flight initiation (flight initiation distance: FID, viz. the bird attempting to walk away with its egg on its feet). We quantified relative local neighbor density, centrality within the colony (rank), and the number of aggressions the focal bird emitted towards neighbors during the approach. We found that birds engaged in aggressive conflicts with neighbors were less likely to flee, and that increasing relative local neighbor density at low and medium overall colony density resulted in a decrease in bird AD, both supporting the "distracted prey" hypothesis. However, at maximal overall colony density, increasing relative local neighbor density resulted in longer AD, supporting the "many-eyes" hypothesis. We found no support for the "dilution effect" and "selfish herd" hypotheses, and no effects of any hypothesis on FID.

Non-angry aggressive arousal and angriffsberietschaft: A narrative review of the phenomenology and physiology of proactive/offensive aggression motivation and escalation in people and other animals

Human aggression typologies largely correspond with those for other animals. While there may be no non-human equivalent of angry reactive aggression, we propose that human proactive aggression is similar to offense in other animals' dominance contests for territory or social status. Like predation/hunting, but unlike defense, offense and proactive aggression are positively reinforcing, involving dopamine release in accumbens. The drive these motivational states provide must suffice to overcome fear associated with initiating risky fights. We term the neural activity motivating proactive aggression "non-angry aggressive arousal", but use "angriffsberietschaft" for offense motivation in other animals to acknowledge possible differences. Temporal variation in angriffsberietschaft partitions fights into bouts; engendering reduced anti-predator vigilance, redirected aggression and motivational over-ride. Increased aggressive arousal drives threat-to-attack transitions, as in verbal-to-physical escalation and beyond that, into hyper-aggression. Proactive aggression and offense involve related neural activity states. Cingulate, insular and prefrontal cortices energize/modulate aggression through a subcortical core containing subnuclei for each aggression type. These proposals will deepen understanding of aggression across taxa, guiding prevention/intervention for human violence.

Age-dependent genetic variation in aggression

Understanding the extent to which behavioural variance is underlain by genotypic, environmental and genotype-by-environment effects is important for predicting how behavioural traits might respond to selection and evolve. How behaviour varies both within and among individuals can change across ontogeny, leading to differences in the relative contribution of genetic and environmental effects to phenotypic variation across ages. We investigated among-individual and among-genotype variation in aggression across ontogeny by measuring, twice as juveniles and twice as adults, both approaches and attacks against a three-dimensional-printed model opponent in eight individuals from each of eight genotypes (N = 64). Aggression was only significantly repeatable and heritabile in juveniles. Additionally, how aggression changed between juvenile and adult life-history stages varied significantly among individuals and genotypes. These results suggest that juvenile aggression is likely to evolve more rapidly via natural selection than adult aggression and that the trajectory of behavioural change across the lifespan has the potential to evolve. Determining when genetic variation explains (or does not explain) behavioural variation can further our understanding of key life-history stages during which selection might drive the strongest or swiftest evolutionary response.

Damage from intraspecific combat is costly

When individuals engage in fights with conspecifics over access to resources, injuries can occur. Most theoretical models suggest that the costs associated with these injuries should influence an individual’s decision to retreat from a fight. Thus, damage from intraspecific combat is frequently noted and quantified. However, the fitness-related costs associated with this damage are not. Quantifying the cost of fighting-related damage is important because most theoretical models assume that it is the cost associated with the damage (not the damage itself) that should influence an individual’s decision to retreat. Here, we quantified the cost of fighting-related injuries in the giant mesquite bug, Thasus neocalifornicus. We demonstrate that experimentally simulated fighting injuries result in metabolic costs and costs to flight performance. We also show that flight costs are more severe when the injuries are larger. Overall, our results provide empirical support for the fundamental assumption that damage acquired during intraspecific combat is costly.

Submission signals in animal groups

Aggression is costly, and animals have evolved tactics to mitigate these costs. Submission signals are an underappreciated example of such adaptations. Here we review submissive behaviour, with an emphasis on non-primates. We highlight the design of submission signals and how such signals can reduce costs. Animal societies necessitate frequent social interactions, which can increase the probability of conflict. Where maintaining group proximity is essential, animals cannot avoid aggression by fleeing. Mutual interest between group members may also select for efficient conflict avoidance and resolution mechanisms. As a result, submission signals may be especially well developed among group living species, helping social animals to overcome potential costs of recurring conflict that could otherwise counter the benefits of group living. Therefore, submission signalling can be a crucial aspect of social living and is deserving of specific attention within the broader context of social evolution and communication.

