Simulated Predation Risk Research Articles

Predator identity and time of day interact to shape the risk-reward trade-off for herbivorous coral reef fishes.

Non-consumptive effects (NCEs) of predators occur as prey alters their habitat use and foraging decisions to avoid predation. Although NCEs are recognized as being important across disparate ecosystems, the factors influencing their strength and importance remain poorly understood. Ecological context, such as time of day, predator identity, and prey condition, may modify how prey species perceive and respond to risk, thereby altering NCEs. To investigate how predator identity affects foraging of herbivorous coral reef fishes, we simulated predation risk using fiberglass models of two predator species (grouper Mycteroperca bonaci and barracuda Sphyraena barracuda) with different hunting modes. We quantified how predation risk alters herbivory rates across space (distance from predator) and time (dawn, mid-day, and dusk) to examine how prey reconciles the conflicting demands of avoiding predation vs. foraging. When we averaged the effect of both predators across space and time, they suppressed herbivory similarly. Yet, they altered feeding differently depending on time of day and distance from the model. Although feeding increased strongly with increasing distance from the predators particularly during dawn, we found that the barracuda model suppressed herbivory more strongly than the grouper model during mid-day. We suggest that prey hunger level and differences in predator hunting modes could influence these patterns. Understanding how context mediates NCEs provides insight into the emergent effects of predator-prey interactions on food webs. These insights have broad implications for understanding how anthropogenic alterations to predator abundances can affect the spatial and temporal dynamics of important ecosystem processes.

Cautious versus desperado males: predation risk affects courtship intensity but not female choice in a wolf spider

Understanding the decisions that animals make in the presence of predation risk can reveal their assessment of current and future reproductive options. The wolf spider, Pardosa milvina, has a conspicuous courtship display that is an indicator of male quality. In an initial experiment, we determined that male courtship was highly repeatable and correlated with symmetry. We then used symmetry as an indicator of quality to explore the reactions of males when presented with simulated predation risk and an attractive virgin female. There were no differences in courtship features between risk and no risk treatments unless we took into account male quality. High-quality males reduced courtship intensity under risk conditions suggesting that they estimated their long-term fitness prospects as strong. This response is consistent with the asset protection principle, which suggests that animals with high value traits should behave cautiously. Low-quality males courted at high levels under predation risk, which seemed to be reckless attempts to attract females. This response is analogous to the effect, which is one explanation for vigorous fighting by inferior animals in contests. Although females used behavior to select males when no predator information was present, they seemed to use symmetry in the presence of predator cues. The ability of females to select the most attractive males suggests that they can accurately assess male quality under laboratory conditions. High courting desperado males may occasionally be selected in more complex environments and/or when they are in competition with other males.

Fixed behavioural plasticity in response to predation risk in the three-spined stickleback

I experimentally tested the repeatability and plasticity of two antipredator behaviours, shoaling and risk taking, in a sample of 443 juvenile three-spined sticklebacks, Gasterosteus aculeatus . I quantified between-individual variation in these behaviours as well as behavioural changes over time in two groups of sticklebacks that were either exposed or not exposed to simulated predation pressure. Shoaling and risk taking were repeatable within individuals in both experimental and control fish. Individual willingness to shoal increased over time in both experimental and control groups, but there was no evidence that shoaling changed in response to predation risk. Risk taking also showed temporal changes: sticklebacks exposed to simulated predation risk became increasingly fearful, unlike the control fish, suggesting that this behaviour is plastic. There was, however, no evidence of between-individual variation in the behavioural changes over time in either the control or experimental condition, suggesting that behavioural plasticity is a fixed response in the individuals of this population.

Reproduction in Risky Environments: The Role of Invasive Egg Predators in Ladybird Laying Strategies.

