Predator Stimulus Research Articles

Dynamic colour change in zebrafish (Danio rerio) across multiple contexts.

Many animals are capable of rapid dynamic colour change, which is particularly well represented in fishes. The proximate mechanisms of dynamic colour change in fishes are well understood; however, less attention has been given to understanding its ecological relevance. In this study, we investigate dynamic colour change in zebrafish (Danio rerio) across multiple contexts, using a protocol to image the colouration of live fish without anaesthesia under standardized conditions. We show that zebrafish respond to different visual environments by darkening their overall colouration in a dark environment and lightening in a light environment. This is consistent with crypsis through background matching as a function of dynamic colour change. Additionally, we find that zebrafish use dynamic colour change to increase the internal contrast of their striped patterning in the presence of conspecifics. We speculate that this may function in social signalling and/or dazzle colouration. We find no effect of a predator stimulus on dynamic colour change. Finally, we discuss the potential for zebrafish to use multiple colouration strategies simultaneously as distance-dependent effects, considering the typical viewing distances of zebrafish and their predators.

Context or arousal? Function of drumming in Mongolian gerbils (Meriones unguiculatus)

Drumming is a non-vocal auditory display producing airborne as well as seismic vibrations by tapping body extremities on a surface. It is mostly described as an alarm signal but is also discussed to signal dominance or mating quality. To clarify the function of drumming in Mongolian gerbils (Meriones unguiculatus), we compared the occurrence of drumming during predator, opposite-sex and same-sex encounters. We tested 48 captive Mongolian gerbils (Meriones unguiculatus) in three experiments. In predator experiments, subjects were exposed alone or with their cagemate to aerial and terrestrial predator dummies. In social encounter experiments, familiar and unfamiliar male–female dyads and same-sex dyads were confronted. For the same-sex encounters, a dominance index was calculated for each subject based on the number of won and lost conflicts. Drumming and drumming-call combinations were counted, and a multi-parametric sound analysis was performed. In all experiments drumming and drumming-call combinations occurred. In predator experiments, more subjects drummed when confronted with the predator stimulus than in the habituation phase. In social encounter experiments, more subjects drummed when facing an unfamiliar than a familiar conspecific. In addition, the accompanying call type and body posture of the sender differed between experiments. Thus, we suggest that whereas drumming signals an increased arousal state of the sender, the accompanying call type and the body posture signal context specific information.

Offshore windfarm construction elevates metabolic rate and increases predation vulnerability of a key marine invertebrate

A global increase in offshore windfarm development is critical to our renewable energy future. Yet, widespread construction plans have generated substantial concern for impacts to co-occurring organisms and the communities they form. Pile driving construction, prominent in offshore windfarm development, produces among the highest amplitude sounds in the ocean creating widespread concern for a diverse array of taxa. However, studies addressing ecologically key species are generally lacking and most research is disparate, failing to integrate across response types (e.g., behavior, physiology, and ecological interactions), particularly in situ. The lack of integrative field studies presents major challenges to understand or mitigate actual impacts of offshore wind development. Here, we examined critical behavioral, physiological, and antipredator impacts of actual pile driving construction on the giant sea scallop (Placopecten magellanicus). Benthic taxa including bivalves are of particular concern because they are sound-sensitive, cannot move appreciable distances away from the stressor, and support livelihoods as one of the world's most economically and socially important fisheries. Overall, pile driving sound impacted scallops across a series of behavioral and physiological assays. Sound-exposed scallops consistently reduced their valve opening (22%), resulting in lowered mantle water oxygen levels available to the gills. Repeated and rapid valve adductions led to a 56% increase in metabolic rates relative to pre-exposure baselines. Consequently, in response to predator stimuli, sound-exposed scallops displayed a suite of significantly weaker antipredator behaviors including fewer swimming events and shorter time-to-exhaustion. These results show aquatic construction activities can induce metabolic and ecologically relevant changes in a key benthic animal. As offshore windfarm construction accelerates globally, our field-based study highlights that spatial overlap with benthic taxa may cause substantial metabolic changes, alter important fisheries resources, and ultimately could lead to increased predation.

