Evasive Prey Research Articles

Evasive mimicry: when (if ever) could mimicry based on difficulty of capture evolve?

We elucidate the conditions under which an easy-to-catch edible prey species may evolve to resemble another edible species that is much more difficult to capture ('evasive Batesian mimicry'), and the conditions under which two or more edible but hard-to-catch species evolve a common resemblance ('evasive Mullerian mimicry'). Using two complementary mathematical models, we argue that both phenomena are logically possible but that several factors will limit the prevalence of these forms of mimicry in nature. Evasive Batesian mimicry is most likely to arise when it is costly in time or energy for the predator species to pursue evasive prey, when mimics are encountered less frequently than evasive models and where there are abundant alternative prey. Evasive Mullerian mimicry, by contrast, is most likely to arise when evasive prey species differ in abundance, predators are slow to learn to avoid evasive prey and evading capture is costly to the prey. Unequivocal evidence for evasive Batesian or Mullerian mimicry has not yet been demonstrated in the field, and we argue that more empirical work is needed to test whether putative examples are indeed a result of selection to signal difficulty of capture.

Size‐dependent, Spatial, and Temporal Variability of Juvenile Walleye Pollock (Theragra chalcogramma) Feeding at a Structural Front in the Southeast Bering Sea

Abstract. The waters surrounding the Pribilof Islands are an important nursery ground for juvenile walleye pollock (Theragra chalcogramma), an important forage fish in the pelagic food web of the productive Bering Sea shelf region. The diet of juvenile pollock was studied in two consecutive years along a transect line crossing from a well‐mixed coastal domain, through a frontal region to stratified water farther offshore. Variability in stomach fullness was high and evidence for increased feeding intensity in the front was weak. Prey diversity and prey size generally increased with increasing fish size, shifting from predominantly small copepods to larger, more evasive prey items such as euphausiids, crab megalopae and fish. The diet of the fish reflected changes in the relative abundance of copepods and euphausiids in the prey fields between years. Juvenile pollock showed increased feeding rates at dusk, and stomach fullness as well as prey condition were generally lowest just before sunrise; however, the proportion of euphausiids increased in the diet of pollock caught at night, suggesting that some food was also ingested during darkness. Juvenile pollock and their euphausiid prey both vertically migrated above the thermocline at night, although each had a different daytime depth.

Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance

It has been shown that freshwater pelagic mysid shrimps capture zooplankton at a higher rate in light than in darkness. This has been suggested to be due to facilitation of visual predation on evasive zooplankton prey. To test this hypothesis with Baltic mysid shrimps, and to see whether pelagic (migrating) and littoral (non-migrating) mysids differ in this respect, we compared the feeding rates of Mysis mixta and Praunus flexuosus on the copepod Acartia spp. As light conditions change radically from the beginning of summer to late autumn at the Baltic latitudes, we conducted the experiments during three different times of the year to determine if there was a seasonal response to light in mysids. The feeding rates of pelagic mysids were significantly higher in total darkness than in light. In contrast, the feeding rates of littoral mysids did not differ in the dark and the light in the three different seasons. The decreased feeding of pelagic mysids under well-lit conditions may be an adaptation to avoid visual predation by pelagic fish. In contrast, littoral mysids, which live in the well-lit layer throughout the day, do not show a similar response. The fact that light did not increase feeding in either of the mysid species indicates that these mysid species do not use vision in capturing prey, but rely mainly on mechano-reception.

Diet of the zooplanktivorous icefish Neosalanx pseudotaihuensis Zhang

The icefish Neosalanx pseudotaihuensis (Salangidae) is a small transparent fish with a life cycle of about one year. This fish species is increasingly introduced to waterbodies in China in order to enhance fish production. Diet of the icefish was investigated from the 1996 cohort (June 1996–April 1997) in Xujiahe Reservoir (central China). The gut content analysis of 1001 specimens (size range: 21–72 mm in total length) revealed that the major components of the icefish diet were Leptodora kindti, Daphnia galeata, Diaphanosoma dubia, calanoids and cyclopoids. L. kindti and D. dubia were strongly selected for by the fish. However, the icefish did not show strong positive selection for Daphnia galeata, although it was a very important contributor to fish diet. Bosmina spp. were always negatively selected for by the icefish. The larger-sized fish showed a stronger preference for copepods, suggesting improved capture efficiency for evasive prey with increasing fish size.

