Prey vulnerability to a gape-size limited predator: behavioural and morphological impacts on northern pike piscivory

The northern pike ( Esox lucius ) is a selective and important predator in lake ecosystems. Prey size in pike is limited by pike gape size, which is a linear function of pike body length. Here we show that the absolute gape-size limit in pike is greater than previously considered, and that maximum ingestible prey size is limited by prey body depth. Further, we experimentally show that pike prefer shallow-bodied roach before deeper-bodied common bream, and small prey sizes within each prey species. Handling time in pike increases with prey body depth, and since common bream are deeper-bodied than roach, handling time is longer for bream than for roach of the same length, but equal considering body depth. Prey handling time is suggested to be a major cost to the pike, since it increases the risk of losing the prey, as well as exposure to predation, kleptoparasitism and cannibalism. Consequently, prey vulnerability is determined by risk of predation and intraspecific interactions, and behavioural preferences in the pike, and not by pike gape-size limits. The consequences for natural populations is evaluated by analysing size structures of predator and prey fish populations in a eutrophic lake.     

Responses of larval dragonflies to conspecific and heterospecific predator cues

Abstract.  1. In cannibalistic populations, smaller individuals are subject to predation by larger conspecifics, and small individuals commonly alter their behaviour in response to cannibals. Little is known, however, about the underlying cues that trigger such responses and how the behavioural responses to conspecific cannibals differ from heterospecific predators.
2. This study tests which cues are used for the detection of conspecific predators in the larva of the dragonfly Plathemis lydia and how the behavioural response to cannibals differed from the response to heterospecific predators.
3. Individuals were exposed to chemical cues, visual cues, and a combination of both cues from conspecifics as well as no predator and heterospecific predator controls during which their activity and feeding rates were observed.
4. Individuals increased their activity, spatial movement and feeding behaviour in response to either visual or chemical cues from conspecific predators, which was opposite to responses displayed with cues from heterospecific predators. Interestingly, the responses to visual and chemical cues from conspecifics combined were weaker than to either cue in isolation and similar to the no cue control.
5. The results clearly indicate that individuals are able to use chemical and visual cues to detect even very subtle differences in phenotype of conspecific predators.
6. The opposite response in behaviour when exposed to conspecific cannibals vs. heterospecific predators suggests that the presence of cannibals will increase the mortality risk of small individuals due to heterospecific predation. This risk-enhancement is likely to have important consequences for the dynamics of predator–prey interactions.     

Learned Recognition of Intraspecific Predators in Larval Long-Toed Salamanders Ambystoma macrodactylum

The ability of prey to detect predators and respond accordingly is critical to their survival. The use of chemical cues by animals in predator detection has been widely documented. In many cases, predator recognition is facilitated by the release of alarm cues from conspecific victims. Alarm cues elicit anti‐predator behavior in many species, which can reduce their risk of being attacked. It has been previously demonstrated that adult long‐toed salamanders, Ambystoma macrodactylum, exhibit an alarm response to chemical cues from injured conspecifics. However, whether this response exists in the larval stage of this species and whether it is an innate or a learned condition is unknown. In the current study, we examined the alarm response of naïve (i.e. lab‐reared) larval long‐toed salamanders. We conducted a series of behavioral trials during which we quantified the level of activity and spatial avoidance of hungry and satiated focal larvae to water conditioned by an injured conspecific, a cannibal that had recently been fed a conspecific or a non‐cannibal that was recently fed a diet of Tubifex worms. Focal larvae neither reduced their activity nor spatially avoided the area of the stimulus in either treatment when satiated, and exhibited increased activity towards the cannibal stimulus when hungry. We regard this latter behavior as a feeding response. Together these results suggest that an anti‐predator response to injured conspecifics and to cannibalistic conspecifics is absent in naïve larvae. Previous studies have shown that experienced wild captured salamanders do show a response to cannibalistic conspecifics. Therefore, we conducted an additional experiment examining whether larvae can learn to exhibit anti‐predator behavior in response to cues from cannibalized conspecifics. We exposed larvae to visual, chemical and tactile cues of stimulus animals that were actively foraging on conspecifics (experienced) or a diet of Tubifex (naïve treatment). In subsequent behavioral treatments, experienced larvae significantly reduced their activity compared to naive larvae in response to chemical cues of cannibals that had recently consumed conspecifics. We suggest that this behavior is a response to alarm cues released by consumed conspecifics that may have labeled the cannibal. Furthermore, over time, interactions with cannibals may cause potential prey larvae to learn to avoid cannibals regardless of their recent diet.     