Habituation to a predatory stimulus in a harvester (Arachnida, Opiliones)

Several studies have investigated habituation in a defensive context, but few have addressed responses to dangerous stimuli. In such cases, animals should not habituate since this could cost their lives. Here we have stimulated individuals of the harvester Mischonyx squalidus with a predatory stimulus (squeezing with tweezers) in repeated trials within and between days, and measured the occurrence and magnitude of nipping, a defensive behavior. Contrary to our expectations, they did habituate to this stimulus. The probability and magnitude of response declined over trials during each of three days of testing in a typical habituation pattern. During the trials we also observed other defensive behaviors. We discuss our results mainly considering alternative defensive responses. Our data show that we lack information on (1) the role played by the ambiguity of stimuli, (2) the role played by subsequent stimuli and (3) the importance of the array of defensive behaviors of a species in understanding habituation. Although ubiquitous across animals and therefore expected, habituation is described for the first time in the order Opiliones.

Variation in Predator Recognition Across Three Species of Jumping Spiders (Salticidae)

The ability to recognize threats and to respond in a timely and appropriate manner carries significant benefits. Depending on the recognition task, this can be cognitively demanding. The zebra jumping spider (Salticus scenicus) is capable of visually recognizing static predator stimuli and reacts via a robust “freeze and retreat,” a potentially innate response in this species. Here, we extend this finding, asking whether the ability of spiderlings to recognize a static predator and to initiate an escape response is common across juvenile salticids, and if so, whether there is species-specific variation of anti-predator responses. We found that captive-reared spiderlings of three European salticid species from different genera (Heliophanus cf. cupreus, Evarcha arcuata, Marpissa muscosa) were able to robustly recognize and retreat from a stationary predator stimulus. Additionally, we found differences in the reaction times between the species as well as different behavioral repertoires associated with the escape response which may reflect species-specific predator avoidance strategies.

Naphthenic acid fraction compounds, produced by the extraction of bitumen from oil sands, alter survival and behaviour of juvenile yellow perch (Perca flavescens)

We evaluated whether naphthenic acid fraction compounds (NAFCs) extracted from oil sand tailings adversely affect fish survival and behaviour. Following a before–after-control-impact design, we housed wild-caught juvenile yellow perch ( Perca flavescens) in outdoor mesocosms to assess survival and behaviour under baseline conditions, then exposed fish to one of three treatments: negative control, 2 mg/L NAFC, or 15 mg/L NAFC. We performed behavioural assays (no-stimulus activity, food stimulus, and predator stimulus using a model bird) and assessed a comprehensive suite of endpoints (equilibrium losses, activity, shoaling, burst swimming, freezing, and space use). We found that exposure to 15 mg/L NAFCs substantially reduced fish survival and impaired fish equilibrium in all three behavioural tests. Furthermore, exposure to NAFCs impaired anti-predator behaviour: while the activity of control fish increased by two-fold in response to a predator stimulus, fish exposed to 2 or 15 mg/L NAFC did not change their activity levels after stimulation. No significant changes were observed in other behavioural endpoints. Overall, our findings suggest that a week-long exposure to NAFCs at concentrations commonly found in tailings ponds, constructed wetlands, and other mining-impacted waters may affect multiple facets of fish behaviour that could ultimately lead to reduced fitness in fish populations.

Overcoming the phantoms of the past: Influence of predatory stimuli on the antipredator behavior of island pitvipers.

The reduction of predation is a potentially important factor for the evolution of the traits of an island animal species. By relaxed selection, insular animals tend to lose their antipredator behaviors. A monophyletic group of pitvipers (genus Bothrops) in southeastern Brazil, which have high genetic affinity and dwell on the mainland and adjacent islands, provide an appropriate setting to study the evolution of antipredator behavior and how different predatory stimuli can influence this behavior. The mainland Bothrops jararaca has several terrestrial and aerial predators, whereas B. insularis and B. alcatraz, restricted to two small islands, Queimada Grande and Alcatrazes, respectively, have a smaller range of aerial predators. Terrestrial predators are absent on Queimada Grande, but one potential snake predator occurs on Alcatrazes. We observed that the defensive repertoire of island snakes has not been lost, but they display different frequencies of some antipredator behaviors. The type of predatory stimuli (terrestrial and aerial) influenced the defensive response. Bothrops insularis most often used the escape strategies, especially against terrestrial predatory stimuli. Bothrops alcatraz displayed the highest rate of strike for both terrestrial and aerial stimuli. Our results indicate that even though relaxed selection may occur in island environments as compared to mainland environments, these pitvipers still retain their antipredator behaviors but with different response degrees to the two predator types.