Reproductive environments are variable and the resources available for reproduction are finite. If reliable cues about the environment exist, mothers can alter offspring phenotype in a way that increases both offspring and maternal fitness (‘anticipatory maternal effects’—AMEs). Strategic use of AMEs is likely to be important in chemically defended species, where the risk of offspring predation may be modulated by maternal investment in offspring toxin level, albeit at some cost to mothers. Whether mothers adjust offspring toxin levels in response to variation in predation risk is, however, unknown, but is likely to be important when assessing the response of chemically defended species to the recent and pervasive changes in the global predator landscape, driven by the spread of invasive species. Using the chemically defended two-spot ladybird, Adalia bipunctata, we investigated reproductive investment, including egg toxin level, under conditions that varied in the degree of simulated offspring predation risk from larval harlequin ladybirds, Harmonia axyridis. H. axyridis is a highly voracious alien invasive species in the UK and a significant intraguild predator of A. bipunctata. Females laid fewer, larger egg clusters, under conditions of simulated predation risk (P+) than when predator cues were absent (P-), but there was no difference in toxin level between the two treatments. Among P- females, when mean cluster size increased there were concomitant increases in both the mass and toxin concentration of eggs, however when P+ females increased cluster size there was no corresponding increase in egg toxin level. We conclude that, in the face of offspring predation risk, females either withheld toxins or were physiologically constrained, leading to a trade-off between cluster size and egg toxin level. Our results provide the first demonstration that the risk of offspring predation by a novel invasive predator can influence maternal investment in toxins within their offspring.

Visible implant elastomer (VIE) tagging and simulated predation risk elicit similar physiological stress responses in three-spined stickleback Gasterosteus aculeatus.

The effect of visible implant elastomer (VIE) tagging on the immediate physiological stress response was tested in female three-spined stickleback Gasterosteus aculeatus, using non-invasive waterborne cortisol analysis. Post-tagging cortisol levels were significantly higher compared with pretreatment baseline concentrations; however, when comparing post-tagging cortisol levels with cortisol levels after exposure to a simulated aerial predator, no significant differences were found. This study indicates that VIE tagging elicits a physiological stress response similar to those occurring in the everyday lives of this important biological model organism.

Parasite- and predator-induced maternal effects in the great tit ( Parus major )

Both predators and parasites can elicit behavioral and physiological responses in prey and hosts, respectively. These responses may involve the reallocation of resources and may thus limit each other. We investigated the effects of concurrent pre-laying exposure of great tit females ( Parus major ) to both a simulated predation risk and a nest-based ectoparasite, the hen flea ( Ceratophyllus gallinae ), on nestling growth and development. We manipulated perceived predation risk using models and vocalizations of sparrowhawks ( Accipiter nisus ). At the start of incubation, we swapped whole clutches between treated and untreated nests to separate pre-laying maternal effects from posthatching effects. Since costs and benefits of maternal responses to parasites need to be assessed under parasite pressure, we infested half of the rearing nests with hen fleas. Parasites had negative effects on mass gain and wing growth, both via maternal effects and via direct exposure of nestlings, whereas maternal predation risk had no significant effect. The interaction between predator and parasite treatments was not significant and, thus, suggests the absence of a trade-off between the 2 stressors operating at the level of maternal effects. Alternatively, the complexity of the design, despite a relatively large sample size, may have limited the power for detection of this expected trade-off.

Antipredatory Response and Food Intake in Wood Mice (Apodemus sylvaticus) under Simulated Predation Risk by Resident and Novel Carnivorous Predators

AbstractChemical signals left by predators are a potential source of information about the risk of predation, and small mammals are known to take them into account when making decisions. We investigated whether wood mice (Apodemus sylvaticus) are more likely to avoid the faeces of resident predators (red fox Vulpes vulpes and common genet Genetta genetta) vs. a novel predator (European pine marten Martes martes). Odour recognition would increase perceived predation risk and reduce food intake by individual mice. Wood mice response to predators was analysed by live‐trapping using two untreated controls (baited/non‐baited) and traps experimentally manipulated with three predator treatments (faeces of red fox, common genet or pine marten). Traps were baited with 4 g of toasted corn, and food intake by wood mice was determined as the amount of bait remaining in each trap. We found that traps treated with faeces of resident predators were the most avoided, and the number of captures in traps treated with pine marten faeces was similar to the control‐baited traps. The variation found in food intake was explained by the interaction between the types of treatment and breeding condition. Food intake was similar in control‐baited traps and in traps with faeces of pine marten, but when predation risk by resident predators (red fox and common genet) was simulated, breeders reduced food intake significantly as compared to non‐breeders. These results indicate that predator recognition and feeding behaviour under predation risk depend on individual factors and the balance of costs‐benefits in each particular predation risk situation at a given place and time.

