The development of schooling behaviour in Atlantic herring Clupea harengus

The timing of the development of schooling behaviour in Atlantic herring Clupea harengus and the quantitative and qualitative evolution of such behaviour in the larval and post-metamorphic stages were investigated with a simulation model and experimental observations of laboratory-reared fish. Herring larvae started to develop schooling behaviour between the lengths of 35 and 40 mm. This coincided with the beginning of metamorphosis, characterized by ontogenetic changes which require a switch to a different antipredator strategy from that employed by early larvae, while providing the sensory and anatomical mechanisms necessary for the formation of schools. Schooling behaviour was established at the end of metamorphosis (50–55 mm) and its characteristics continued to evolve throughout the early juvenile stage. A critical period in terms of vulnerability to predation is expected between the end of the larval and beginning of the juvenile stages, before schooling behaviour is fully developed. The implications of some of the results for the methodology of future studies are also discussed.     

Predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the cyclopoid copepod Cyclops vicinus: a laboratory investigation with mixed and single prey

We tested the outcome of predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the similar sized cyclopoid copepod Cyclops vicinus in laboratory experiments with mixed and single-prey. When ovigerous females and adult non-ovigerous females of the calanoid copepod and the cyclopoid copepod were offered in equal numbers to the fish in the mixed-prey experiments, the cyclopoid copepods were significantly more heavily preyed upon than the calanoids. Between 14 and 16 females of the cyclopoid copepod and only between 2 and 4 calanoids had been consumed after the observation period of two hours. The single-prey experiments revealed that the conspicuousness of the calanoid vs the cyclopoid depended on sex and gravidity. Ovigerous females of the cyclopoid copepod were slightly earlier detected by the fish than ovigerous females of the calanoid, probably a result of the highly visible egg-sacs carried by the cyclopoid females. Females without eggs and males of the cyclopoid copepod were recognized later by the predator than females without eggs or males of the calanoid, probably a result of the different behaviours of the different copepod taxa. Cyclopoids frequently congregated near the aquarium bottom while calanoids were closer to the water surface where they were better visible. The calanoid copepod could better escape the fish's attacks than the cyclopoid copepod. Egg-bearing females of both the calanoid and the cyclopoid copepod could significantly better escape than non-ovigerous females or males. Probably ovigerous females react very early to water disturbances caused by the predator. Activity measurements showed that the cyclopoid copepod displayed 2 to 4 times more hops per time unit than the calanoid. Probably the high number of jerky movements displayed by the cyclopoid attracted attention of the predator and contributed to its greater vulnerability.     

Susceptibility of Atlantic herring and plaice larvae to predation by juvenile cod and herring at two constant temperatures

Predation encounters were staged in the laboratory to compare prey responsiveness, predator error rate, and predator capture success for juvenile cod Gadus morhua (a suction feeder) and herring Clupea harengus (a biting predator) preying on herring and plaice Pleuronectes platessa larvae. Trials were conducted at near natural temperature extremes for the larvae (8 and 13°C) to assess the importance of water temperature to the interaction. Herring larvae were significantly more responsive to attacks by herring than were plaice larvae (5·7 vs 0'0%). The two prey species were equally responsive to attacks by cod (2–6 vs 10%). Cod caught 91% of herring larvae attacked and juvenile herring caught 87%. Cod were successful in 96% of attacks on plaice, but juvenile herring caught significantly fewer (83%) plaice larvae. For each predator species, capture success did not vary significantly with prey species. Overall capture success for herring was significantly lower than that for cod. Responsiveness of herring larvae to attacks by juvenile herring increased with temperature, but predator error rate and capture success were not altered by water temperature.     

Differential predation on colour morphs of the Midas cichlid, Cichlasoma citrinellum

Predation has often been invoked as a selective agent affecting colour patterns in a wide range of organisms, but data relating susceptibility to predation and objective measures of conspicuousness are rare. To investigate relative crypticity in free-swimming fishes, two colour morphs of the Midas cichlid were exposed to predation by largemouth bass, Micropterus salmoides. The two colour morphs, normal and amelanic golds, were viewed by predators in ponds against a uniform, natural background. Swimming pools, 4 m in diameter, were stocked with equal numbers of gold and normal juvenile Midas cichlids. Bass took a significantly higher proportion of normal-coloured fishes (69·2% of fish) than gold morphs overall, and in a significantly higher proportion of trials (68·8% of experiments). No differences were found between colour morphs in their behaviour or in the willingness of bass to attack either morph. The significance of these results are discussed in relation to the visual system of the predator and the relative conspicuousness of the prey colour patterns.     