Shock or jump: deimatic behaviour is repeatable and polymorphic in the Apennine yellow-bellied toad, Bombina pachypus

Abstract Inter-individual variation in antipredatory behaviour has long attracted curiosity among scientists. Deimatism is a complex, multi-component antipredatory strategy consisting of prey suddenly unleashing unexpected defences to startle predators and stop their attack. Deimatic species provide an appealing system to investigate the trade-offs between alternative behavioural phenotypes when faced with a threat. Yet, inter-individual variation in deimatic behaviour is almost unexplored. Here, we simulated a predator attack on 71 yellow-bellied toads, Bombina pachypus, to investigate the extent and pattern of inter-individual variation in the unken reflex behaviour, a deimatic display performed by some amphibians. Results show that deimatic displays consistently differ among B. pachypus individuals. Only about half of the individuals reacted to the simulated predation stimuli by exhibiting the display, which varied in responsiveness, duration and intensity. All the behavioural descriptors were highly repeatable. Finally, we found significant correlations between the measured parameters, defining two prevalent behavioural profiles: individuals quickly doing the unken reflex, with high intensity and long duration of the display, and individuals avoiding the unken reflex but instead escaping. This dichotomy resembles the proactive and reactive patterns in coping styles, and raises intriguing questions on the evolutionary processes shaping multiple adaptive responses to predation within populations.

Interspecific differences in the effects of masking and distraction on anti-predator behavior in suburban anthropogenic noise.

Predation is a common threat to animal survival. The detection of predators or anti-predator communication signals can be disrupted by anthropogenic noise; however, the mechanism by which responses are affected is unclear. Masking and distraction are the two hypotheses that have emerged as likely explanations for changes in behavior in noise. Masking occurs when the signal and noise fall within the same sensory domain; noise overlapping the energy in the signal reduces signal detection. Distraction can occur when noise in any sensory domain contributes to a greater cognitive load, thereby reducing signal detection. Here, we used a repeated measures field experiment to determine the relative contributions of masking and distraction in mediating reduced anti-predator responses in noise. We recorded the approaches and vocalizations of black-capped chickadees (Poecile atricapillus), tufted titmice (Baeolophus bicolor), and white-breasted nuthatches (Sitta carolinensis) to both visual and acoustic cues of predator presence, either with or without simultaneous exposure to anthropogenic noise. Titmice increased their calling to both visual and acoustic cues of predator presence. However, there was no significant effect of noise on the calling responses of titmice regardless of stimulus modality. Noise appeared to produce a distraction effect in chickadees; however, this effect was small, suggesting that chickadees may be relatively unaffected by low levels of anthropogenic noise in suburban environments. White-breasted nuthatch calling behavior was affected by the interaction of the modality of the predator stimulus and the noise condition. Nuthatches had a delayed response to the predator presentations, with a greater calling rate following the presentation of the acoustic stimulus in quiet compared to the presentation of the acoustic stimulus in noise. However, there was no difference in calling rate between the quiet and noise conditions for the visual stimulus. Together this suggests that even moderate levels of noise have some masking effect for white-breasted nuthatches. We suggest that the mechanisms through which noise influences anti-predator behavior may depend on the social roles, foraging ecology and auditory capabilities of each species.

Effects of amplitude and duration of noise exposure on the hearing and anti-predator behaviour of common roach (Rutilus rutilus) and sand goby (Pomatoschistus minutus).