Living with failure: the prey capture success of young brook charr in streams

Abstract – We examined the prey capture success of recently emerged brook charr (Salvelinus fontinalis) foraging in shallow, clear, still‐water pools along the edges of streams. Fewer than 42% of attacks ended with ingestion either because of difficulty distinguishing suitable prey from unsuitable items or because of difficulty capturing evasive prey. Probabilities of capture upon attack and ingestion upon capture depended upon where attacks were directed in the water column, the fish's level of activity at the time of attack and its fork length, and the sampling date. In general, success was higher for larger, sedentary fish attacking prey in the lower portion of the water column than for smaller, active fish attacking prey at the water surface. The frequency of items attacked was only a moderate predictor of the frequency of prey ingested. Poor capture success is an important aspect of the early life history of brook charr in particular and probably of young salmonines in general.NOTE

Differences in predation among morphotypes of the rotifer Asplanchna silvestrii

Summary1. Interactions were observed between three morphotypes of the predatory rotifer Asplanchna silvestrii and six different prey (Brachionus plicatilis, B. rotundiformis, B. pterodinoides, B. satanicus, Hexarthra jenkinae and copepod nauplii) to understand the differences in feeding abilities among morphotypes that may have led to the evolution of this predator polymorphism. The outcome of predation events was affected significantly, both by predator morphotype and prey type. Predator morphotypes also interacted differently with different prey types.2. The two smaller morphotypes, the saccate and the cruciform, responded similarly to prey overall, except that the smallest morphotype (saccate) was unable to ingest the most mobile prey (nauplii) and less able to ingest relatively large prey (B. plicatilis). The largest morphotype, the campanulate, had the highest encounter rate with prey, but the lowest probability of attack after encounter, so that it consumed far fewer prey per feeding bout than did the smaller morphotypes. This may have been because campanulates prefer larger prey than used in this study.3. Highly mobile prey (H. jenkinae and copepod nauplii) were much less susceptible to predation than the less mobile Brachionus species. While evasiveness reduced attacks by saccates and cruciforms, campanulates did not have a significantly lower attack rate on H. jenkinae and copepod nauplii than on less evasive prey. Large body size moderately defended B. plicatilis against ingestion by saccates only. The short‐spined B. satanicus was the only prey that was rejected after capture, resulting in lower ingestion probabilities for B. satanicus than other brachionid prey.

Optimal foraging as the criteria of prey selection by two centrarchid fishes

The nature of prey selection by two centrarchids (white crappie and bluegill) is presented as a model incorporating optimal foraging strategies. The visual field of the foraging fish as represented by the reactive distance is analysed in detail to estimate the number of prey encounters per search bout. The predicted reactive distances are compared with experimental data. The energetic cost associated with fish foraging behaviour is calculated based on the sequence of events that takes place for each prey consumed. Comparisons of the relative abundance of prey species and size categories in the stomach to the lake environment indicated that both white crappie and bluegill (length < 100 mm) strongly select prey utilising an energy optimization strategy. In most cases, the fish exclusively selected large Daphnia ignoring evasive prey types (Cyclops, Diaptomids) and small cladocera. This selectivity is the result of fish actively avoiding prey with high evasion capabilities even though they appear to be high in energetic content and having translated this into optimal selectivity through capture success rates. The energy consideration and visual system, apart from the forager's ability to capture prey, are the major determinants of prey selectivity for large-sized bluegill and white crappie still at planktivorous stages.

Visual fields in the Black‐crowned Night Heron Nycticorax nycticorax: nocturnality does not result in owl‐like features