Acoustic and visual adaptations to predation risk: a predator affects communication in vocal female fish

Predation is an important ecological constraint that influences communication in animals. Fish respond to predators by adjusting their visual signaling behavior, but the responses in calling behavior in the presence of a visually detected predator are largely unknown. We hypothesize that fish will reduce visual and acoustic signaling including sound levels and avoid escalating fights in the presence of a predator. To test this we investigated dyadic contests in female croaking gouramis (Trichopsis vittata, Osphronemidae) in the presence and absence of a predator (Astronotus ocellatus, Cichlidae) in an adjoining tank. Agonistic behavior in T. vittata consists of lateral (visual) displays, antiparallel circling, and production of croaking sounds and may escalate to frontal displays. We analyzed the number and duration of lateral display bouts, the number, duration, sound pressure level, and dominant frequency of croaking sounds as well as contest outcomes. The number and duration of lateral displays decreased significantly in predator when compared with no-predator trials. Total number of sounds per contest dropped in parallel but no significant changes were observed in sound characteristics. In the presence of a predator, dyadic contests were decided or terminated during lateral displays and never escalated to frontal displays. The gouramis showed approaching behavior toward the predator between lateral displays. This is the first study supporting the hypothesis that predators reduce visual and acoustic signaling in a vocal fish. Sound properties, in contrast, did not change. Decreased signaling and the lack of escalating contests reduce the fish’s conspicuousness and thus predation threat.

Habituation is not neutral or equal: Individual differences in tolerance suggest an overlooked personality trait

In behavioral studies, observer effects can be substantial, even for habituated animals, but few studies account for potential observer-related phenomenon empirically. We used wild, habituated chacma baboons to explore two key assumptions of behavioral ecology (i) that observers become a "neutral" stimulus and (ii) that habituation is "equal" across group members. Using flight initiation distance (FID) methods within a personality paradigm, the behavioral responses of baboons suggested that observers were not perceived as neutral but instead viewed as a high-ranking social threat. Habituation was also not equal across group members, with repeatable individual differences more important than contextual factors (e.g., habitat) in determining the distance at which baboons visually oriented or displaced from observers. A strong correlation between individual visual tolerance and displacement tolerance (i.e., convergent validity) indicated a personality trait. We offer several suggestions for how to account for these factors and minimize potential bias in future studies.

Adaptive Maternal Investment in the Wild? Links between Maternal Growth Trajectory and Offspring Size, Growth, and Survival in Contrasting Environments

Life-history theory predicts that investment per offspring should correlate negatively with the quality of the environment that offspring are anticipated to encounter; parents may use their own experience as juveniles to predict this environment and may modulate offspring traits, such as growth capacity and initial size. We manipulated nutrient levels in the juvenile habitat of wild Atlantic salmon (Salmo salar) to investigate the hypothesis that the egg size that maximizes juvenile growth and survival depends on environmental quality. We also tested whether offspring traits were related to parental growth trajectory. Mothers that grew fast when young produced more offspring and smaller offspring than mothers that grew slowly to reach the same size. Despite their size disadvantage, offspring of faster-growing mothers grew faster than those of slower-growing mothers in all environments, counter to the expectation that they would be competitively disadvantaged. However, they had lower relative survival in environments where the density of older predatory/competitor fish was relatively high. These links between maternal (but not paternal) growth trajectory and offspring survival rate were independent of egg size, underscoring that mothers may be adjusting egg traits other than size to suit the environment their offspring are anticipated to face.