This study investigates whether an exposure to two different received sound pressure levels at equal cumulative energy affects anti-predator behaviour and auditory detection thresholds of common roach (Rutilus rutilus) and sand goby (Pomatoschistus minutus) differently. This was examined in regard to a vessel slowdown as a management strategy to decrease vessel noise impact on fishes. Using continuous broadband noise, we found significant temporary threshold shifts (TTS) in roach, with 11.9 and 13.4 dB at 250 and 1000 Hz respectively, for the louder exposure. In contrast, gobies exhibited a non-significant shift of 6.6 dB at 125 Hz. Group cohesion increased in roach exposed to an artificial predator in the control group, but not during noise exposures. Gobies showed an initial freezing reaction towards the predator stimulus remaining motionless regardless of treatment. Our results show that a reduction in vessel speed with a corresponding reduction in source level could mitigate the effects on the auditory senses of sensitive fish, but does not appear to have any mitigating effect on their noise-induced behavioural changes. Further studies should investigate the effects of multiple vessel passages, but also the ecological consequences of the described effects on hearing and behaviour at individual and population level.

Guppies in large groups cooperate more frequently in an experimental test of the group size paradox.

The volunteer's dilemma, in which a single individual is required to produce a public good, predicts that individuals in larger groups will cooperate less frequently. Mechanistically, this could result from trade-offs between costs associated with volunteering and costs incurred if the public good is not produced (nobody volunteers). During predator inspection, one major contributor to the cost of volunteering is likely increased probability of predation; however, a predator also poses a risk to all individuals if nobody inspects. We tested the prediction that guppies in larger groups will inspect a predator less than those in smaller groups. We also predicted that individuals in larger groups would perceive less threat from the predator stimulus because of the protective benefits of larger groups (e.g. dilution). Contrary to prediction, we found that individuals in large groups inspected more frequently than those in smaller groups, but (as predicted) spent less time in refuges. There was evidence that individuals in intermediate-sized groups made fewest inspections and spent most time in refuges, suggesting that any link between group size, risk and cooperation is not driven by simple dilution. Extensions of theoretical models that capture these dynamics will likely be broadly applicable to risky cooperative behaviour.

Anthropogenic and biological sound effects on the maternal care behavior of a key crab species

IntroductionMaternal care in marine decapods involves eggs caring in the brood compartment until the larvae hatch. This behavior mainly allows embryo mass oxygen supply, ensuring healthy embryonic development. The present study aimed to analyze the effect of different sound sources (anthropogenic and biologic) and their temporal patterns (low and high rate: 1 min of the sound stimulus + 5 min of silence and 1 min of the sound stimulus + 1 min of silence, respectively) on the maternal care of the key crab species, Neohelice granulata.MethodsIn the laboratory, three acoustic stimuli were played back: an artificial white noise (10 Hz – 20 kHz), and two sounds obtained from the crabs´ natural habitat, motorboat passages and biological signals from a crabs’ predator fish. Three behavioral variables were quantified: still position, and two maternal care behaviors: abdominal flapping and chelae probing.ResultsResults demonstrated that the high rate anthropogenic stimuli, white noise and motorboat, affected all behavioral variables, increasing the still position and diminishing the maternal care behaviors. Otherwise, the predatory stimulus did not affect the still position although diminished the maternal care behaviors (high rate).DiscussionThe different behavioral response depending on the sound stimuli may indicate that crabs distinguish sound sources. The anthropogenic noise is suggested to cause distraction that is linked to the increased still position, while the predator stimulus would be associated with an alert behavior not affecting the locomotion behavior. The sound stimuli effect on the maternal care behavior revealed a negative effect that potentially could affect offspring survival. This is important considering the ecosystem engineering function of the studied key crab species. The reduction of the noise emission pattern rate is suggested as a mitigation action to diminish sound impact effects in the crab’s natural habitat. The study contributes the first to assessing the effect of different sound sources on the maternal care behavior of a crustacean species.

Primate–Predator Interactions: Is There a Mismatch Between Laboratory and Ecological Evidence?