Compared with diurnally active species, the eyes of nocturnally active herons (Ardeidae) are relatively larger and more widely separated. These features are found in comparisons between the nocturnally foraging Black‐crowned Night Heron Nycticorax nycticorax and the diurnally active Cattle Egret Bubulcus ibis. Casual viewing of the Black‐crowned Night Heron gives the impression of a somewhat owl‐like appearance, with an apparently wide degree of binocular overlap. Visual fields and eye movements were determined in two alert, restrained Black‐crowned Night Herons with the use of an ophthalmoscopic technique. A wide degree of binocular overlap was not confirmed, and the Black‐crowned Night Heron's visual field closely resembles those of diurnally foraging herons (Western Reef Heron Egretta gularis schistacea, Squacco Heron Ardeola ralloides, Cattle Egret). The Black‐crowned Night Heron's binocular field is vertically long and narrow (maximum width 22d̀), with the bill placed approximately at the centre. Monocular and cyclopean field widths in the horizontal plane equal 171d̀ and 320d̀, respectively. Retinal binocular overlap can be abolished by eye movements. There is a blind sector (10–13d̀ wide) at the margin of each eye's optical field, and this results in the functional retinal binocular field being much narrower than the optical binocular fields. It is these blind sectors which give rise to the appearance of a much wider binocular field. The visual field characteristics of this heron species may be best understood in relation to the foraging technique of capturing agile, evasive prey directly in the bill. The comparatively large size of the Black‐crowned Night Herons' eyes may be associated with activity over a wide range of natural light levels but does not give rise to binocular fields larger than those of diurnal heron species.

Echolocation Calls of the Long-Tailed Bat: A Quantitative Analysis of Types of Calls

The echolocation calls of long-tailed bats (Chalinolobus tuberculatus) were recorded in the Eglinton Valley, Fiordland, New Zealand, and digitized for analysis with signal-processing software. Univariate and multivariate analyses of measured features facilitated a quantitative classification of the calls. Cluster analysis was used to categorize calls into two groups equating to search and terminal-buzz calls described qualitatively for other species. When moving from search to terminal phases, the calls decrease in bandwidth, maximum and minimum frequency of call, and duration. Search calls begin with a steep-downward FM sweep followed by a short, less-modulated component. Buzz calls are FM sweeps. Although not found quantitatively, a broadband pre-buzz group of calls also was identified. Ambiguity analysis of calls from the three groups shows that search-phase calls are well suited to resolving the velocity of targets, and hence, identifying moving targets in stationary clutter. Pre-buzz and buzz calls are better suited to resolving range, a feature that may aid the bats in capture of evasive prey after it has been identified.

Transmission of force and velocity in the feeding mechanisms of labrid fishes (Teleostei, Perciformes)

The feeding mechanisms of four species of the teleostean family Labridae (Cheilinus fasciatus, C. trilobatus, Oxycheilinus bimaculatus, and O. unifasciatus) were modeled using four-bar linkage theory from mechanical engineering. The predictions of four-bar linkage models regarding the kinematics of feeding were compared to the movements observed with high speed cinematography (200 frames/s). A four-bar linkage was an accurate model of the mechanism by which upper jaw protrusion, maxillary rotation, and gape increase occur in each species. A four-bar mechanism of hyoid depression was an accurate predictor of hyoid depression when simultaneous cranial elevation and sternohyoideus contraction were simulated. Morphometrics of the linkage systems of the jaws and hyoid were collected for 12 labrid species. These data were used to calculate the transmission of force and motion through the musculoskeletal linkages. Several measures of mechanical advantage and displacement advantage were compared, including both traditional lever ratios and transmission coefficients of four-bar linkages. Alternative designs of the feeding mechanisms maximize force or velocity for the capture of different prey types. High velocity transmission of both the jaw and hyoid systems is characteristic of those species that feed on evasive prey, whereas species that feed on benthic invertebrates favor increased force transmission in both systems. Quantitative models of biomechanical systems supply criteria for functionally relevant morphometrics, and aid in calculating the capacity for transmission of force and velocity in musculoskeletal systems.

Foraging tactic as a potential selection pressure influencing geographic differences in body shape among populations of dace (Phoxinus eos)

Geographic differences in body shape among 18 populations of northern redbelly dace in three regions of Ontario (Sudbury, Algonquin Provincial Park, and Kingston) were quantified with truss analysis. Principal component analysis of size-adjusted residuals of the trusses revealed that 35.11% of the morphological variation in body shape is explained on principal component I. This axis describes the major trend in body shape variation, ranging from populations in which fish are relatively deep-bodied to those in which fish are more shallow-bodied. Shape variation among populations in Algonquin Park spanned the range observed among all three regions in Ontario. Experiments were thus conducted on two Algonquin populations differing considerably in body shape, and it was found that deeper bodied fish from one population were significantly more adept at capturing evasive prey than were relatively shallow-bodied fish from another population. This finding is not only consistent with functional analysis of fish shape but also fits predictions from foraging theory. Fish catching plentiful but evasive prey should rely on ambush predation, and thus have a large body depth, which increases body area relative to mass, in order to optimize acceleration. Conversely, fish catching less evasive food occurring at low density should use a tactic of wide foraging, for which they need a streamlined anterior body suitable for cruising over extended periods of time. Samples from the natural population revealed that deep-bodied fish had a significantly greater number of evasive prey in their guts, and that the more shallow-bodied fish had relatively few. Furthermore, the pond in which the deeper bodied fish occurred had significantly higher prey density than the pond inhabited by the shallow-bodied fish. Because the Algonquin populations experience sufficient gene flow to prevent differentiation due to drift alone, the most likely explanation for population differentiation resides in selection on body shape for foraging tactic.