Ecology of sound communication in fishes

Fishes communicate acoustically under ecological constraints which may modify or hinder signal transmission and detection and may also be risky. This makes it important to know if and to what degree fishes can modify acoustic signalling when key ecological factors-predation pressure, noise and ambient temperature-vary. This paper reviews short-time effects of the first two factors; the third has been reviewed recently (Ladich, 2018). Numerous studies have investigated the effects of predators on fish behaviour, but only a few report changes in calling activity when hearing predator calls as demonstrated when fish responded to played-back dolphin sounds. Furthermore, swimming sounds of schooling fish may affect predators. Our knowledge on adaptations to natural changes in ambient noise, for example caused by wind or migration between quiet and noisier habitats, is limited. Hearing abilities decrease when ambient noise levels increase (termed masking), in particular in taxa possessing enhanced hearing abilities. High natural and anthropogenic noise regimes, for example vessel noise, alter calling activity in the field and laboratory. Increases in sound pressure levels (Lombard effect) and altered temporal call patterns were also observed, but no switches to higher sound frequencies. In summary, effects of predator calls and noise on sound communication are described in fishes, yet sparsely in contrast to songbirds or whales. Major gaps in our knowledge on potential negative effects of noise on acoustic communication call for more detailed investigation because fishes are keystone species in many aquatic habitats and constitute a major source of protein for humans.

Fight, fatigue, and flight: narrowing of attention to a threat compensates for decreased anti-predator vigilance

Fighting carries predation risk because animals have limited attention, constraining their ability to simultaneously engage in aggression and anti-predator vigilance. However, the influence of interspecific aggression and fatigue on the predation cost of fighting is seldom examined, although both are unignorable aspects of fighting. Here, I incorporated both factors in a series of field experiments on the cichlid Lamprologus ocellatus. If territorial males respond more strongly to conspecific territorial intruders than heterospecific intruders, then they should delay escape more frequently during intraspecific fighting than interspecific fighting. Consequently, although I predict that vigilance would be decreased as fighting progresses in both fighting, intraspecific aggression should decrease vigilance more than interspecific aggression. Males were also exposed to a predator approaching at different (slow or fast) speeds during these fighting bouts. Delays in predator detection and flight initiation were quantified and these predictions were tested. As predicted, males were responded more strongly to intraspecific intruders, resulting in slower predator detection and fleeing times than when encountering interspecific intruders. Furthermore, flight latency decreased with increasing fight duration, suggesting that fatigue negatively influenced escape responses. However, contrary to prediction, the vigilance decrement rate was faster in response to slow predators than to rapid predators, and was not influenced by intruder identity. This suggests that fighting males reserve their attention for information critical to their survival and are less vigilant toward a less-threatening (slow approaching) predator. This cognitive allocation may be an adaptive compensation for fatigue-related low vigilance during fighting.

Opposite companion effect on flight initiation distance in sympatric species: plateau pika (Ochotona curzoniae) and White-rumped Snowfinch (Onychostruthus taczanowskii)

There are many factors influencing prey’s risk perception and escape decision during predator–prey encounters. The distance at which animals move away from perceived danger (often quantified as flight initiation distance or FID) has been used by behavioral ecologists to understand the economics of antipredator behavior. Using general linear models, we investigated escape decision-making processes in plateau pika (Ochotona curzoniae (Hodgson, 1858)) and White-rumped Snowfinch (Onychostruthus taczanowskii (Prjevalsky, 1876), formerly known as Montifringilla taczanowskii Przewalski, 1876) together and we found that (i) there are significant positive correlations between starting distance and FID in both species; (ii) pika escapes at a longer distance from an approaching intruder when it is far from its burrow; (iii) foraging animals tolerate closer distances than watching ones, both in pikas and in Snowfinches; (iv) conspecifics seem to have no effect on pikas’ escaping behavior, while the appearance of Snowfinches dramatically decreases the FID of pikas. On the contrary, conspecifics significantly decrease the FID of Snowfinches, while the presence of pikas has no effects. These findings provide new evidence, which are consistent with optimal escape theory. These novel results in multispecies interactions prompt us to pay attention to the potential relationship between pikas and Snowfinches, as well as the “bird–pika in one hole” phenomenon.

Early development and allometric growth in Nannacara anomala Regan, 1905 (Perciformes: Cichlidae) under laboratory conditions

Morphological development and allometric growth of laboratory reared Nannacara anomala were studied from hatching to the loss of larval characters and beginning of squamation (18 days post-hatching) at 26°C. The mean total length (TL) of larvae and juveniles increased from 3.74 mm at hatching to 9.60 mm at metamorphosis. Morphogenesis and differentiation were most intense during the first week of development. During this period (TL interval = 3.74 - 4.84 mm) there was an evident priority to enhance the feeding and swimming capabilities by promoting accelerated growth in the head and tail regions. Following this period, there was a major decrease in growth coefficients, indicating a change in growth priorities. Observations on the early development of Nannacara anomala confirmed the basic uniformity development of a substrate brooding cichlid.