Opposite latitudinal patterns for bird and arthropod predation revealed in experiments with differently colored artificial prey

The strength of biotic interactions is generally thought to increase toward the equator, but support for this hypothesis is contradictory. We explored whether predator attacks on artificial prey of eight different colors vary among climates and whether this variation affects the detection of latitudinal patterns in predation. Bird attack rates negatively correlated with model luminance in cold and temperate environments, but not in tropical environments. Bird predation on black and on white (extremes in luminance) models demonstrated different latitudinal patterns, presumably due to differences in prey conspicuousness between habitats with different light regimes. When attacks on models of all colors were combined, arthropod predation decreased, whereas bird predation increased with increasing latitude. We conclude that selection for prey coloration may vary geographically and according to predator identity, and that the importance of different predators may show contrasting patterns, thus weakening the overall latitudinal trend in top‐down control of herbivorous insects.     

Loss of body mass under predation risk: cost of antipredatory behaviour or adaptive fit-for-escape?

Predation risk may compromise the ability of animals to acquire and maintain body reserves by hindering foraging efficiency and increasing physiological stress. Locomotor performance may depend on body mass, so losing mass under predation risk could be an adaptive response of prey to improve escape ability. We studied individual variation in antipredatory behaviour, feeding rate, body mass and escape performance in the lacertid lizard Psammodromus algirus. Individuals were experimentally exposed to different levels of food availability (limited or abundant) and predation risk, represented by reduced refuge availability and simulated predator attacks. Predation risk induced lizards to reduce conspicuousness behaviourally and to avoid feeding in the presence of predators. If food was abundant, alarmed lizards reduced feeding rate, losing mass. Lizards supplied with limited food fed at near-maximum rates independently of predation risk but lost more mass when alarmed; thus, mass losses experienced under predation risk were higher than those expected from feeding interruption alone. Although body mass of lizards varied between treatments, no component of escape performance measured during predator attacks (endurance, speed, escape strategy) was affected by treatments or by variations in body mass. Thus, the body mass changes were consistent with a trade-off between gaining resources and avoiding predators, mediated by hampered foraging efficiency and physiological stress. However, improved escape efficiency is not required to explain mass reduction upon predator encounters beyond that expected from feeding interruption or predation-related stress. Therefore, the idea that animals may regulate body reserves in relation to performance demands should be reconsidered.     

The interplay between multiple predators and prey colour divergence

Evolutionary divergence in the coloration of toxic prey is expected when geographic variation in predator composition and behavior favours shifts in prey conspicuousness. A fundamental prediction of predator‐driven colour divergence is that the local coloration should experience lower predation risk than novel prey phenotypes. The dorsal coloration of the granular poison frog varies gradually from populations of conspicuous bright red frogs to populations of dull green and relatively cryptic frogs. We conducted experiments with clay models in four populations to examine the geographic patterns of taxon‐specific predation. Birds avoided the local phenotype while lizards consistently selected for decreased conspicuousness and crab predation did not depend on frog coloration. Importantly, birds and lizards favoured low conspicuousness in populations where relatively cryptic green morphs have evolved. This study provides evidence for the interplay among distinct selective pressures, from multiple‐predator taxa, acting on the divergence in protective coloration of prey species. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 580–589.     