Zooplankton capture by a coral reef fish: an adaptive response to evasive prey

High-speed cinematography and video using modified Schlieren optics and laser illumination helped elicit details of prey capture mechanisms used by Chromis viridis while feeding on calanoid copepods and Artemia. Chromis viridis is capable of a ram-jaw, low-suction feeding, as well as a typical suction feeding behavior described for other species of planktivores. By adjusting the degree of jaw protrusion and amount of suction used during a feeding strike, this fish can modulate its feeding strikes according to the prey type being encountered. The ram-jaw feeding mode enables C. viridis to capture highly evasive calanoid copepods within 6 to 10 msec. The use of specialized feeding behavior for evasive prey and the ability to vary feeding behavior are adaptations for feeding on evasive prey.

Feeding performance in aquatic postmetamorphic newts (Urodela: Salamandridae): Are bidirectional flow systems necessarily inefficient?

There are two basic designs of suction-feeding mechanisms in aquatic salamanders. Premetamorphic salamanders use a unidirectional flow system in which water is drawn into the mouth anteriorly and expelled posteriorly through the gill slits. Postmetamorphic salamanders use a bidirectional flow system in which water enters and then exits through the mouth. It has been suggested that bidirectional flow results in an inefficient feeding mechanism. To test this hypothesis, we compared the structure of the feeding system and the proficiency of aquatic prey capture among postmetamorphic individuals from (i) predominantly terrestrial (Tylototriton verrucosus), (ii) amphibious (Pleurodeles waltl, Cynops pyrrhogaster, Notophthalmus viridescens, Taricha granulosa, and Paramesotriton hongkongensis), and (iii) aquatic (Pachytriton brevipes) species of newts (family Salamandridae). Ossification of the hyolingual skeleton, size of the labial lobes, and taper of the snout increase with increasing preference for an aquatic habitat. All species studied except T. verrucosus captured prey in water. Feeding efficiency was similar among all the amphibious species; each was highly successful at capturing immobile prey (90–98%), successful at capturing mobile prey (40–58%), and less successful at capturing evasive prey (18–23%). Fully aquatic P. brevipes was highly successful in taking immobile (100%) and mobile (90%) prey and successful at capturing evasive prey (65%). We attribute the superior feeding performance of this newt to several morphological specializations of its feeding system, including a strongly tapered snout and greatly enlarged labial lobes. We reject the hypothesis that bidirectional flow is always inefficient for prey capture by metamorphosed salamanders. The degree of feeding proficiency in water is influenced by the morphological and behavioral attributes of aquatic salamanders, rather than just by the presence or absence of gill slits and, hence, by unidirectional flow.

Ontogenetic Changes in Feeding Habits of Juvenile Cutthroat Trout

Abstract Laboratory experiments demonstrated an ontogenetic shift in feeding habits of juvenile cutthroat trout Salmo clarki. Fish smaller than 6.0 cm in standard length preferred planktonic prey Daphnia pulex to each of three benthic prey (species of Hyalella, Callibaetis, and Enallagma). Larger fish consumed benthic prey significantly more often. Diet of cutthroat trout reared in a field enclosure also shifted from primarily planktonic prey to more frequent use of benthic prey once fish grew to about 7.0 cm. Changes in feeding habits were not due to differences in ability to capture evasive prey or to handle large prey. Rather, the motivation of cutthroat trout to feed on daphnia seems to be different between small and large fish. Concurrent changes in morphology of the mouth suggest that onset of a new phase of physical development may be responsible for the shift in feeding habits of juvenile cutthroat trout.