Abundant empirical and theoretical studies indicate that predation is a key driver of primate evolution. The Snake Detection Theory (SDT) posits that snakes have been the main predators of primates since the late Cretaceous and that they influenced the diversification and evolution of primates. Laboratory research focusing on the innate ability of primates to detect snakes amid complex visual stimuli has provided strong support for key tenets of the SDT. While this theory has greatly contributed to our knowledge of primate evolution, supporting experimental studies may have overly focused on snakes and disregarded other important predators. This potential sampling bias weakens the conclusion that primates respond with a specific (high) intensity to snakes compared to other predators. We reviewed the literature about primate-predator interactions under natural and experimental conditions. We listed the primate and predator species involved in natural versus experimental studies. Predation events on primates recorded in the field mainly involved other primates, then raptors and carnivorans. SDT-related experimental studies heavily focused on snakes as predator stimuli and did not include raptors. Other experimental studies largely used snakes and primates and to a lesser extent carnivorans. Apes were the most often tested primates in experimental studies, whereas other primate taxa were neglected. Moreover, predators used as stimuli in experimental studies were inaccurately identified, notably snakes. Altogether, our results show that SDT-related studies neglected most of the major natural predators of primates. SDT studies also focused on a handful of primate species, whereas the theory relies on comparisons among taxa. Finally, poor taxonomic information on snakes used as stimuli blurs the interpretation of their relationship with primates. We suggest that future studies test the SDT by presenting a wide range of predators to different primate species to improve our understanding of the complexity of predator–prey interactions.

Exploring behavioral traits over different contexts in four species of Australian funnel-web spiders.

Australian funnel-web spiders are arguably the most venomous spiders in the world, with much research focusing on this aspect of their biology. However, other aspects related to their life history, ecology and behaviour have been overlooked. For the first time, we assessed repeatability, namely risk-taking behaviour, aggressiveness and activity in the contexts of predation, conspecific tolerance and exploration of a new territory in four species of Australian funnel-web spiders: two are closely related, Hadronyche valida and H. infensa, and two have overlapping distributions but occupy different habitats, H. cerberea and Atrax robustus. We also compared behaviors between species. At the species level, we found that H. valida showed consistency in risk-taking behavior when exposed to a predator stimulus, aggressiveness against conspecifics, and exploration of a new territory. In contrast, in the other species, only A. robustus showed repeatability in the context of exploration of a new territory. These results suggest that some behavioral traits are likely more flexible than others, and that the repeatability of behaviors may be species-specific in funnel-webs. When we compared species, we found differences in risk-taking behavior and defensiveness. This study provides novel insights to understanding variation in behavioral traits within and between species of funnel-web spiders, suggesting that some behavioral traits are likely context and/or species dependent, as a result of their evolutionary history. These findings provide key insights for understanding the ecological role of behavior and venom deployment in venomous animals, and a greater understanding of behavior in these medically significant and iconic spiders that are of conservation concern.

Multimodal Cues Do Not Improve Predator Recognition in Green Toad Tadpoles.

Simple SummaryTadpoles are known to use their sense of smell to detect the presence of predators, but some studies showed their reliance on vision during social interaction, suggesting that vision might have a role in predatory contexts as well. Here, we investigated how chemical or visual cues of a native predator, or a combination of both, influence the defensive behaviour of green toad tadpoles. We expected tadpoles to reduce their activity when exposed to chemical cues and avoid the area of the experimental arena near to the caged predator when exposed to the visual ones. With both cues, we expected tadpoles to show both responses and with greater intensity. Our results indicate that visual cues alone do not elicit any apparent defensive response, suggesting that tadpoles mainly rely on chemical cues to assess predation risk.The anti-predator behaviour of green toad (Bufotes balearicus) tadpoles was investigated by exposing them to only the visual or chemical cues, or a combination of both, of a native predator, southern hawker Aeshna cyanea. We collected green toad egg strings in the field and tadpoles did not receive any predatory stimulus before the onset of the experiment. To manipulate chemical and visual cues independently, dragonfly larvae were caged inside a transparent plastic container, while chemical cues (odour of tadpole-fed dragonfly larvae) were injected into the surrounding arena. An empty container and water were used, respectively, as controls. The behaviour of individually tested tadpoles was videorecorded for 40 min, of which 20 were before their exposure to stimuli. Five second-distance frames were compared to assess both tadpole activity and position within the arena with respect to the visual stimulus. The tadpole level of activity strongly decreased after exposure to either chemical cues alone or in combination with visual cues, while visual cues alone apparently did not elicit any defensive response. The position of tadpoles inside the arena was not affected by visual cues, suggesting that green toad tadpoles mainly rely on olfactory cues to assess the level of predation risk.