Direct and indirect cues of predation risk influence behavior and reproduction of prey: a case for acarine interactions

Little is known about the flow of chemical information fromhigher to lower levels within the animal food chain. However,this information may determine the behavior and distributionof many animals (e.g., that of potential prey) when exposedto direct and indirect cues of predation risk. We used herbivorousspider mites, Tetranychus urticae Koch (Tetranychidae) as amodel to examine the foraging and oviposition decisions thatprey make when exposed to these cues. We conducted laboratorytests to determine if the previous presence of predators (directcues) on leaf discs or the presence of injured conspecifics(indirect cues) alters the distribution of adults and eggs ofT. urticae. When given a choice, after 24 h, fewer adults and eggswere found on leaf discs that had previously contained specialistspider mite predators, Phytoseiulus persimilis Athias-Henriot (Phytoseiidae),than on discs unexposed to predators. Also, more T. urticaeemigrated from predator-exposed discs than from unexposed discs orfrom those that had previously contained nonpredatory mites(Tyrophagus putrescentiae, Acaridae). Finally, fewer T. urticaeforaged and laid eggs on predator-exposed discs or on thosewith artificially damaged conspecifics (eggs or dead adults)than on discs with intact conspecifics. Tetranychus urticaeprobably recognizes infochemicals (kairomones) from its predatorsor cues from injured spider mites and consequently avoids feedingor ovipositing in areas exposed to these cues. Recognition and avoidanceof kairomones from specialist predators by this prey are likelyto be hereditary, but avoidance of injured conspecifics maybe an adaptation to avoid predators that are not inherentlyrecognized. We discuss the behavioral and ecological implicationsof our findings.     

Pathogens as a factor limiting the spread of cannibalism in tiger salamanders

Summary Intraspecific predation is taxonomically widespread, but few species routinely prey on conspecifics. This is surprising as conspecifics could be a valuable resource for animals limited by food. A potential cost of cannibalism that has been largely unexplored is that it may enhance the risk of acquiring debilitating pathogens or toxins from conspecifics. We examined how pathogens affect variation in the incidence of cannibalism in tiger salamander larvae (Ambystoma tigrinum nebulosum), which occur as two environmentally-induced morphs, typicals and cannibals. Salamanders from one population were more likely than those in another to develop into cannibals, even when reared under identical conditions. Variation in the propensity to become a cannibal may be caused by variation in pathogen density. In the population with cannibals at low frequency, bacterial blooms in late summer correlated with massive die-offs of salamanders. The frequency of cannibals correlated significantly negatively with bacterial density in ten different natural lakes. In the laboratory, cannibals exposed to a diseased conspecific always preyed on the sick animal. As a result, cannibals wre more likely to acquire and die from disease than were typicals that were similarly exposed, or cannibals that were exposed to healthy conspecifics. Since conspecifics often share lethal pathogens, enhanced risk of disease may explain why cannibalism is generally infrequent. Pathogens may constrain not only the tendency to be behaviorally cannibalistic, but also the propensity to develop specialized cannibal morphologies.     

The Role of gastric evacuation rate in handling time of equal-mass rations of different prey sizes in northern pike

In a laboratory experiment, northern pike Esox lucius gastric evacuation rates did not differ between equal-mass rations of small and large prey. In a comparison between intermediate and large prey, the pike were unable to fit two intermediate prey completely into the stomach at the same time, resulting in two consecutive evacuations, and changes in patterns of gastric evacuation. Thus, total gastric evacuation time was not affected by prey size composition in equal-mass rations, but patterns in evacuation rate may depend on the size ratio between predator and prey. Cumulative manipulation time between strike and complete swallowing of prey differed between equal-mass rations of small, intermediate and large prey, in that small prey took the shortest time to manipulate. Pike had problems striking and redirecting intermediate prey to swallow them head first, and the manipulation times for intermediate prey were as long as for large prey. Since an increased time manipulating prey in the mouth increases risk of predation and intraspecific interactions in pike, it is concluded that risks associated with long manipulation times, and not only energy per total handling time, determine prey value and prey size preference in this piscivore.     

