Function Of Prey Size Research Articles

Evaluating the Use of Chemical Weapons for Capturing Prey by a Venomous Mammal, the Greater Slow Loris (Nycticebus coucang).

Few mammals are venomous, including one group of primates-slow (Nycticebus spp.) and pygmy (Xanthonycticebus spp.) lorises. Hypotheses for the evolutionary function of venom in these primates include defense from predators or ectoparasites, communication or competition with conspecifics, and the capture of prey. We tested the prey capture hypothesis in 75 trials with 22 wild-caught greater slow lorises (N. coucang) housed in a rescue center in Java, Indonesia. We experimentally offered the slow lorises arthropod prey items varying in size, escape potential, and toxicity and recorded venom-related and predatory behaviors using live and video observations. The slow lorises visually targeted arthropod prey, approached it quickly and efficiently, and captured it with a manual grasping motion. They rarely performed venom-related behaviors and seemed to do so in a defensive context. The slow lorises exhibited little variation in pre-capture behavior as a function of prey size or escape potential. In response to noxious prey, the slow lorises performed tongue-flicking and other investigative behaviors that indicate they are using chemosensory input to assess prey characteristics. These data suggest it is unlikely that slow lorises use chemical weapons to subdue arthropod prey and may support, instead, a defensive function for slow loris venom.

Observations on the predation of the snail Huttonella bicolor on the snail Allopeas gracile

The predatory snail Huttonella bicolor (Hutton 1834) (Gastropda: Streptaxidae) was encountered along with Allopeas gracile (Hutton 1834) (Gastropoda: Subulinidae) during a survey of small land snail species from several terrestrial habitats in Kolkata, India. An evaluation of the predation of H. bicolor as a function of prey size and predator density was carried out using A. gracile as a model prey snail. The predatory interactions were noted with an increasing ratio of 1, 2 and 4 H. bicolor against 10 A. gracile of varied size classes in a defined terrarium. At the end of a 48 h period of exposure, H. bicolor was observed to consume on an average 5.32 ± 0.50 snails depending on the size class and the predator density. The predation pattern varied significantly with the prey size class, as revealed through the logistic equation, y (prey-consumed) = 1 / (1 + exp (-(0.97–0.71*size class-prey))). In a separate experiment, it was observed that the presence of H. bicolor induced a reduction in the fecundity in A. gracile, as revealed through the logistic regression, y (egg laid) = 1 / (1 + exp(-(3.45–0.67*predator-density))). The direct effect of predation and indirect effect of oviposition reduction reflect the efficacy of H. bicolor on population regulation of A. gracile. In view of conservation biological control, the use of the snail H. bicolor as a biocontrol agent may prove beneficial in situations where A. gracile is a pest.

The influence of prey size, sediment thickness and fish size on consumption in common sole (Solea solea L.)

Summary This study determined prey consumption in common sole as a function of prey size (0–0.5, 1–1.5, 2–2.5 and 4–5 g), sediment thickness (20 cm and 2 cm) and fish size (50 g, 125 g or 300 g). Prey consumption (in numbers of prey eaten per fish per day) was reduced with increasing prey size and sediment thickness, and was increased with increasing fish size (p < .001 for all factors). All 3 factors showed significant two way interactions (p < .001) when expressed in numbers of prey eaten. Prey consumption decreased with prey size when prey could not escape by burying (2 cm of sediment thickness) irrespective of fish size. We suggest that increasing effort to ingest and handle larger prey played a role. Prey consumption increased with fish size when prey could not bury (2 cm of sediment thickness). However, when prey was able to bury (at 20 cm sediment thickness) prey consumption was similar irrespective of fish size (p < .001 for interaction fish size × sediment). This interaction suggests that with increasing fish size there is an increasing mismatch between foraging adaptation and prey burial depth. This may explain the dominance of crustaceans in the diet of adult common sole in nature, despite the high abundance of polychaetes.

Impact of zooplankton food selectivity on plankton dynamics and nutrient cycling

Within models, zooplankton grazing is typically defined as being dependent on total prey concentration, with feeding selectivity expressed only as a function of prey size. This behavior ignores taxonomic preferences shown by the predators and the capacity of some zooplankton to actively select or reject individual prey items from mixtures. We carried out two model experiments comparing impacts of zooplankton displaying passive and active selection, which resulted in contrasting dynamics for the pelagic system. Passive selection by the grazer resulted in a top down control on the prey with a fast turn-over of nutrients. Active selection, on the other hand led to a bottom-up control, with slower nutrient turnover constraining primary production by changing the system toward export of particulate matter. Our results suggest that selective feeding behavior is an important trait, and should be considered alongside size and taxonomy when studying the role of zooplankton impact in the ecosystem.

Diet of two insectivorous bats, Myotis lucifugus and Myotis keenii, in relation to arthropod abundance in a temperate Pacific Northwest rainforest environment

We assessed the diet of two morphologically similar bats ( Myotis lucifugus (LeConte, 1831) and Myotis keenii (Merriam, 1895)), which both used hydrothermally heated nursery roosts at Gandll K’in Gwaayaay (Hotspring Island), Haida Gwaii (Queen Charlotte Islands), British Columbia, in 1998 and 1999. Our purpose was to determine if they fed opportunistically or actively selected prey, and whether they partitioned prey resources. We determined diet by analyzing feces collected from captured bats and compared it with the relative abundance of insects captured in light traps. Myotis lucifugus fed mainly on lepidopterans, medium-sized to large dipterans, neuropterans, and hymenopterans, while M. keenii fed on lepidopterans, arachnids, medium-sized to large dipterans, and neuropterans. We found that both species were selecting prey, although selection may have been more a function of prey size than particular taxa. Arachnids occurred in feces of both species, implying that both were capable of gleaning prey from surfaces, although only M. keenii regularly fed on spiders. We concluded from the preponderance of flying insects in the diet of M. lucifugus that it was primarily aerial hawking prey, while we took the frequent occurrence of both flying insects and spiders in the diet of M. keenii to indicate that it was both aerial hawking and gleaning prey. Weather conditions between years affected relative abundance of insects and bat diet, with species diversity being lower in light-trap samples and diet of M. lucifugus in 1999, which was cooler and wetter than in 1998. Species diversity in the diet of M. keenii was higher in 1999. Similarities in diet indicated that some interspecific competition was occurring, although this competition was likely minimized by their different foraging strategies.