The influence of hunger and predation risk on group cohesion in a pelagic fish, walleye pollock Theragra chalcogramma

Variation in the intensity of schooling behavior in fishes suggests that the benefits of aggregation are balanced by certain costs. We examined the proximity of group members to each other in juvenile walleye pollock, Theragra chalcogramma, under different environmental conditions. Food availability, simulated by a gradient of six ration treatments, had a major influence on group cohesion, with increasing dispersion as food level decreased. Group cohesion also decreased at night relative to daytime levels. Small juveniles (x=53 mm TL) maintained on high rations were highly responsive to the potential threat of a predator, with groups becoming more cohesive and remaining so for up to an hour after the initial threat. A chronic threat (continual presence of predators) resulted in tighter group cohesion than an acute threat (single simulated attack). Small juveniles maintained on low rations were less responsive to predation threats and recovered quickly, supporting the hypothesis that hunger induces risk-taking behavior. Large juveniles (x=149 mm TL) did not change their degree of aggregation in response to either type of predation threat. An overall plasticity in the degree of cohesiveness among group members indicates that walleye pollock are capable of gradually modifying their schooling behavior according to the environmental context.     

Levels of diurnal predation on a school of flat-iron herring,Harengula thrissina

Synopsis The incidence of diurnal predation (0800–1600h) on schools of several hundred thousand adult flat-iron herring,Harengula thrissina, was quantified. Nine species of piscine predators perpetrated 653 attacks and 158 captures during 42.5 h of observation. Four predators: cabrilla,Mycteroperca rosacea, cornetfish,Fistularia commersonii, green jack,Caranx caballus, and black skipjack,Euthynnus lineatus, were responsible for 92% of the attacks and 96% of the successes. Among these predators attack rates (attacks × time^–1) and success rates (successes × attacks^–1) varied with respect to time of day, although all predators were active throughout the day. As a consequence, the herring school experienced fairly homogeneous attack and success rates. These rates were estimated as roughly half the crepuscular attack rates but they were sustained over a much longer period. Piscivorous predators, particularly those species frequently found in association with schooling prey, may be focusing their attacks on opportunities, not specific times of day and/or light levels. More direct information needs to be gathered on diurnal as well as crepuscular attack and success rates in order to assess the impact that piscine predators have on schooling prey.     

Ontogeny of swimming speed, schooling behaviour and jellyfish avoidance by Japanese anchovy Engraulis japonicus

The ontogeny of swimming speed, schooling behaviour and jellyfish avoidance was studied in hatchery-reared Japanese anchovy Engraulis japonicus to compare its life-history strategy with two other common pelagic fishes, jack mackerel Trachurus japonicus and chub mackerel Scomber japonicus. Cruise swimming speed of E. japonicus increased allometrically from 1·4 to 3·9 standard length (L(S) ) per s (L(S) s(-1) ) from early larval to metamorphosing stage. Burst swimming speed also increased from 6·1 to 28 L(S) s(-1) in these stages. Cruise speed was inferior to that of S. japonicus, as was burst speed to that of T. japonicus. Engraulis japonicus larvae were highly vulnerable to predation by moon jellyfish Aurelia aurita and were readily eaten until they reached 23 mm L(S) , but not at 26 mm L(S) . Schooling behaviour (indicated by parallel swimming) started at c. 17 mm L(S) . Average distance to the nearest neighbour was shorter than values reported in other pelagic fishes. The relatively low predator avoidance capability of E. japonicus may be compensated for by their transparent and thus less conspicuous body, in addition to their early maturation and high fecundity.     

Shifting the balance between foraging and predator avoidance: the importance of food distribution for a schooling pelagic forager

It is widely held that when predator avoidance conflicts with other activities, such as feeding, avoidance of predators often takes precedence. In this study, we examine how predation risk and food distribution interact to influence the schooling behavior and swimming speed of foraging juvenile walleye pollock, Theragra chalcogramma. Fish were acclimated to either spatially and temporally clumped, or spatially and temporally dispersed food for 3 weeks. Fish were then monitored while feeding in the absence and presence of predatory sablefish, Anoplopoma fimbria. Fish foraging for clumped food swam rapidly in a loose school when predators were absent, but swam more slowly and adopted more cohesive schooling in the presence of predators, trading-off foraging opportunity for decreased vulnerability to predators. Fish foraging for dispersed food swam about slowly and did not engage in cohesive schooling in either the absence or presence of predators. These fish accepted greater predation risk in order to continue foraging, suggesting that the cost of schooling, in terms of decreased foraging opportunity, was greater when food was dispersed than when it was clumped. This lower responsiveness to predators among fish receiving dispersed food demonstrates that predator avoidance does not always take precedence over other activities, but rather, that a balance is maintained between predator avoidance and feeding, which shifts as food distribution changes.     