Linking Morphological and Behavioural Defences: Prey Fish Detect the Morphology of Conspecifics in the Odour Signature of their Predators

Predation is a strong selective force acting on both morphology and behaviour of prey animals. While morphological defences (e.g. crypsis, presence of armours or spines or specific body morphologies) and antipredator behaviours (e.g. change in foraging or reproductive effort, or hiding and fleeing behaviours) have been widely studied separately, few studies have considered the interplay between the two. The question raised in our study is whether antipredator behaviours of a prey fish to predator odours could be influenced by the morphology of prey conspecifics in the diet of the predator. We used goldfish (Carassius auratus) as our test species; goldfish exposed to predation risk significantly increase their body depth to length ratio, which gives them a survival advantage against gape‐limited predators. We exposed shallow‐bodied and deep‐bodied goldfish to the odour of pike (Esox lucius) fed either form of goldfish. Deep‐bodied goldfish displayed lower intensity antipredator responses than shallow‐bodied ones, consistent with the hypothesis that individuals with morphological defences should exhibit less behavioural modification than those lacking such defences. Moreover, both shallow‐ and deep‐bodied goldfish displayed their strongest antipredator responses when exposed to the odour of pike fed conspecifics of their own morphology, indicating that goldfish are able to differentiate the morphology of conspecifics through predator diet cues. For a given individual, predator threat increases as the prey become more like the individual eaten, revealing a surprising level of sophistication of chemosensory assessment by prey fish.     

Predator recognition and defence strategies in crucian carp, Carassius carassius

Crucian carp from populations that lack piscivores are extremely vulnerable to predation. However, in the presence of piscivores these fish develop an inducible morphological defence, a deep body. This switch from a vulnerable, shallow-bodied morph to a morphologically defended morph makes this species very suitable for investigations of anti-predator strategies, and trade-offs between morphological and behavioural defences. To address these questions, we performed eight different experiments. We found that crucian carp exhibited fright responses to chemical cues from unfamiliar predators (northern pike, perch) when these were fed prey that contained alarm substance (for northern pike: crucian carp, roach; for perch: crucian carp). Cues from small pike that were fed prey that lacked alarm substance (swordtails) caused no significant fright response whereas cues from larger pike with the same diet did. Perch on a chironomid diet elicited weaker but significant fright responses. Starved predators caused as strong fright reactions as recently fed ones did, whereas no response was exhibited towards nonpredatory fish (roach, crucian carp). Crucian carp were able to detect the presence of pike after cues had been diluted to an equivalent of 21 000 l, and larger predators elicited stronger fright responses. Prior experience of predators decreased fright responses. In particular, individuals from populations that coexisted with northern pike responded less to chemical cues from northern pike than individuals without prior experience did. Thus, crucian carp may use both alarm-substance related and predator-related cues to identify predators. Further, they were able to discriminate between large and small predators. Finally, individuals from populations that coexist with predators exhibit less pronounced fright responses. These fish have an induced morphological defence, a deep body, which most likely decreases the need for strong antipredator behaviour.     

Analysis of sibling cannibalism among pike,Esox lucius,juveniles reared under semi-natural conditions

Synopsis Sibling cannibalism in pike, Esox lucius, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae m^–2, equivalent to 12 and 36 larvae m^–3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae m^–2 (36 larvae m^–3), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60 mm) onwards. At a mean total length of 100 mm and for initial stocking densities of 6 and 18 larvae m^–2, (12 and 36 larvae m^–3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.     

Influence of prey availability and conspecifics on patch quality for a cannibalistic forager: laboratory experiments with the wolf spider Schizocosa

 Because cannibals are potentially both predator and prey, the presence of conspecifics and alternative prey may act together to influence the rate at which cannibals prey upon each other or emigrate from a habitat patch. Wolf spiders (Lycosidae) are cannibalistic-generalist predators that hunt for prey with a sit-and-wait strategy characterized by changes in foraging site. Little information is available on how both prey abundance and the presence of conspecifics influence patch quality for these cursorial, non-web-building spiders. To address this question, laboratory experiments were conducted with spiderlings and older juveniles of the lycosid genus Schizocosa. The presence of insect prey consistently reduced rates of spider emigration when spiders were housed either alone or in groups. Solitary juvenile Schizocosa that had been recently collected from the field exhibited a median giving-up time (GUT) of 10 h in the absence of prey (Collembola); providing Collembola increased the median GUT to 64 h. For solitary spiders, the absence of prey increased by about fourfold the rate of emigration during the first 24 h. In contrast, for spiders in patches with a high density of conspecifics, the absence of prey increased the 24-h emigration rate by only 1.6-fold. For successful cannibals in the no-prey patches, the presence of conspecifics improved patch quality by providing a source of food. Mortality by cannibalism was affected by both prey availability and openness of the patch to net emigration. In patches with no net emigration, the presence of prey reduced rates of cannibalism from 79% to 57%. Spiders in patches open to emigration but not immigration experienced a rate of cannibalism (16%) that was independent of prey availability. The results of these experiments indicate that for a cannibalistic forager such as the wolf spider Schizocosa, (1) the presence of conspecifics can improve average patch quality when prey are absent, and (2) cannibalism has the potential to be a significant mortality factor under natural field conditions because cannibalism persisted in prey patches that were open to emigration. Received: 12 April 1996 / Accepted: 14 August 1996     