Mammal responses to predator scents across multiple study areas

AbstractAntipredator behaviors allow prey to mitigate the impacts of their predators. We investigated antipredator responses of two herbivore species, roe deer and European hare, and one mesopredator, red fox, toward predation risk imposed by lynx and wolf. We collected data (using camera traps) on visitation frequency and vigilance behavior to olfactory predator stimuli during 158 standardized scent trials in five areas across Europe, where lynx and wolves either occurred or had been absent for centuries. After a period without scent, trial sites were either marked with lynx or wolf urine, or butyric acid (unspecific scent used to contrast species‐specific scent responses). We expected the two herbivores to respond aversively (reduced visitation frequency, and increased vigilance) to predator urine, while red foxes (scavengers) might adopt a risk‐sensitive exploration strategy by increasing vigilance near predator urine without reducing visitation frequency. For all species, we expected stronger responses toward the ambush predator lynx than to the pursue predator wolf (cryptic predator hypothesis). If prey responds more strongly to predator stimuli when coexisting with the predator, we expected stronger reactions to predator urine in areas where predators occurred (naïve prey hypothesis). Roe deer significantly avoided lynx urine and butyric acid, but not wolf urine. However, roe deer visitation frequency did not differ between scent treatment with large carnivore urine and butyric acid (suggesting that roe deer generally avoid areas with unfamiliar scent), or between areas where large carnivores were present and absent. Hares did not significantly avoid predator urine. Red foxes were attracted to lynx urine in sympatry, but not in allopatry with large carnivores. They increased vigilance rates in the presence of lynx urine independent of sympatry/allopatry with large carnivores. These findings generally confirm our expectations of predator avoidance by herbivores, and attraction combined with increased vigilance of mesopredators. In all species, lynx urine elicited stronger responses than wolf urine (although not significantly different), which to some extent conforms to the cryptic predator hypothesis. We found no support for the naïve prey hypothesis. However, higher attraction of red foxes to lynx urine in sympatric situations might indicate that positive responses by scavengers are learning based.

Risk in one is not risk in all: snails show differential decision making under high- and low-risk environments

Predation is a major force that shapes prey phenotypes, species interactions and ecological communities. Prey respond to variation in predation risk and intensity by either increasing or decreasing vigilance and balancing energy requirements under high- or low-risk scenarios. We test the possible mechanism of sensory adaptation and decision making while responding to risk in the pond snail Lymnaea stagnalis . We maintained three risk treatment groups (no risk, low risk and high risk) for 20 days before testing them under multiple cognitive and behavioural challenges involving a predatory stimulus. Under a configural learning procedure where snails learned to forgo feeding when encountering food in the presence of a predatory threat, the high-risk group snails did not show learning/memory formation. However, when high-risk group individuals were transferred to a no-risk environment for 20 days, they were able to form configural learning memory. In a social interaction trial, snails from the no-risk group reduced interactions under threat whereas the high-risk group showed no change between risk environments. All risk groups responded similarly to an operant conditioning training under predation threat that enhanced memory. The decreased antipredator response of the high-risk group in comparison to the low-risk groups shows that snails are able to respond to risk based on past experience, especially when there exists a trade-off between survival and antipredator response. The variation in response to predation threat under different circumstances provides evidence of decision making rather than sensory adaptation as the mechanism involved in prey response. Our study combines multiple phenotypic responses to elucidate animal decision making under different intensities of risk and highlights the importance of experience in shaping antipredator responses. • We tested snails' responses (sensory adaptation/decision making) to predation risk. • Two cognitive tests and one social behavioural assay were used as response. • High-risk group animals showed weaker antipredator responses. • Response variation across risk groups support a decision-making mechanism at work.

