The paradox of risk assessment: comparing responses of fathead minnows to capture-released and diet-released alarm cues from two different predators

Summary. Many aquatic prey are known to use chemical alarm cues to assess their risk of predation. In fishes, such alarm cues can be released either through damage of the epidermis during a predatory attack (capture-released) or through release from the predator feces (diet-released). In our study, we compared the importance of capture- versus diet-released alarm cues in risk assessment by fathead minnows (Pimephales promelas) that were na?ve to fish predators. We utilized two different fish predators: a specialized piscivore, the northern pike (Esox lucius) and a generalist predator, the brook trout (Salvelinus fontinalis). Handling time of pike consuming minnows was much shorter than for trout consuming minnows, likely resulting in less epidermal damage to the minnows during attacks by pike. In accordance with this, minnows showed a less intense antipredator response to capture-released cues from pike than capture-released cues from trout. This represents a paradox in risk assessment for the minnows as they respond to the specialized piscivore, the more dangerous predator, with a less intense antipredator response. In contrast, the minnows showed a stronger antipredator response to the specialized piscivore than to the generalist when given diet cues. This work highlights the need for researchers to carefully consider the nature of the information available to prey in risk assessment.     

Predation risk assessment by green frog (<Emphasis Type="Italic">Rana clamitans</Emphasis>) tadpoles through chemical cues produced by multiple prey

Many prey assess predation risk through predator chemical cues. Numerous studies have shown that (1) prey sometimes respond to chemical cues produced by heterospecifics and (2) that many species are capable of associative learning. This study extends this research by focusing on predation risk assessment and antipredator behavior in environments containing chemical cues produced by multiple prey species. The results show that green frog (Rana clamitans) tadpoles (1) assess risk from the chemical cue produced during predation by a heterospecific (gray tree frog, Hyla versicolor, tadpoles) and (2) can exhibit similarly strong behavioral responses to a mix of conspecific and heterospecific cues compared to conspecific cue alone, depending on their conditioning environment. I then discuss how the prey choice of the predators and the relative abundances of the prey species should influence the informational value of heterospecific cues.     

The role of learning in the acquisition of threat-sensitive responses to predator odours

The supposition that prey animals respond to a predator with an intensity that matches the risk posed by the predator is known as the threat-sensitive predator avoidance hypothesis. Many studies have provided support for this hypothesis; yet, few studies have attempted to determine how such abilities are acquired by prey species. In this study, we investigated whether fathead minnows (Pimephales promelas) could learn to recognize an unknown predator (northern pike, Esox lucius) in such a way that they could match the intensity of their antipredator response with the threat posed by the predator. We exposed pike-naïve minnows to conspecific alarm cues paired with either a high or low concentration of pike odor. The following day, both groups were tested for a response to either high or low concentration of pike odor alone. We found that minnows conditioned with alarm cues paired with a given concentration of pike odor subsequently responded with a higher intensity to higher concentrations of pike odor, and with a lower intensity to lower concentrations of pike odor. These results demonstrate that during a single conditioning trial, minnows learn the identity of the predator in a threat-sensitive manner. Minnows use predator odor concentrations that they experience in subsequent interactions to adjust the intensity of their antipredator behavior.     

Degradation of chemical alarm cues under natural conditions: risk assessment by larval woodfrogs

Summary. Many aquatic species use chemosensory information to assess predation risk. The cues used in such risk assessment can come either from the predator (predator odour) or from injured prey (alarm cues). The information conveyed through chemicals may, however, be inaccurate both spatially and temporally, as chemicals may persist in the environment long after the predator is gone. Thus, the level of accuracy of the cues for risk assessment may depend on the persistency of the chemicals in the habitat. Here, we investigated the persistency of alarm cues of a larval amphibian, the woodfrog (Rana sylvatica) in a ephemeral pond, their natural habitat. We introduced either alarm cues or control water in enclosed sleeves (~10 L) installed in the pond. The sleeve water was then sampled after 5 min and every two hours for eight hours. We used the behavioural response of woodfrog tadpoles to alarm cues as a bioassay to assess how long the alarm cues persisted in the environment. We found that tadpoles responded with an antipredator response to the pond water containing alarm cues 5 min after the injection of the cues in the sleeves but did not respond to that same pond water after two hours. Our results indicate that biodegradation and/or photodegradation of alarm cues in natural habitats might occur relatively quickly as the loss of a response to the cues in our experiment was independent of a dilution effect. This contrasts with previous laboratory results indicating that chemicals may be active after several hours.     