Emergent impacts of cannibalism and size refuges in prey on intraguild predation systems

Many organisms undergo ontogenetic niche shifts due to considerable changes in size during their development. These ontogenetic shifts can alter the trophic position of individuals, the type and strength of ecological interactions across species, and allow for cannibalism within species. In this study we ask if and how the interaction of a size refuge and cannibalism in the prey alters the dynamics of intraguild predation (IGP) systems. By manipulating the composition of large cannibalistic (Aeshna umbrosa) and predatory (Anax junius) dragonfly larvae in mesocosms we show that the interaction of cannibals and predators was non-linear and increased the survival of prey. The structure of the final resource community shared by prey and predator differed between small and large dragonfly treatments but not within size classes across species. In general, the small prey stage showed similar shifts in microhabitat use and refuge use when exposed to either conspecific cannibals or predators, while large cannibals showed no clear anti-predator response. However, further behavioral experiments revealed that specific behavioral components, such as distances between individuals or number of movements, differed when individuals were exposed to either cannibals or predators. This indicates that individuals discriminated between conspecific or heterospecific predators. Furthermore, in similar experiments large cannibals and predators showed different behaviors when exposed to conspecifics rather than to each other. These changes in behavior are consistent with the observed increase in prey survival. In general, the results indicate that cannibalism and ontogenetic niche shifts can result in behavior-mediated indirect interactions that reduce the impact of the predator on the mortality of its prey and alter the interactions of IGP systems. However, they also indicate that size is not the sole determinant and that we also need to account for the species identity when predicting the dynamics of communities.     

Predation risk suppresses the positive feedback between size structure and cannibalism

1.?Cannibalism can play a prominent role in the structuring and dynamics of ecological communities. Previous studies have emphasized the importance of size structure and density of cannibalistic species in shaping short- and long-term cannibalism dynamics, but our understanding of how predators influence cannibalism dynamics is limited. This is despite widespread evidence that many prey species exhibit behavioural and morphological adaptations in response to predation risk. 2.?This study examined how the presence and absence of predation risk from larval dragonflies Aeshna nigroflava affected cannibalism dynamics in its prey larval salamanders Hynobius retardatus. 3.?We found that feedback dynamics between size structure and cannibalism depended on whether dragonfly predation risk was present. In the absence of dragonfly risk cues, a positive feedback between salamander size structure and cannibalism through time occurred because most of the replicates in this treatment contained at least one salamander larvae having an enlarged gape (i.e. cannibal). In contrast, this feedback and the emergence of cannibalism were rarely observed in the presence of the dragonfly risk cues. Once salamander size divergence occurred, experimental reversals of the presence or absence of dragonfly risk cues did not alter existing cannibalism dynamics as the experiment progressed. Thus, the effects of risk on the mechanisms driving cannibalism dynamics likely operated during the early developmental period of the salamander larvae. 4.?The effects of dragonfly predation risk on behavioural aspects of cannibalistic interactions among hatchlings may prohibit the initiation of dynamics between size structure and cannibalism. Our predation trials clearly showed that encounter rates among hatchlings and biting and ingestion rates of prospective prey by prospective cannibals were significantly lower in the presence vs. absence of dragonfly predation risk even though the size asymmetry between cannibals and victims was similar in both risk treatments. These results suggest that dragonfly risk cues first suppress cannibalism among hatchlings and then prevent size variation from increasing through time. 5.?We suggest that the positive feedback dynamics between size structure and cannibalism and their modification by predation risk may also operate in other systems to shape the population dynamics of cannibalistic prey species as well as overall community dynamics.     