Different escape tactics of two vole species affect the success of the hunting predator, the least weasel

In the ongoing evolutionary arms race between predators and their prey, successful escape from the predator leads to the evolution of improved escape tactics in prey, but also predators become more effective in following and attacking the prey. Antipredatory behavior of prey is considered to be the strongest towards their most dangerous predators. However, prey species can differ both in vulnerability and efficiency of escape to a shared predator. We studied escape reactions of two vole species, the bank vole (Myodes glareolus) and the field vole (Microtus agrestis), under a simulated predation risk of the least weasel (Mustela nivalis nivalis). We conducted a laboratory experiment where a vole was given a possibility to escape from a weasel by fleeing to a horizontal tunnel or climbing the tree. Subsequently to the vole escape decision, we released a weasel to the same tunnel system to test how the weasel succeeded in following the vole. Weasel presence changed the behavior of voles as especially bank voles escaped by climbing. Instead, the majority of field voles fled into the ground-layer tunnel. The different escape tactics of the voles affected the success of the weasel, because climbing voles were less often successfully followed. We suggest that the difference in escape tactics has evolved as an adaptation to different habitats; meadow-exploiting field voles using ground-level escape while bank voles living in three-dimensional forest habitat frequently use arboreal escape tactics. This is likely to lead to different habitat-dependent vulnerabilities to predation in Microtus and Myodes vole species.

Voices in the dark: predation risk by owls influences dusk singing in a diurnal passerine

Predation is an important cost of communication in animals and thus a potent selection pressure on the evolution of signaling behavior. Heterospecific eavesdropping by predators may increase the vulnerability of vocalizing prey, particularly during low light, such as at dusk when nocturnal predators are actively hunting. Despite the risk it entails, dawn and dusk chorusing is common in passerines. However, the dusk chorus has not been studied much, neglecting the opportunity for understanding how eavesdropping between predators and prey may shape communication in birds. Here, we report the first demonstration of simulated predation risk (playback of owl vocalizations) altering the dusk chorus of a diurnal passerine, the veery (Catharus fuscescens). Veeries have a pronounced dusk chorus, singing well after sunset and potentially exposing themselves to predation by owls. In response to brief playbacks of owl calls (~30 s of calls presented three times over 25 min), veeries sang fewer songs post-sunset and stopped singing earlier relative to control trials. These changes in singing remained evident 30 min after the last owl stimulus. Although the avian dusk chorus has received relatively little attention to date, our results suggest that the dusk chorus may pose a higher predation risk to singing males that may influence the evolution of singing behavior in diurnal birds.

A test of maternal programming of offspring stress response to predation risk in threespine sticklebacks

Non-genetic maternal effects are widespread across taxa and challenge our traditional understanding of inheritance. Maternal experience with predators, for example, can have lifelong consequences for offspring traits, including fitness. Previous work in threespine sticklebacks showed that females exposed to simulated predation risk produced eggs with higher cortisol content and offspring with altered anti-predator behavior. However, it is unknown whether this maternal effect is mediated via the offspring glucocorticoid stress response and if it is retained over the entire lifetime of offspring. Therefore, we tested the hypothesis that maternal exposure to simulated predation risk has long-lasting effects on the cortisol response to simulated predation risk in stickleback offspring. We measured circulating concentrations of cortisol before (baseline), 15min after, and 60min after exposure to a simulated predation risk. We compared adult offspring of predator-exposed mothers and control mothers in two different social environments (alone or in a group). Relative to baseline, offspring plasma cortisol was highest 15min after exposure to simulated predation risk and decreased after 60min. Offspring of predator-exposed mothers differed in the cortisol response to simulated predation risk compared to offspring of control mothers. In general, females had higher cortisol than males, and fish in a group had lower cortisol than fish that were by themselves. The buffering effect of the social environment did not differ between maternal treatments or between males and females. Altogether the results show that while a mother's experience with simulated predation risk might affect the physiological response of her adult offspring to a predator, sex and social isolation have much larger effects on the stress response to predation risk in sticklebacks.