Novel features of an inducible defense system in larval tree frogs (Hyla chrysoscelis)

Organisms in aquatic ecosystems must often tolerate variable environmental conditions, including an uncertain risk of predation. Individuals that can maintain plastic defenses against predation will increase their survival when predators are present, but will not incur the costs of these defenses when the risk of predation is low and the defense is not induced. Larvae of the pond-breeding anuran Hyla chrysoscelis develop a conspicuous phenotype in the presence of predators consisting of a brightly colored tail and a deeper tail fin. In this study, I attempted to identify the source of the chemical signal that induces this defensive morphology in this species. I tested whether metabolites alone, originating from the prey but passing through the predator, were able to induce the same morphological response as the combination of alarm signals released directly by attacked conspecifics, and metabolites. I used morphometric and tail conspicuousness data to assess tadpole response to the perceived risk of predation by larval odonate predators (Anax junius). I also tested whether this inducing cue could be recognized across species by measuring the morphological response of H. chrysoscelis tadpoles exposed to cues emitted when tadpoles of a closely related genus (Pseudacris crucifer) were consumed. Tadpoles exhibited a clean graded response of both overall shape and tail morphology in response to all cues, corresponding to their relative reliability as indicators of a risk of predation. H. chrysoscelis tadpoles were also able to respond to cues emitted when tadpoles of a closely related genus were consumed by predators. These results illustrate that tadpoles of this species are able to respond to metabolites alone without alarm signals, and that interspecific chemical communication is a primary mechanism for predator avoidance in this inducible defense system.     

Chemical communication in green and golden bell frogs: do tadpoles respond to chemical cues from dead conspecifics?

Captive bred animals often lack the ability of predator recognition and predation is one of the strongest causes of failure of breed and release projects. Several tadpole and fish species respond defensively to chemical cues from injured or dead conspecifics, often referred to as alarm pheromones. In natural conditions and in species that school, the association of chemical cues from predators to alarm pheromones released by attacked conspecifics may lead to the learning of the predator-related danger without experiencing an attack. In the laboratory, this chemical communication can also be used in associative learning techniques to teach naïve tadpoles to avoid specific predators and improve survivorship of released animals. In our experimental trials, tadpoles of the threatened green and golden bell frog (Litoria aurea) did not avoid or decrease their activity when exposed to solutions of conspecific macerate, suggesting that the chemicals released into the water by dead/injured conspecifics do not function as an alarm pheromone. This non-avoidance of dead conspecific chemicals may explain why green and golden bell frog tadpoles have seemingly not developed any avoidance behaviour to the presence of introduced mosquito fish, and may render attempts to teach naïve tadpoles to avoid this novel predator more difficult.     

Variable predation risk and the dynamic nature of mosquito antipredator responses to chemical alarm cues

Summary. Predation is a pervasive selective agent highly variable in space and time. Due to the costs associated with antipredator responses, prey would be at a selective advantage if they respond to predation threats with an intensitfy matching the threat posed by the predator. Many aquatic organisms have been shown to use chemical alarm cues present in the water to assess the level of risk in their environment. This includes mosquito larvae which show antipredator responses to conspecific alarm cues. In this study, we investigated the nature of the responses of larval mosquitoes Culex restuans to those cues. In our initial observations, we showed pond/population differences in the response intensity of C. restuans to alarm cues. In experiment 1, we showed that the response intensity to alarm cues could be increased by increasing the background level of risk in the mosquitoes’ environment (by adding salamander predators) and once turned on, the response intensity to alarm cues was likely maintained for the remainder of the mosquitoes’ aquatic life. In experiments 2 and 3, we investigated if the increase in response intensity to alarm cues was directly correlated with the level of background risk in the mosquitoes’ environment. When given increasing levels of background risk, mosquito larvae subsequently showed a graded response to conspecific alarm cues. This series of experiments demonstrates that the response intensity of larval mosquitoes to a standard concentration of alarm cues is not fixed, but rather dependent on the background level of risk in the environment. An understanding of the background level of risk is particularly important for comparing antipredator responses of prey between habitats.     