Prey Selection by Freshwater Predators with Different Foraging Strategies

We observed several freshwater predators, including the odonate larvae Pachydiplax longipennis and Anax junius, the hemipterans Notonecta unifasciata and Buenoa scimitra, the dytiscid larva Acilius semisulcatus, and juvenile Gambusia affinis, feeding on a variety of microcrustacean prey and determined the frequency of the component parts of predator–prey interactions (encounter, attack, capture, ingestion). Encounter rates were the most important determinant of predator selectivity when predators were presented with a variety of microcrustacean prey. When only copepod species were used as prey, however, both encounter rates and capture success were important in determining predator diets. We used our data to test hypotheses concerning relationships between predator foraging mode and patterns of prey selection: mobile predators exhibited stronger selection for sedentary prey than did sit-and-wait predators; our own and literature data also indicated that macroinvertebrate sit-and-wait predators are better able to capture, and have higher selectivity for evasive prey than do mobile predators. A predator's attack acceleration, however, may be a better predictor of its selectivity for evasive versus nonevasive prey than its mean swimming speed.

Selective impact of filter‐feeding gizzard shad on zooplankton community structure1

Comparison of ponds containing gizzard shad with control ponds without fish showed that gizzard shad predation suppressed populations of Keratella sp., Diaphanosoma brachyurum, copepod nauplii, Chaoborus sp., and cyclopoid copepodids and adults, but enhanced populations of Diaptomus pallidus copepodids and adults. These results confirm previous laboratory studies which showed that filter‐feeding gizzard shad are not size‐selective predators, as is typically assumed of planktivorous fish, but instead feed on those animals which have little ability to escape, so that their predation results in a community shift toward the more evasive prey.

Diet choice, resource depression, and the regular nest spacing of birds of prey

The hypothesis is suggested that resource depression is one important factor behind the observed regular nest spacing of many birds of prey. It predicts that the proportion of evasive prey in the predator's diet should be reflected in the degree of regularity in its nest spacing. The idea is tested on data from 24 raptorial bird populations of 19 species. The proportion of supposed evasive prey (birds and large mammals) in the diet was positively correlated with the degree of regularity in spacing of the nests, considering all populations or species. No such significant correlation was found for the six highly territorial species, whereas the correlation was highly significant for the other 13 species. Thus, the hypothesis seems most applicable to species with widely overlapping home ranges. However, resource depression could be one reason for territoriality.

Planktivore Preference by Prey Size

The reactive field volume model of prey encounter was shown to give a close approximation to the apparent size model of prey encounter rate for prey size distributions consisting of many prey types. The two models of prey encounter give almost identical predictions for the Lake Washington zooplankton community. The prey encounter models do not account for observed patterns of prey selection by Lake Washington juvenile sockeye salmon. This suggests an active preference for large nonevasive prey. This preference is dynamic. Small as well as evasive prey are pursued and eaten at times in the year when the large nonevasive prey are rare or absent from the water column.

Variable Kinematics of Sacramento Perch (Archoplites interruptus) Capturing Evasive and Nonevasive Prey

Kinematic analysis of the prey attack behavior of Sacramento perch (Archoplites interruptus) for familiar nonevasive prey (Daphnia magna), and for novel evasive prey (Diaphanosoma brachyurum) was undertaken using video taping. The fish use two distinct attack modes. The attack on nonevasive prey is characterized by leisurely approach and relatively conservative attack motions. Attacks on evasive prey are very vigorous and energetically more demanding. Vigorous attacks on the evasive prey which were successful 55% of the time required approximately four times as much energy per attack as did the always successful attacks on the nonevasive prey. Differences between the evasive and nonevasive prey were rapidly learned as shown by the development of the vigorous attack in naive fish, which generally occurred in 10–20 encounters.Key words: predation, behavior, learning, evasion, kinematics, Archoplites interruptus

Differential Prey Vulnerability and Predator Selectivity: Effects of Evasive Prey on Bluegill (Lepomis macrochirus) and Pumpkinseed (L. gibhosus) Predation

Sunfish will select Daphnia preferentially over copepods even though the apparent size of the copepods may be considerably larger. This selectivity is probably the result of the fishes having learned differences in the evasion capabilities of the two different prey types, and having translated this into a differential selectivity. Sunfish have the ability to correlate information about the evasion capabilities of prey with visual appearances of the prey, and to select less evasive prey. Such information may have considerable significance in defining the observed selectivity of these fish in nature, and such behavioral responses should be considered in future modeling efforts.Key words: predation, Lepomis, selectivity, behavior