Social and Ecological Regulation of a Decision-Making Circuit

Ecological context, sensory inputs, and the internal physiological state are all factors that need to be integrated for an animal to make appropriate behavioral decisions. However, these factors have rarely been studied in the same system. In the African cichlid fish Astatotilapia burtoni, males alternate between two phenotypes based on position in a social hierarchy. When dominant (DOM), fish display bright body coloration and a wealth of aggressive and reproductive behavioral patterns that make them conspicuous to predators. Subordinate (SUB) males, on the other hand, decrease predation risk by adopting cryptic coloration and schooling behavior. We therefore hypothesized that DOMs would show enhanced startle-escape responsiveness to compensate for their increased predation risk. Indeed, behavioral responses to sound clicks of various intensities showed a significantly higher mean startle rate in DOMs compared with SUBs. Electrophysiological recordings from the Mauthner cells (M-cells), the neurons triggering startle, were performed in anesthetized animals and showed larger synaptic responses to sound clicks in DOMs, consistent with the behavioral results. In addition, the inhibitory drive mediated by interneurons (passive hyperpolarizing potential [PHP] cells) presynaptic to the M-cell was significantly reduced in DOMs. Taken together, the results suggest that the likelihood for an escape to occur for a given auditory stimulus is higher in DOMs because of a more excitable M-cell. More broadly, this study provides an integrative explanation of an ecological and social trade-off at the level of an identifiable decision-making neural circuit.

Context-dependent variability in the components of fish escape response: integrating locomotor performance and behavior

Escape responses are used by most fish species in order to avoid predation. Escape responses include a number of behavioral and kinematic components, such as responsiveness, reaction distance, escape latency, directionality, and distance-derived performance. All of these components can contribute to escape success. Work on the context-dependent variability has focused on reaction distance, and suggests that this component is largely determined by the relative cost and benefits of escaping (economic hypothesis). For example, reaction distance was found to depend on many factors related to perceived risk and cost of escaping, such as the attack speed and size of the predators, the proximity to refuges, and engagement in other activities (e.g., feeding). Evidence from many behavioral, kinematic, and physiological studies suggest that performance in other components of the escape response is also not always maximized. For example, escape latencies may increase in the presence of schooling neighbors, and escape speed is higher in fish that have been subject to higher predation pressure. In addition, all escape components are further modulated by the effect of environmental factors. Variability in escape components can be interpreted by using both ultimate and proximate explanations, for example, the effect of stimulus strength on escape latency can be interpreted as the triggering neural threshold varying with stimulus strength (proximate explanation) and high intensity stimuli representing higher risk to the prey (ultimate explanation). An integrative approach is suggested for a full, ecologically relevant, assessment of escape performance in fish.

Xiphophorus: carving a niche towards a broader understanding of aggression and dominance

A review of some exciting new directions for Xiphophorus research in behavioral ecology and integrative animal behavior is presented. The primary objective of this review is to identify areas of research in this teleost genus that, if studied further, could yield insights into the manifold causes and consequences of dominance encounters that will have broad relevance in the behavioral biology community. First described is the phenomenon of social eavesdropping--the ability of animals to extract information from signaling interactions between others-as it applies to Xiphophorus systems, and how exploring the context--dependency of individual responses to watching fights will be of benefit. A brief discussion follows of the overwhelming tendency for research on Xiphophorus to focus on individual responses to visual cues available in their social environment, and to promote advancements towards a multimodal approach to understanding social dynamics. Lastly, historical studies on neuroendocrinology in this genus are reviewed, and recent advances in the molecular realm are highlighted that might serve as a springboard for integrative research addressing the behavioral impacts of direct (overt fighting) and indirect (eavesdropping) experiences in Xiphophorus.