Predation as a cost of sexual communication in nocturnal seabirds: an experimental approach using acoustic signals

Conspicuous behaviour, such as sexual advertisement, exposes animals to predation; mate attraction thus often conflicts with antipredator behaviour. We investigated whether an avian predator, the brown skua, Catharacta antarctica l?nnbergi, uses the mate attraction calls of colonial seabirds, the petrels. The majority of petrels attract mates at night and vocalizations are their main way of communicating. At our study sites, skua predation on nocturnal petrels was heavy, and concentrated particularly on a single species, the blue petrel, Halobaena caerulea. Using playback experiments, we showed that skuas can use male petrel calls as a cue for prey location and selection. This listening behaviour of skuas probably imposes a major constraint on advertising petrels, and especially on single males which face a trade-off between attracting females (which respond by calling in flight) and avoiding predation. We also investigated the consequences of this predation risk on the behaviour of petrels: a second set of playback experiments showed that the most heavily preyed on petrel species could use skua territorial calls to infer predation risk and stop calling thereafter, which may reduce conspicuousness and predation risk. Copyright 2000 The Association for the Study of Animal Behaviour.     

Influence of temperature on evasive responses of Atlantic herring larvae attacked by yearling herring, Clupea harengus L.

Predation encounters were staged in the laboratory between yearling herring, Clupea harengus L., 66 to 104 mm t.l. , and herring larvae, 8 to 30 mm t.l. ., at 8,11 and 14 o C. Video records were used to quantify prey behaviour. Prey responsiveness, reactive distance, response latency, and apparent looming threshold were not affected by temperature. Response speeds increased with temperature. Predator error rate and capture success showed no consistent thermal effects. Although the experiments could not fully evaluate the influence of temperature on the predators, results suggest that the predator's performance largely governs the outcome of an attack on a larva and that higher temperatures favour the predator by increasing the frequency of its encounter with prey.     

The response of a selfish herd to an attack from outside the group perimeter

According to the selfish herd hypothesis, animals can decrease predation risk by moving toward one another if the predator can appear anywhere and will attack the nearest target. Previous studies have shown that aggregations can form using simple movement rules designed to decrease each animal's Domain of Danger. However, if the predator attacks from outside the group's perimeter, these simple movement rules might not lead to aggregation. To test whether simple selfish movement rules would decrease predation risk for those situations when the predator attacks from outside the flock perimeter, we constructed a computer model that allowed flocks of 75 simulated fiddler crabs to react to one another, and to a predator attacking from 7 m away. We attacked simulated crab flocks with predators of different sizes and attack speeds, and computed relative predation risk after 120 time steps. Final trajectories showed flight toward the center of the flock, but curving away from the predator. Path curvature depended on the predator's size and approach speed. The average crab experienced a greater decrease in predation risk when the predator was small or slow moving. Regardless of the predator's size and speed, however, predation risk always decreased as long as crabs took their flock-mates into account. We conclude that, even when flight away from an external predator occurs, the selfish avoidance of danger can lead to aggregation.     

Ontogenetic shifts in a prey’s chemical defences influence feeding responses of a snake predator

Foraging theory suggests that predator responses to potential prey should be influenced by prey chemical defences, but the effects of ontogenetic variation in such defences on prey vulnerability to predators remain unclear. Cane toads (Rhinella marina) are toxic to anurophagous snakes, including the keelback (Tropidonophis mairii, a natricine colubrid that occurs within the toads' invasive range in Australia). Toxin levels and diversity change through toad ontogeny, decreasing from the egg stage to metamorphosis, then increasing in postmetamorphic toads. If the toxin content of a prey item influences predator responses, we predict that keelbacks should exhibit selective predation on toads close to metamorphosis. The results of our laboratory trials on adult (field-collected, and thus toad-experienced) and hatchling (laboratory-incubated, and thus toad-naive) keelbacks supported this prediction. The snakes selectively consumed later-stage rather than earlier-stage tadpoles, and earlier-stage rather than later-stage metamorphs. Our data are thus consistent with the hypothesis that ontogenetic changes in toxin content can affect individuals' vulnerability to predation.     