Foraging behaviour of the Peaceful Dove (Geopelia striata) in relation to predation risk: group size and predator cues in a natural environment

Foraging behaviour is directly influenced by the risk of predation, which is in turn influenced by distance to cover, height of vegetation and predator cues. In this study, we used giving-up density (GUD) to investigate the foraging behaviour of the Peaceful Dove (Geopelia striata) in relation to predation risk. We simulated predation risk by placing feeding patches in grass of various heights, and visual and auditory predator cues were also simulated. GUD was higher near tall grass and minimum GUD of individuals remained similar among different group sizes. Both visual cues and the combination of visual and acoustic cues altered the patterns of GUD significantly, but the effects of the two treatments did not differ from each other. We conclude that changes in vegetation structure and visual predator cues can strongly increase the assessment of predation risk by ground-feeding birds. Grouping behaviour in this species was not entirely a result of clumped food resources in their natural environment but was influenced by anti-predator strategies and birds could maintain higher food intake in more dangerous places when in groups. Our study used a rare, but important, field experimental approach to determine factors that affect foraging behaviour in a bird.

Mammalian nest predator feces as a cue in avian habitat selection decisions

Breeding habitat selection is expected to be adaptive. Animals should respond to strong agents of natural selection, such as expected offspring mortality due to nest predators, in their settlement decisions. In birds, mammalian nest predators are a significant mortality source and birds are known to respond to their presence. However, the mechanism used by birds to perceive mammalian nest predators and estimate the nest predation risk remains unknown, in particular at larger spatial scales while comparing potential breeding habitat patches. We experimentally tested whether the farmland bird community can detect and perceive cues of a mammalian nest predator (urine and feces), and how this perception affected the habitat selection and community structure of birds. The experiment was conducted at a large habitat patch scale by simulating a high abundance of nest predator by spraying on the ground water with dissolved mink excrements, whereas water was sprayed in the control treatment. Birds avoided settling in the plots treated with nest predator excrement. The number of migratory passerine species and their total density were lower in the simulated predation risk treatment than in the control treatment. Our results revealed a novel antipredator behavioral mechanism in birds; passerine birds can detect the excrements of mammalian nest predators and thereby assess the relative nest predation risk among potential breeding habitat patches. This mechanism has direct impacts on the structure of avian breeding bird community. Key wor ds: community structure, habitat selection, information use, nest preda tion. [Behav Ecol]

Environmental and genetic effects shape the development of personality traits in the mangrove killifishKryptolebias marmoratus

Consistent individual differences in behaviour are well documented, for example, individuals can be defined as consistently bold or consistently shy. To date our understanding of the mechanisms underpinning consistent individual differences in behaviour (also termed behavioural types (BTs)) remains limited. Theoretical work suggests life‐history tradeoffs drive BT variation, however, empirical support is scarce. Moreover, whilst life‐history is known to be phenotypically plastic in response to environmental conditions during ontogeny, the extent to which such plasticity drives plasticity in behavioural traits and personality remains poorly understood. Using a natural clonal vertebrate,Kryptolebias marmoratus, we control for genetic variation and investigate developmental plasticity in life‐history and three commonly studied behavioural traits (exploration, boldness, aggression) in response to three ecologically relevant environments; conspecific presence, low food and perceived risk. Simulated predation risk was the only treatment that generated repeatable behaviour i.e. personality during ontogeny. Treatments differed in their effects on mean life‐history and behavioural scores. Specifically, low food fish exhibited reduced growth rate and exploration but did not differ from control fish in their boldness or aggression scores. Conspecific presence resulted in a strong negative effect on mean aggression, boldness and exploration during ontogeny but had minimal effect on life‐history traits. Simulated predation risk resulted in increased reproductive output but had minimal effect upon average behavioural scores. Together these results suggest that life‐history plasticity/variation may be insufficient in driving variation in personality during development. Finally, using offspring derived from each rearing environment we investigate maternal effects and find strong maternal influence upon offspring size, but not behaviour. These results highlight and support the current understanding that risk perception is important in shaping personality, and that social experience during ontogeny is a major influence upon behavioural expression.