Acoustic communication in territorial butterflyfish: test of the sound production hypothesis

Butterflyfishes are conspicuous members of coral reefs and well known for their visual displays during social interactions. Members of the genus Chaetodon have a unique peripheral arrangement of the anterior swim bladder that connects with the lateral line (the laterophysic connection) and in many species projects towards the inner ear. This morphology has lead to the proposal that the laterophysic connection and swim bladder system may be a specialized structure for the detection of sound. However, the relevant stimuli, receiver mechanisms and functions for these putative hearing structures were unknown because butterflyfishes were previously not recognized to produce sounds during natural behavior. We performed field experiments to test the hypothesis that Chaetodon produces sounds in natural social contexts. Acoustic and motor behaviors of the monogamous multiband butterflyfish, C. multicinctus, were evoked and recorded by placement of bottled fish into feeding territories of conspecific pairs. We demonstrate that territory defense includes the production of agonistic sounds and hydrodynamic stimuli that are associated with tail slap, jump, pelvic fin flick and dorsal-anal fin erection behaviors. In addition, grunt pulse trains were produced by bottled intruders and are tentatively interpreted to function as an alert call among pair mates. Acoustic behaviors include low frequency hydrodynamic pulses <100 Hz, sounds with peak energy from 100 Hz to 500 Hz, and a broadband high frequency click (peak frequency=3.6 kHz), which is produced only during the tail slap behavior. These results provide a biological framework for future studies to interpret the proximate function of the acoustico-lateralis sensory system, the evolution of the laterophysic mechanism and their relevance to butterflyfish social behavior.

Coping with divided attention: the advantage of familiarity

The ability of an animal to perform a task successfully is limited by the amount of attention being simultaneously focused on other activities. One way in which individuals might reduce the cost of divided attention is by preferentially focusing on the most beneficial tasks. In territorial animals where aggression is lower among familiar individuals, the decision to associate preferentially with familiar conspecifics may therefore confer advantages by allowing attention to be switched from aggression to predator vigilance and feeding. Wild juvenile brown trout were used to test the prediction that familiar fishes respond more quickly than unfamiliar fishes to a simulated predator attack. Our results confirm this prediction by demonstrating that familiar trout respond 14% faster than unfamiliar individuals to a predator attack. The results also show that familiar fishes consume a greater number of food items, foraging at more than twice the rate of unfamiliar conspecifics. To the best of our knowledge, these results provide the first evidence that familiarity-biased association confers advantages through the immediate fitness benefits afforded by faster predator-evasion responses and the long-term benefits provided by increased feeding opportunities.

Comprehensive description of newborn distress behavior in response to acute pain (newborn male circumcision)

One of the most difficult challenges still facing researchers and clinicians is assessing pain in the newborn. Behaviors provide one of the most promising avenues for deepening our fundamental understanding of complex phenomenon like newborn pain, and are key to developing descriptive-level knowledge to further newborn pain assessment efforts. In this ethologically based research, we report on the duration and frequency of neonatal distress behavior to seven distinct noxious and non-noxious but distress-provoking events including baseline (diaper change, post-diaper change, application of arm and leg restraints, post-application of arm and leg restraints, circumcision, post-circumcision) associated with newborn surgical pain. Approximately 67 min of videotaped data, involving four neonates who had undergone newborn male circumcision, were coded at 1-s intervals (4010 s in total). A reliably established coding scheme was used to code behaviors as they were observed on videotape for the duration of the seven designated events. This led to the identification of (1) 40 distress behaviors as they occurred along the continuum of distress, (2) eight distress behaviors specific to surgery, (3) 11 classes of behaviors occurring within the five sub-phases of circumcision, and (4) a description of 25 distinct post-distress behaviors. Findings support the ability to distinguish distress behaviors specific to pain and the ability to detect prolonged distress as well as individual differences in distress-related pain expression. Findings also justify ongoing use of ethological approaches to further newborn pain assessment and to investigate poorly understood topics such as infant self-regulation within the context of pain (pain recovery).

Behavioural and ecological consequences of limited attention

Ecological research in the past few decades has shown that most animals acquire and respond adaptively to information that affects survival and reproduction. At the same time, neurobiological studies have established that the rate of information processing by the brain is much lower than the rate at which information is encountered in the environment, and that attentional mechanisms enable the brain to focus only on the most essential information at any given time. Recent integration of the ecological and neurobiological approaches helps us to understand key behaviours with broad ecological and evolutionary implications. Specifically, current data indicate that limited attention affects diet choice and constrains animals' ability simultaneously to feed and attend to predators. Recent experiments also suggest that limited attention influences social interactions, courtship and mating behaviour.