Cannibalism in pike fry, Esox lucius L.: some experiments with fry densities

Pike fry were kept in 0.054-m³ tanks at densities of 50, 100 and 150 (277, 555, 833 fry m⁻²) for seven weeks during which the development of social and feeding behaviour was observed. Zooplankton, macro-invertebrates and perch fry were provided sequentially as food; the pike fry were allowed to feed ad libitum.
Zooplanktivorous fry stopped growing at 22 mm whilst, in the presence of abundant suitable alternative prey, 1–4% turned cannibalistic at 5 weeks of age; cannibals subsequently grew rapidly (mean 1.88 mm day⁻¹). Cannibalism ensued in all tanks when the ratio of predator size: prey size was c .2:1.
Fry tended to space-out evenly in the tanks with no overt aggression or territoriality. Behaviour was typified by remaining still for long periods, particularly subsequent to the onset of cannibalism. Cannibals were attracted by fry movements which often initiated attacks.
Daily per capita mortality rates showed no crowding effects before cannibalism but significant density-dependent mortality due to cannibals. Consumption rates of cannibals varied between 0.63 and 6.0 fry per cannibal per day. Cannibals accounted for 54–96% of daily mortality in the experimental tanks. These results are discussed in relation to proposed mechanisms of pike population density regulation.     

The Diel Activity of Crucian Carp, Carassius Carassius, in Relation to Chemical Cues from Predators

Chemical cues from piscivorous fish and prey alarm substances often cause rapid fright responses in prey. However little is known of how piscivore-related chemical cues affect prey behaviour over periods longer than a few hours. Here we have investigated how chemical cues from piscivorous northern pike, Esox lucius, affect habitat choice and diel activity of crucian carp, Carassius carassius, over an extended period 11 days. At the beginning of the experiment control fish were nocturnal while fish in the pike cue treatment were aperiodic. After 11 days, control fish had become more strongly nocturnal and displayed two activity peaks during early and late night whereas fish in the pike cue treatment were still aperiodic with no activity peaks. Habitat choice was aperiodic in both treatments throughout the experiment. In both treatments, more fish were found in the vegetation zone than in the open habitat. This was most pronounced when pike cues were present. These results demonstrate that short-term anti-predator responses to chemical cues from predators can translate into long-term adjustments of diel periodicity. Further, the results did not support the idea that crucian carp should switch to nocturnal activity in response to visually hunting predators. Control fish were nocturnal and chemical cues from pike did not make this pattern more pronounced.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

Comparison of mouth morphology and prey size selection among three esocid taxa

Aquatic organisms, especially fishes, exhibit exceptional diversity in mouth morphology and this variation has been shown to influence foraging patterns. We compared mouth morphology among muskellunge Esox masquinongy, northern pike Esox lucius and their hybrid, tiger muskellunge E. masquinongy x E. lucius. Head and mouth size among the three taxa were similar for juveniles (<400 mm total length), but diverged with increasing length, being greater for northern pike than muskellunge. Tiger muskellunge had a head and mouth size intermediate to the two, but more similar to northern pike than muskellunge. Morphological differences among taxa were related to data examining prey size selection in laboratory and field experiments. In the laboratory, northern pike selected prey that were smaller than their maximum mouth width (widest point between outside corners of mouth), tiger muskellunge selected larger prey, and muskellunge size-selection was intermediate between the other two taxa. Among the three esocids, muskellunge had the smallest increase in handling time with increasing prey body depth relative to predator mouth width. In a common garden field experiment in three lakes containing mainly deep-bodied prey, results generally followed morphological patterns, with northern pike selecting larger prey compared to muskellunge. Although morphology predicted most of the variation in greatest body depth of prey consumed, the best predictor of prey size was a model that included predator mouth width, taxon, and interaction. Information comparing prey size selection among esocid taxa is useful for understanding how to manage esocid populations based on system-specific prey characteristics and also for understanding how variations in morphological characteristics of apex predators can influence prey vulnerability and ecosystem structure.     