Provenance and threat-sensitive predator avoidance patterns in wild-caught Trinidadian guppies

The antipredator behaviour of prey organisms is shaped by a series of threat-sensitive trade-offs between the benefits associated with successful predator avoidance and a suite of other fitness-related behaviours such as foraging, mating and territorial defence. Recent research has shown that the overall intensity of antipredator response and the pattern of threat-sensitive trade-offs are influenced by current conditions, including variability in predation risk over a period of days to weeks. In this study, we tested the hypothesis that long-term predation pressure will likewise have shaped the nature of the threat-sensitive antipredator behaviour of wild-caught Trinidadian guppies (Poecilia reticulata). Female guppies were collected from two populations that have evolved under high- and low-predation pressure, respectively, in the Aripo River, Northern Mountain Range, Trinidad. Under laboratory conditions, we exposed shoals of three guppies to varying concentrations of conspecific damage-released chemical alarm cues. Lower Aripo (high-predation) guppies exhibited the strongest antipredator response when exposed to the highest alarm cue concentration and a graded decline in response intensity with decreasing concentrations of alarm cue. Upper Aripo (low-predation) guppies, however, exhibited a nongraded (hypersensitive) response pattern. Our results suggest that long-term predation pressure shapes not only the overall intensity of antipredator responses of Trinidadian guppies but also their threat-sensitive behavioural response patterns.     

Bold minnows consistently approach danger in the field and lab in response to either chemical or visual indicators of predation risk

A behavioral syndrome is a suite of behaviors correlated across multiple social contexts. In this study, boldness in the face of predation risk was assessed twice in fish across two different sensory modalities in both the field and lab to ascertain the biological relevance and complexity of this attribute. Individual fathead minnows were captured from a natural field population using traps that either contained chemical alarm cues (conspecific skin extract) or control (well water) and their responses to the presence of predator behind a glass partition assessed in the laboratory. Although fewer fish were captured in alarm cue-labeled traps, these bold fish performed longer predator inspections than shy fish captured in control traps. Thus, a shy/bold behavioral syndrome was expressed consistently across field and lab settings in response to both chemical and visual indicators of danger. Shy and bold individuals did not differ in sex, body length, secondary sexual characteristics, or parasite load but were of more robust physical condition.     

Threat-sensitive learning of predators by larval mosquitoes <Emphasis Type="Italic">Culex restuans</Emphasis>

A prerequisite for prey to show adaptive behavioural responses to predators is that the prey has the ability to recognise predators as threats. While predator recognition can be innate in many situations, learning is often essential. For many aquatic species, one common way to learn about predators is through the pairing of a novel predator odour with alarm cues released from injured conspecifics. One study with fish demonstrated that this mode of learning not only allows the prey to recognise the predatory cues as a threat, but also mediates the level of threat associated with the predator cues (i.e. threat-sensitive learning). When the prey is exposed to the novel predator with a high concentration of alarm cues, they subsequently show a high intensity of antipredator response to the predator cues alone. When exposed to the predator with a low concentration of alarm cues, they subsequently show a low-intensity response to the predator cues. Here, we investigated whether larval mosquitoes Culex restuans have the ability to learn to recognise salamanders as a threat through a single pairing of alarm cues and salamander odour and also whether they would learn to respond to salamander cues in a threat-sensitive manner. We conditioned individual mosquitoes with water or a low, medium or high concentration of crushed conspecific cues (alarm cues) paired with salamander odour. Mosquitoes exposed to salamander odour paired with alarm cues and subsequently exposed to salamander odour alone responded to the salamander as a threat. Moreover, the intensity of antipredator response displayed during the conditioning phase matched the response intensity during the testing phase. This is the first demonstration of threat-sensitive learning in an aquatic invertebrate.     

Localized defecation by pike: a response to labelling by cyprinid alarm pheromone?