Territorial intrusion risk and antipredator behaviour: a mathematical model

In territorial animals that hide to avoid predators, a predatory attack creates a conflict because a hiding animal cannot defend its territory from conspecific intruders. When intruders are persistent, a past conspecific intrusion informs a territorial resident that future intrusions by the same animal are likely. Using a mathematical model, I examine the effects that past territorial intrusions can have on antipredator behaviour. Past territorial intrusions rarely affect a resident animal's time to hide (the optimal behaviour is to hide as soon as the predator initiates its attack). In contrast, past intrusions should shorten the length of time during which territory holders remain in hiding, with the magnitude of this effect depending on the time of the predator's attack, the re-intruder's pattern of return, and the intrusion rates of other conspecifics. The results of the model show that we need more information on patterns of re-intruders' behaviour, and emphasize that a similar functional explanation could underlie other behavioural changes following territorial and/or aggressive encounters (such as winner/loser effects or changes in display frequency and territorial vigilance). Differences between my findings and those from previous studies suggest that the trade-off between antipredator behaviour and territorial defence can involve different costs from the trade-off between antipredator behaviour and foraging.

Theory and method in studies of vigilance and aggregation

Predation is considered one of the most important selective pressures on free-ranging animals. Our understanding of it derives mainly from studies of individual vigilance (visual scanning of the surroundings beyond the immediate vicinity) and aggregation in prey. Vigilance bears a direct relationship to aggregation, because animals in groups may rely on associates for early warning of danger. This review addresses the relationship between vigilance and aggregation with particular attention to the prediction that individual vigilance declines with increasing group size. Contrary to most other animals studied, primates do not support the prediction. Exploring this, I examined the assumptions underlying vigilance theory in the light of primate behaviour. First I tested whether manual harvesting and upright processing of food as seen among primates might permit them to feed and scan simultaneously. I found no support for this idea. Next I examined the targets of primate vigilance and found that one component (within-group vigilance) might explain the differences between primates and other animals. Finally, I evaluated whether individual primates in large groups face a lower risk of predation than those in small groups. A conclusion was impossible, but by separating group-level from individual-level risk, I was able to identify several common circumstances in which group size would not predict individual risk or vigilance. These circumstances arose for primates and nonprimates alike. I concluded that the relationship of vigilance to aggregation is not straightforward. The absence of a group-size effect on vigilance among primates is probably due to functional differences in vigilance behaviour or safety in groups, not to methodological differences. Furthermore, future work on animal vigilance and aggregation must fully consider both the targets of glances, and the assumption that larger groups are safer from predators. I predict that animals will not relax vigilance in larger groups if conspecific threat increases with group size. Group size will not predict individual risk of predation nor individual vigilance rates when predators do not rely on surprise, or when predators select a small subset of highly vulnerable group members. Copyright 2000 The Association for the Study of Animal Behaviour.

Anti-predator behaviour changes following an aggressive encounter in the lizard Tropidurus hispidus

Avoiding predators may conflict with territorial defence because a hiding territorial resident is unable to monitor its territory or defend it from conspecific intrusions. With persistent intruders, the presence of an intruder in the near past can indicate an increased probability of future intrusions. Therefore, following a conspecific-intrusion, territorial residents should minimize costs from future intrusions at the cost of higher predation risks. I conducted experiments with males of the territorial lizard Tropidurus hispidus recording approach distance (distance between predator and prey when the prey escapes) and time to re-emergence from a refuge after hiding. Past aggressive interactions affected anti-predator behaviour: lizards re-emerged sooner (compared to a control) when the predator attacked 5 min after an aggressive encounter. If the predator attacked while an aggressive encounter was ongoing, there was also a reduction in approach distance. The results are consistent with an economic hypothesis which predicts that T. hispidus incur greater predation risks to minimize future territorial intrusion; additionally they show that the effects of past and ongoing aggressive interactions are different, consistent with the minimization of present intrusion costs. These results are relevant for studies of the changes in aggressive behaviour due to changes in the social environment and for studies of the costs and (co) evolution of aggressive and anti-predator strategies.