Interactions between predator- and diet-induced phenotypic changes in body shape of crucian carp

Predator cues and diet, when studied separately, have been shown to affect body shape of organisms. Previous studies show that the morphological responses to predator absence/presence and diet may be similar, and hence could confound the interpretation of the causes of morphological differences found between groups of individuals. In this study, we simultaneously examined the effect of these two factors on body shape and performance in crucian carp in a laboratory experiment. Crucian carp (Carassius carassius) developed a shallow body shape when feeding on zooplankton prey and a deep body shape when feeding on benthic chironomids. In addition, the presence of chemical cues from a pike predator affected body shape, where a shallow body shape was developed in the absence of pike and a deep body shape was developed in the presence of pike. Foraging activity was low in the presence of pike cues and when chironomids were given as prey. Our results thereby suggest that the change in body shape could be indirectly mediated through differences in foraging activity. Finally, the induced body shape changes affected the foraging efficiency, where crucians raised on a zooplankton diet or in the absence of pike cues had a higher foraging success on zooplankton compared to crucian raised on a chironomid diet or in the presence of pike. These results suggest that body changes in response to predators can be associated with a cost, in terms of competition for resources.     

Benefits of a predtor-induced morphology in crucian carp

Crucian carp (Carassius carassius) develop a deeper body in response to chemical cues from piscivores. This change in body morphology has been suggested to be a predator-induced defence. Here we investigate the possible benefits of the induced body morphology in laboratory experiments. Pike foraging behaviour when feeding on crucian carp of different body depths was recorded using video. Further, in a preference experiment pike were allowed to choose between shallow-bodied and deep-bodied crucian carp of similar lengths. Crucian carp body morphology did not affect predatory behaviours (activity, searching, following, observing, capture success) in northern pike, but an increase in crucian carp body depth led to an increase in handling time in pike. In the preference experiment, pike preferred shallow-bodied crucian carp over deep-bodied. Thus, a change in body morphology, induced by the presence of piscivores, benefits crucian carp by increasing piscivore handling times and an avoidance of the deep-bodied phenotype.     

Effects of northern pike on patterns of nest use and resproductive behavior of male fathead minnows in a boreal lake

We conducted a two-part study to assets predator avoidance byreproductive male fathead minnows (Pimephales promelos) subjectedto predation threat from northern pike (Esox lucius). First,we determined if patterns of nest use by egg-guarding male minnowsin a boreal lake were related to pike densities. We samplednorthern pike and identified four areas of "high pike-density"and three areas of "low pike-density." We censused natural nestsand placed nest boards in these areas. We found eggs on naturalnests more frequently in areas with low densities of pike thanin areas with high densities of pike. However, we could notfully explain the distribution of nests by predation risk. Second,we evaluated the behavioral response of egg-guarding males toa control stimulus (a piece of wood) or a live pike in a wirecage. We used time to return to the nest after a stimulus asa measure of risk taking. Males took different amounts of riskbased on predation threat; males in the predator treatment tooklonger to return to their nests than control males. Risk takingwas not related to the number or age of the eggs but to distanceto nearest egg-guarding neighbor; males with close neighborsreturned sooner than more isolated males. Males in the predatortreatment had lower total activity and egg rubbing than controlmales after they returned to their nests. We conclude that malefathead minnows altered their reproductive behavior in waysthat reduced predation risk, but the cost of predator avoidancemight include egg predation, lost mating opportunities, or usurpationof nests     

Subtidal food thieves: interactions of four invertebrate kleptoparasites with the sea star Leptasterias polaris

We used time-lapse underwater video systems in the subtidal zone of the Mingan Islands, eastern Canada, to examine behavioural interactions of four kleptoparasites with the sea star Leptasterias polaris when it was feeding on infaunal (buried) clams. Departures of L. polaris from its prey coincided with interactions with kleptoparasites in 10 out of 10 filmed feeding bouts on the large clam Spisula polynyma, compared with only four out of 10 of filmed bouts on the smaller clam Mya truncata. The sea star's abandoning of the prey was most often caused by interactions with another sea star, Asterias vulgaris, a potential predator. The whelk (a carnivorous snail), Buccinum undatum, and the crabs Hyas araneus and Cancer irroratus, also kleptoparasitized L. polaris, especially when A. vulgaris was present. Comparisons of feeding bouts in the presence and absence of kleptoparasites showed that at least 10.4% of the prey mass captured by L. polaris was lost to kleptoparasites. Simultaneous current meter data showed that the movement of A. vulgaris and B. undatum to sites where L. polaris was feeding on S. polynyma was upstream, suggesting chemodetection of food odours. Crabs in the vicinity may also use visual cues. Leptasterias polaris is the only carnivore that can efficiently extract large clams from the sediments and therefore its foraging may supply a substantial part of the diet of kleptoparasites. Copyright 2000 The Association for the Study of Animal Behaviour.