Fathead minnows (Pimephales promelas) that have never encountered a predatory pike (Esox lucius), are able to detect conspecific alarm pheromone in a pike's diet if the pike has recently consumed minnows. It remains unclear how this minnow alarm pheromone is secreted by pike and if a pike is able to avoid being labelled as a potential predator by localizing these cues away from its foraging range. The first experiment determined that minnow alarm pheromone is present in pike feces when pike are fed minnows. Individual fathead minnows exhibited a fright response to a stimulus of pike feces if the pike had been fed minnows, but not if the pike had been fed swordtails, which lack alarm pheromone. Individual minnows also exhibited a fright reaction to alarm pheromone in the water (which contained no feces) housing pike which had been fed minnows, suggesting that alarm pheromone is also released in urine, mucous secretions and/or via respiration. The second experiment determined that test pike spent a significantly greater proportion of time in the home area of the test tanks (i.e. where they were fed) but the majority of feces were deposited in the opposite end of the test tank. By localizing their defecation away from the home or foraging area, pike may be able to counter the effects of being labelled as a predator by the alarm pheromone of the prey species.     

Ontogenetic differences in chemical alarm cue production determine antipredator responses and learned predator recognition

How individuals assess, respond and subsequently learn from alarm cues is crucial to their survival and future fitness. Yet this information is not constant through time; many individuals are exposed to different predators throughout their life as they outgrow some predators or move to habitats containing different predators. To maximise overall fitness, individuals should discriminate between different cues and respond and learn from only those that are relevant to their current ontogenetic stage. We tested whether juvenile spiny chromis, Acanthochromis polyacanthus, could distinguish between chemical alarm cues from conspecific donors of different ontogenetic stages and whether the cue ontogenetic stage of the cue donor affected the efficacy of learning about predators. Juveniles displayed a significant antipredator response when conditioned with juvenile chemical alarm cues paired with predator odour but failed to respond when conditioned with predator odour paired with either adult alarm cues or with saltwater. Subsequently, individuals only recognised the predator odour alone as a threat when conditioned with juvenile alarm cues. This demonstrates that prey may be highly specific in how they use information from conspecific alarm cues, selectively responding to and learning from only those cues that are relevant to their developmental stage.     

Synergistic interactions between chemical alarm cues and the presence of conspecific and heterospecific fish shoals

Chemical and visual sources of information are used by aquatic prey during risk assessment. Here, we test the behavioral response of littoral prey fish to combinations of chemical alarm cues (skin extract) and the visual presence of a fish shoal. We scented minnow traps with either alarm cues or water (control) placed inside the trap, a jar that contained either a fish shoal or nothing (control), and recorded the number and species of fish captured. We predicted that chemical alarm cues would reduce the number of fish captured and that a fish shoal would increase the number of fish captured. The predicted effect of chemical and visual cues combined depended on the nature of the interaction. We found that the lowest catch rate was for the combination of alarm cue + no shoal, but the highest catch rate occurred for the combination of alarm cue + shoal. Fish shoal + water had the second highest catch rate and no shoal + water had the second lowest catch rate. We conclude that chemical alarm cues induce area avoidance in the absence of a shoal, but a strong behavioral proclivity to increase shoal cohesion in the presence of a shoal. The presence of a shoal in the traps induced alarmed fish to shoal with them and thus, enter the traps. This occurred even though traps were the source of the alarm cue.Communicated by A. Mathis     

Tradeoffs between courtship, fighting, and antipredatory behavior by a lizard, Eumeces laticeps