Is risk taking during foraging a personality trait? A field test for cross-context consistency in boldness

During foraging, animals have to balance the risk of predation with the energy gain. The amount of risk animals take for a given resource depends on their energy budget but is expected also to vary between individuals of different personality types. We tested whether individuals of free-ranging grey mouse lemurs, Microcebus murinus, forage risk-sensitively and are consistent in their risk-taking behaviour. Furthermore, we tested whether boldness towards a novel object predicts risk taking in a foraging task. In a field experiment, we simulated low and high predation risk at artificial feeding sites. During focal platform observations, we quantified behaviours related to exploration and feeding for 36 individuals. Furthermore, we used a novel object test to quantify variation in boldness for 22 of these individuals. As predicted, grey mouse lemurs foraged risk-sensitively indicated by longer latencies to enter a feeding platform and to start feeding as well as by relatively longer feeding time compared to nonfeeding in high-risk situations. Individual differences in risk taking were repeatable and repeatability increased with increasing risk. Individual plasticity was higher for low-risk individuals providing field evidence for coping styles. There was no relationship between individual body condition and risk-taking behaviour. Finally, boldness measured in a novel object test was correlated with risk taking in a foraging task, providing a rare ecological validation for this personality trait. These results suggest that intrinsic individual differences in boldness need to be considered as an important source of variation when testing predictions of risk-sensitive foraging using optimality approaches.

Consistent individual differences in fathering in threespined stickleback Gasterosteus aculeatus.

There is growing evidence that individual animals show consistent differences in behavior. For example, individual threespined stickleback fish differ in how they react to predators and how aggressive they are during social interactions with conspecifics. A relatively unexplored but potentially important axis of variation is parental behavior. In sticklebacks, fathers provide all of the parental care that is necessary for offspring survival; therefore paternal care is directly tied to fitness. In this study, we assessed whether individual male sticklebacks differ consistently from each other in parental behavior. We recorded visits to nest, total time fanning, and activity levels of 11 individual males every day throughout one clutch, and then allowed the males to breed again. Half of the males were exposed to predation risk while parenting during the first clutch, and the other half of the males experienced predation risk during the second clutch. We detected dramatic temporal changes in parental behaviors over the course of the clutch: for example, total time fanning increased six-fold prior to eggs hatching, then decreased to approximately zero. Despite these temporal changes, males retained their individually-distinctive parenting styles within a clutch that could not be explained by differences in body size or egg mass. Moreover, individual differences in parenting were maintained when males reproduced for a second time. Males that were exposed to simulated predation risk briefly decreased fanning and increased activity levels. Altogether, these results show that individual sticklebacks consistently differ from each other in how they behave as parents.

Predation risk increases permissiveness for heterospecific advertisement calls in túngara frogs, Physalaemus pustulosus

Factors that influence the strength of sexual selection within a population may have important consequences for the evolution and maintenance of reproductive isolation between populations. Here we examine the role of one such factor, predation risk, in influencing female permissiveness for heterospecific advertisement calls. Specifically, we tested the effect of simulated predation risk on mate choice in female tungara frogs, Physalaemus pustulosus, using variation in ambient light levels, travel time and the presence of auditory cues of local predators. Simulated predation risk increased female permissiveness for advertisement calls along an artificial gradient of calls intermediate between conspecifics and a congener, Physalaemus enesefae. Across the entire gradient, the presence of auditory cues of the predatory frog Leptodactylus pentadactylus in association with the conspecific call dramatically increased the likelihood of females choosing the intermediate call. In addition, higher ambient light levels and simulation of increased travel distances both increased the likelihood that females would choose intermediate calls over conspecific calls. These results suggest that, although mate choice may be important in causing reproductive isolation between allopatric populations, spatial or temporal variation in predation risk may strongly influence the expression of mate choice and thus the outcome of secondary contact.

Simulated Predation Risk Influences Female Choice in Túngara Frogs, Physalaemus pustulosus

While the presence of predators can influence female mate choice, few studies have investigated how females respond to quantitative variation in predation risk. In addition, we know little of how females respond to multiple, independent cues of risk. In this study, we investigated the effects of simulated predation risk on mate choice in tungara frogs, Physalaemus pustulosus, using the advertisement calls of predatory frogs, variation in ambient light, and simulated distance. Females showed aversion to conspecific calls associated with the calls of predators, and females were significantly less likely to travel perceived longer distances while the calls of predatory frogs were broadcast. In both the laboratory and field, females chose among potential mates significantly faster under higher light levels. Female responses to acoustic cues of predation risk were significantly influenced by light level, but decisions about travel distances were not. These results demonstrate that female choice is strongly influenced by perceived predation risk and that females can simultaneously evaluate quantitative variation in different cues of pre- dation risk. The changes in search behavior and mate evaluation we demonstrate indicate that predation plays a strong role in limiting signal evolution and possibly reproductive isolation.