Inferring Predator Behavior from Attack Rates on Prey-Replicas That Differ in Conspicuousness

Behavioral ecologists and evolutionary biologists have long studied how predators respond to prey items novel in color and pattern. Because a predatory response is influenced by both the predator’s ability to detect the prey and a post-detection behavioral response, variation among prey types in conspicuousness may confound inference about post-prey-detection predator behavior. That is, a relatively high attack rate on a given prey type may result primarily from enhanced conspicuousness and not predators’ direct preference for that prey. Few studies, however, account for such variation in conspicuousness. In a field experiment, we measured predation rates on clay replicas of two aposematic forms of the poison dart frog Dendrobates pumilio, one novel and one familiar, and two cryptic controls. To ask whether predators prefer or avoid a novel aposematic prey form independently of conspicuousness differences among replicas, we first modeled the visual system of a typical avian predator. Then, we used this model to estimate replica contrast against a leaf litter background to test whether variation in contrast alone could explain variation in predator attack rate. We found that absolute predation rates did not differ among color forms. Predation rates relative to conspicuousness did, however, deviate significantly from expectation, suggesting that predators do make post-detection decisions to avoid or attack a given prey type. The direction of this deviation from expectation, though, depended on assumptions we made about how avian predators discriminate objects from the visual background. Our results show that it is important to account for prey conspicuousness when investigating predator behavior and also that existing models of predator visual systems need to be refined.     

Evaluation of in situ enclosures during a study of the importance of starvation to the vulnerability of herring larvae to a piscine predator

Two competing hypotheses about the effect of starvation on the vulnerability of fish larvae to predators were tested in large enclosures (mesocosms). One hypothesis predicts greater vulnerability as starvation weakens larvae. The alternative recognizes that the behaviour of starving larvae may reduce encounters with predators or fail to elicit predatory attacks, thereby decreasing vulnerability. Results of four large-scale experiments, in which progressively starved Atlantic herring Clupea harengus L. larvae were exposed to larger herring as predators, were unable to support either hypothesis because of a large amount of variability. Much of the variability was thought to be due to erratic feeding behaviour of the predators. Additional experimental observations suggest that sunshine and predator satiation can affect these experiments.     

Consequences of temperature and temperature variability on swimming activity,group structure,and predation of endangered delta smelt

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Behavioral plasticity mitigates risk across environments and predators during anuran metamorphosis

Most animals metamorphose, changing morphology, physiology, behavior and ecological interactions. Size- and habitat-dependent mortality risk is thought to affect the evolution and plastic expression of metamorphic timing, and high predation during the morphological transition is posited as a critical selective force shaping complex life cycles. Nonetheless, empirical data on how risk changes across metamorphosis and stage-specific habitats, or how that varies with size, are rare. We examined predator–prey interactions of red-eyed treefrogs, Agalychnis callidryas, with an aquatic predator (giant water bug, Belostoma) and a semi-terrestrial predator (fishing spider, Thaumasia) across metamorphosis. We manipulated tadpole density to generate variation in metamorph size and conducted predation trials at multiple developmental stages. We quantified how frog behavior (activity) changes across metamorphic development, habitats, and predator presence or absence. In aquatic trials with water bugs, frog mortality increased with forelimb emergence, as hypothesized. In semi-terrestrial trials, contrary to predictions, predation by spiders increased, not decreased, with tail resorption. In neither case did frog size affect mortality. Frogs reduced activity upon forelimb emergence in the water, and further with emergence into air, then increased activity with tail resorption. Longer-tailed metamorphs were captured more often in spider attacks, but attacked less, as most attacks followed prey movements. Metamorphs behaviorally compensated for poor escape performance more effectively on land than in water, thus emergence timing may critically affect mortality. The developmental timing of the ecological transition between environments that select for different larval and juvenile phenotypes is an important, neglected variable in studies of complex life cycles.