Male fitness in many species depends strongly on social behaviors needed to obtain fertilizations and prevent loss of fertilizations to other males, but courtship, copulation, and fighting may incur increased risk of predation. When demands for reproductive and antipredatory behaviors conflict, fitness may be maximized by accepting some degree of risk to enhance reproductive success. To examine such tradeoffs, I introduced tethered conspecific males or females to adult male broad-headed skinks, Eumeces laticeps, in the field and observed how close they allowed a simulated predator (me) to approach before fleeing, or their latency to approach an introduced female located at different distances from the predator. When conspecific males were introduced, isolated and mate-guarding males initiated agonistic behaviors and permitted closer approach than control males, and mate-guarding males permitted closer approach than isolated males. When females were introduced, both isolated and mate-guarding males courted the introduced females and isolated males permitted closer approach than did mate-guarding males. These results for introduced males and females suggest that increasing risk was accepted when reproductive benefits were greater. Latency for isolated males to approach a conspecific female was greater when the predator was closer to the female, further suggesting sensitivity to predation risk during a reproductive opportunity. Relationships between reproductive and antipredatory behaviors have been studied much less than those between feeding and antipredatory behaviors, but this study indicates that animals balance increased risk of predation with the opportunity to perform several reproductively important behaviors. Received: 5 March 1999 / Received in revised form: 15 July 1999 / Accepted: 25 July 1999     

Low-level copper exposures increase visibility and vulnerability of juvenile coho salmon to cutthroat trout predators

Copper contamination in surface waters is common in watersheds with mining activities or agricultural, industrial, commercial, and residential human land uses. This widespread pollutant is neurotoxic to the chemosensory systems of fish and other aquatic species. Among Pacific salmonids (Oncorhynchus spp.), copper-induced olfactory impairment has previously been shown to disrupt behaviors reliant on a functioning sense of smell. For juvenile coho salmon (O. kisutch), this includes predator avoidance behaviors triggered by a chemical alarm cue (conspecific skin extract). However, the survival consequences of this sublethal neurobehavioral toxicity have not been explored. In the present study juvenile coho were exposed to low levels of dissolved copper (5-20 microg/L for 3 h) and then presented with cues signaling the proximity of a predator. Unexposed coho showed a sharp reduction in swimming activity in response to both conspecific skin extract and the upstream presence of a cutthroat trout predator (O. clarki clarki) previously fed juvenile coho. This alarm response was absent in prey fish that were exposed to copper. Moreover, cutthroat trout were more effective predators on copper-exposed coho during predation trials, as measured by attack latency, survival time, and capture success rate. The shift in predator-prey dynamics was similar when predators and prey were co-exposed to copper. Overall, we show that copper-exposed coho are unresponsive to their chemosensory environment, unprepared to evade nearby predators, and significantly less likely to survive an attack sequence. Our findings contribute to a growing understanding of how common environmental contaminants alter the chemical ecology of aquatic communities.     

Can the ratio of aromatic skeletons explain cross-species responses within evolutionarily conserved Ostariophysan alarm cues?: testing the purine-ratio hypothesis

Summary. While the response to damage-released chemical alarm cues within the superorder Ostariophysi appears to be highly conserved across species, it is generally observed that the intensity of response to heterospecific alarm cues decreases with increasing phylogenetic distance. Recent studies have demonstrated that purine-N-oxides function as chemical alarm cues within Ostariophyian fishes and that the nitrogen-oxide functional group is conserved as the chief molecular trigger. According to the purine-ratio hypothesis, these cross-species differences may be due to the relative proportion of different carrier compounds associated with the nitrogen-oxide molecular trigger. To test this hypothesis, we exposed glowlight tetras (Hemigrammus erythrozonus, Characidae, Ostariophysi) to one of five synthetic stimuli (hypoxanthine-3-N-oxide (H3NO), pyridine-N-oxide (PNO) or mixed stimuli of 75 % H3NO-25 % PNO, 50 % H3NO-50 % PNO, or 25 % H3NO-75 % PNO), natural conspecific chemical alarm cue or a distilled water control. We quantified changes in shoal cohesion and vertical area use as species typical indicators of an antipredator response. As predicted, response intensity decreased as the ratio of hypoxanthine-3-N-oxide to pyridine-N-oxide decreased and the strongest response was to natural alarm cue. These results suggest that species-specific carrier compounds may account for the well-documented cross-species differences in the response to heterospecific alarm cues within phylogenetically related taxa.     