Gregarious caterpillars shorten their larval development time in response to simulated predation threat

Abstract Group living in gregarious caterpillars may shorten the larval stage, which is thought to reduce predation exposure. In the gregarious caterpillar Battus polydamas archidamas (Papilionidae) we addressed the effects of simulated predation threat on the duration of the larval stage in solitary and gregarious caterpillars. We further evaluated the consequences of group rearing and predation stimuli for pupal mass. We reared caterpillars singly and in groups of ten individuals. Caterpillars were subjected to simulated predation threats (prodding with a paintbrush) and control larvae were untouched. We determined larval stage duration and pupal mass. We also recorded in solitary and gregarious caterpillars the proportion of individuals reacting to predation threats with distinct behavioural responses. Caterpillars in the group completed larval development earlier than solitary caterpillars. This pattern of accelerated development was already evident in the first instar. Simulated predation reduced the duration of both the entire larval stage and the first instar, but only for caterpillars in the group. Pupal mass was not affected by the predation treatment in either solitary or gregarious caterpillars. Solitary caterpillars showed thrashing and osmeterium display + thrashing more frequently than gregarious caterpillars, while individuals in the group reacted with osmeterium display to a greater extent. Accelerated larval development in response to predation risk has been predicted by theoretical models, but–to our knowledge–this is the first report on gregarious caterpillars. This putative adaptive response did not entail a fitness cost in terms of pupal mass.

Capybara responses to varying levels of predation risk

Predators can generate physiological and behavioural responses in prey individuals. Thus, carnivore reintroductions might cause profound changes in communities and ecosystems by modifying antipredator responses. Combining observational and experimental approaches, we compared the short- and long-term antipredator responses of capybaras, Hydrochoerus hydrochaeris , between a landscape with a high density of predators (Pantanal, Brazil) and a landscape where predators became extinct in the mid-20th century but where a jaguar reintroduction project was in progress (Iberá, Argentina). Generalized linear models were used to test whether the presence of natural predators affects capybara behavioural budgets, gregarious behaviour and the associated physiological stress responses and to test whether, in the short term, capybaras increase their vigilance levels after detecting the proximity of a simulated predator (a playback of jaguar, Panthera onca , calls). Capybaras in the Pantanal did not show higher levels of vigilance but they spent a greater proportion of time foraging than did capybaras in Iberá. Pantanal groups were smaller and foraged closer to water. The baseline levels of stress hormones tended to be higher in Iberá. In response to simulated stimuli, both populations responded to predator sound cues by increasing vigilance, but Iberá groups also increased vigilance in response to a control sound, suggesting that they did not recognize the predator stimulus as a greater risk than another stimulus. In areas with predators, capybaras may reduce predation risk by choosing safer areas, where they can spend more time foraging when predators are not nearby. Vigilance may be only a reactive response to cues of the predator's proximity. Understanding the capybara's antipredator responses may help predict the potential effects and the success of jaguar reintroduction in a region where the jaguar has been absent for over 80 years. • Capybaras facing regular jaguar predation spent more time foraging in safe zones. • Capybaras living without predators did not recognize predator sound stimuli. • Vigilance may be only a reactive response to cues of the predator's proximity. • Prey released from predation for generations may lose some antipredator behaviours. • Naïvety and high density of capybaras could predict successful jaguar reintroduction.

Neural correlates of distinct levels of predatory threat in dorsal periaqueductal grey neurons.

The dorsal periaqueductal grey (PAG) is an important site for integrating predatory threats. However, it remains unclear whether predator-related activation in PAG primarily reflects threat itself and thus can distinguish between various degrees of threat, or rather reflects threat-oriented behaviours, with the PAG potentially orchestrating different types of defensive repertoire. To address this issue, we performed extracellular recording of dorsal PAG neurons in freely behaving rats and examined neuronal and behavioural responses to stimulus conditions with distinct levels of predatory threat. Animals were sequentially exposed to a nonthreatening stimulus familiar environment (exposure to habituated environment) and to a novel nonthreatening stimulus (i.e., a toy animal-plush) and to conditions with high (exposure to a live cat), intermediate (exposure to the environment just visited by the cat, with remnant predator scent), and low (exposure on the following day to the predatory context) levels of predatory threat. To test for contributions of both threat stimuli and behaviour to changes in firing rate, we applied a Poisson generalized linear model regression, using the different predator stimulus conditions and defensive repertoires as predictor variables. Analysis revealed that the different predator stimulus conditions were more predictive of changes in firing rate (primarily threat-induced increases) than the different defensive repertoires. Thus, the dorsal PAG may code for different levels of predatory threat, more than it directly orchestrates distinct threat-oriented behaviours. The present results open interesting perspectives to investigate the role of the dorsal PAG in mediating primal emotional and cognitive responses to fear-inducing stimuli.