Behavioural adaptations of mussels to varying levels of food availability and predation risk

Blue mussels Mytilus edulis (n ¼ 14) were studied in the laboratory using Hall sensor systems to record their gaping behaviour when exposed to varying food rations and levels of predation risk. Mussel response to increasing daily algal ration was to increase mean gape angle per day and was associated with copious pseudofaeces production at excessive initial algal concentrations, e.g. 250 cells/ml. Mean gape angle decreased (backward S-shaped curve) when fed a fixed algal ration per day where simulated predation risk (introduced fresh mussel homogenate) increased in the water, presumably as an anti-predation strategy. However, this behaviour is presumed to lower feeding rates. There was a general positive relationship between both gape angle and the extent of valve movement (abduction/ adduction) per event and valve movement speed. However, the fastest valve adduction events, which resulted in valve closure, were recorded independent of gape angle and only when mussels were first exposed to high perceived predation risk. We interpret this as an appropriate response, with high energetic cost, as a first line of defence from predators such as starfish and crabs. Overall, mussel response to predation appears graded and complex, indicating a trade-off between maximizing feeding/pseudofaeces production and minimizing predation risk.

Behavioural responses of voles to simulated risk of predation by a native and an alien mustelid: an odour manipulation experiment

Context. Potential mammalian prey commonly use the odours of their co-evolved predators to manage their risks of predation. But when the risk comes from an unknown source of predation, odours might not be perceived as dangerous, and anti-predator responses may fail, except possibly if the alien predator is of the same archetype as a native predator. Aims. In the present study we examined anti-predator behavioural responses of voles from the outer archipelagos of the Baltic Sea, south-western Finland, where they have had no resident mammalian predators in recent history. Methods. We investigated responses of field voles (Microtus agrestis) to odours of native least weasels (Mustela nivalis) and a recently invading alien predator, the American mink (Mustela vison), in laboratory. We also studied the short-term responses of free-ranging field voles and bank voles (Myodes glareolus) to simulated predation risk by alien mink on small islands in the outer archipelago of the Baltic Sea. Key results. In the laboratory, voles avoided odour cues of native weasel but not of alien mink. It is possible that the response to mink is a context dependent learned response which could not be induced in the laboratory, whereas the response to weasel is innate. In the field, however, voles reduced activity during their normal peak-activity times at night as a response to simulated alien-mink predation risk. No other shifts in space use or activity in safer microhabitats or denser vegetation were apparent. Conclusions. Voles appeared to recognise alien minks as predators from their odours in the wild. However, reduction in activity is likely to be only a short-term immediate response to mink presence, which is augmented by longer-term strategies of habitat shift. Because alien mink still strongly suppresses vole dynamics despite these anti-predator responses, we suggest that behavioural naiveté may be the primary factor in the impact of an alien predator on native prey. Implications. Prey naiveté has long been considered as the root cause of the devastating impacts of alien predators, whereby native prey simply fail to recognise and respond to the novel predation risk. Our results reveal a more complex form of naiveté whereby native prey appeared to recognise alien predators as a threat but their response is ultimately inadequate. Thus, recognition alone is unlikely to afford protection for native prey from alien-predator impacts. Thus, management strategies that, for example, train prey in recognition of novel threats must induce effective responses if they are expected to succeed.

Social learning strategies and predation risk: minnows copy only when using private information would be costly

Animals can acquire information from the environment privately, by sampling it directly, or socially, through learning from others. Generally, private information is more accurate, but expensive to acquire, while social information is cheaper but less reliable. Accordingly, the 'costly information hypothesis' predicts that individuals will use private information when the costs associated with doing so are low, but that they should increasingly use social information as the costs of using private information rise. While consistent with considerable data, this theory has yet to be directly tested in a satisfactory manner. We tested this hypothesis by giving minnows (Phoxinus phoxinus) a choice between socially demonstrated and non-demonstrated prey patches under conditions of low, indirect and high simulated predation risk. Subjects had no experience (experiment 1) or prior private information that conflicted with the social information provided by the demonstrators (experiment 2). In both experiments, subjects spent more time in the demonstrated patch than in the non-demonstrated patch, and in experiment 1 made fewer switches between patches, when risk was high compared with when it was low. These findings are consistent with the predictions of the costly information hypothesis, and imply that minnows adopt a 'copy-when-asocial-learning-is-costly' learning strategy.