Female guppies shorten brood retention in response to predator cues

Predation risk influences the duration of offspring development in many species where embryos develop from externally shed eggs. Surprisingly, such predator-mediated effects on offspring development have rarely been explored in live-bearers. In this paper, we use the guppy (Poecilia reticulata), a live-bearing freshwater fish, to test whether the duration of brood retention (the time from mating to parturition) is influenced by experimental changes in the perceived level of predation. Because the swimming performance of female guppies is impaired during late pregnancy, we predicted that females would withhold broods for shorter periods when they are exposed to cues that signal a heightened risk of predation on adults rather than on juveniles. We therefore simulated increased risk of predation on adults by using a combination of pike-shaped models (resembling natural predators that prey on adult guppies) and ‘alarm substances’ derived from the skin extracts of adult conspecific females. Our results revealed that, under simulated predation risk, female guppies produced broods significantly more quickly than their counterparts assigned to a control group where predator cues were absent. A subsequent evaluation of offspring swimming performance revealed a significant positive association between neonate swimming speeds and the duration of brood retention, suggesting that by accelerating parturition, females may produce offspring with impaired locomotor skills. These findings, in conjunction with similar results from other live-bearing species, suggest that the conditions experienced by gestating females can generate significant variation in the timing of offspring development with potentially important implications for offspring fitness.     

Making sense of predator scents: investigating the sophistication of predator assessment abilities of fathead minnows

According to the threat-sensitive predator avoidance hypothesis, selection favors prey that accurately assess the degree of threat posed by a predator and adjust their anti-predator response to match the level of risk. Many species of animals rely on chemical cues to estimate predation risk; however, the information content conveyed in these chemical signatures is not well understood. We tested the threat-sensitive predator avoidance hypothesis by determining the specificity of the information conveyed to prey in the chemical signature of their predator. We found that fathead minnows (Pimephales promelas) could determine the degree of threat posed by northern pike (Esox lucius) based on the concentration of chemical cues used. The proportion of minnows that exhibited an anti-predator response when exposed to a predator cue increased as the concentration of the pike cue used increased. More surprisingly, the prey could also distinguish large pike from small pike based on their odor alone. The minnows responded more intensely to cues of small pike than to cues of large pike. In this predator–prey system small pike likely represent a greater threat than large pike.Communicated by A. Mathis     

Phylogenetic relatedness and ecological interactions determine antipredator behavior

Interspecific recognition of alarm cues among guild members through "eavesdropping" may allow prey to fine-tune antipredator responses. This process may be linked to taxonomic relatedness but might also be influenced by local adaptation to recognize alarm cues from sympatric species. We tested this hypothesis using antipredator responses of a freshwater gastropod Lymnaea stagnalis (L.) to alarm cues from damaged conspecific and 10 heterospecific gastropod species. As predicted, the magnitude of antipredator response decreased significantly with increasing phylogenetic distance, but increased when species were naturally sympatric (defined as species cohabiting in the same water body) with the source population of L. stagnalis. The responses to sympatric species were higher overall, and the relationship between genetic distance and alarm cue response was stronger when tested with sympatric species. This is the first study to demonstrate that population sympatry influences innate antipredator responses to alarm cues from intraguild members and suggests that responses based on phylogenetic relationships can be modified through local adaptation. Such adaptation to heterospecific alarm cues suggests that species could be at a disadvantage when they encounter novel intraguild members resulting from species invasion or range expansion due to a reduction in the presence of reliable information about predation risk.     

Risk level of chemical cues determines retention of recognition of new predators in Iberian green frog tadpoles

In aquatic environments, many prey rely on chemosensory information from injured (alarm cues) or stressed conspecifics (disturbance cues) to assess predation risk. Alarm cues are considered as a sign of higher risk than disturbance cues. These cues could be used by prey to learn potential new predators. In this study, we tested whether Iberian green frog tadpoles (Pelophylax perezi) exhibited antipredator responses to alarm and disturbance cues of conspecifics and whether tadpoles could associate new predators with alarm or disturbance cues. Tadpoles reduced their activity in the presence of disturbance cues, but only weakly when compared with their response to alarm cues. Also, tadpoles learned to recognize new predators from association with alarm or disturbance cues. However, the period of retention of the learned association was shorter for disturbance than alarm cues. Our results indicate that tadpoles are able to modify their antipredatory behavior according to (1) the degree of risk implied by the experimental cues (2) their previous experience of chemical cues of the predator.