Molecular quantification of differential ingestion and particle trapping rates by the appendicularian Oikopleura dioica as a function of prey size and shape

To investigate whether particles are ingested or merely trapped in the house of the appendicularian Oikopleura dioica, we developed a real‐time quantitative polymerase chain reaction (qPCR) assay and applied it in a controlled study where O. dioica was fed three types of algae at different concentrations. At particle concentrations ranging from 100 to 10,000 cells mL−1, highest ingestion efficiencies (75‐96%) were observed with the smallest algal species (Isochrysis sp.) compared to the larger cryptophyte Rhinomonas sp. (2‐87%) and a similar sized but spiny diatom, Chaetoceros calcitrans (4‐65%). When prey were mixed, the presence of C. calcitrans inhibited ingestion of Rhinomonas sp. but not the smaller Isochrysis sp., suggesting that C. calcitrans clogged the inlet filter and mechanically hindered ingestion of larger algae. O. dioica is adapted to feed on smaller particles at low (nonbloom) concentrations and may thus be subjected to bottom‐up regulation by larger spiny or colonial prey that typically dominate late‐stage phytoplankton blooms.

Functional plasticity of the venom delivery system in snakes with a focus on the poststrike prey release behavior

We explored variations in the morphology and function of the envenomation system in the four families of snakes comprising the Colubroidea (Viperidae, Elapidae, Atractaspididae, and Colubridae) using our own prey capture records and those from the literature. We first described the current knowledge of the morphology and function of venom delivery systems and then explored the functional plasticity found in those systems, focusing on how the propensity of snakes to release prey after the strike is influenced by various ecological parameters. Front-fanged families (Viperidae, Elapidae, and Atractaspididae) differ in the morphology and topographical relationships of the maxilla as well as in the lengths of their dorsal constrictor muscles (retractor vomeris; protractor, retractor, and levator pterygoidei; protractor quadrati), which move the bones comprising the upper jaw, giving some viperids relatively greater maxillary mobility compared to that of other colubroids. Rear-fanged colubrids vary in maxillary rotation capabilities, but most have a relatively unmodified palatal morphology compared to non-venomous colubrids. Viperids launch rapid strikes at prey, whereas elapids and colubrids use a variety of behaviors to grab prey. Viperids and elapids envenomate prey by opening their mouth and rotating both maxillae to erect their fangs. Both fangs are embedded in the prey by a bite that often results in some retraction of the maxilla. In contrast, Atractaspis (Atractaspididae) envenomates prey by extruding a fang unilaterally from its closed mouth and stabbing it into the prey by a downward-backwards jerk of its head. Rear-fanged colubrids envenomate prey by repeated unilateral or bilateral raking motions of one or both maxillae, some aspects of which are kinematically similar to the envenomation behavior in Atractaspis. The envenomation behavior, including the strike and prey release behaviors, varies within families as a function of prey size and habitat preference. Rear-fanged colubrids, arboreal viperids, and elapids tend to hold on to their prey after striking it, whereas atractaspidids and many terrestrial viperids release their prey after striking it. Larger prey are more frequently released than smaller prey by terrestrial front-fanged species. Venom delivery systems demonstrate a range of kinematic patterns that are correlated to sometimes only minor modifications of a common morphology of the jaw apparatus. The kinematics of the jaw apparatus are correlated with phylogeny, but also show functional plasticity relating to habitat and prey.

Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 294:201-212 (2005) - doi:10.3354/meps294201 Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem R. D. Scheinberg1,*, M. R. Landry2, A. Calbet3 1Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA2Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, California 92093-0227, USA3Institut de Ciències del Mar, CMIMA (CSIC), P. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain *Email: scheinbe@hawaii.edu ABSTRACT: The clearance rates of co-occurring appendicularian species, Oikopleura longicauda and O. fusiformis, in Kaneohe Bay, Hawaii, were investigated to evaluate and compare their roles in a tropical food web. Individual appendicularians were captured in situ and allowed to feed on the natural plankton assemblage for 60 to 180 min. Feeding rate estimates were based on flow-cytometry analyses of cell-density changes for heterotrophic bacteria (Hbact), Synechococcus spp. (Syn) and <13.0 µm autotrophic eukaryotes (Aeuks). Despite morphological differences, O. longicauda and O. fusiformis cleared the largest prey size-fraction at statistically indistinguishable rates. For the 3 prey categories (Hbact, Syn and Aeuks), mean clearance rates (±95% CI) were 12 ± 7, 27 ± 6 and 34 ± 18 ml individual (ind.)–1 h–1 and 25 ± 12, 26 ± 15 and 38 ± 20 ml ind.–1 h–1 for O. longicauda and O. fusiformis, respectively. The mean clearance rates of these 2 species on total sub-micron cells in Kaneohe Bay were not significantly different; however, O. fusiformis cleared Hbact at a marginally higher rate (p = 0.07). Only O. longicauda exhibited significantly different retention efficiencies as a function of prey size, clearing the smallest prey (Hbact) at approximately 36% the rate of the largest (Aeuks) (p < 0.01). Despite reduced efficiencies on the smallest prey categories, at high abundances in Kaneohe Bay (often 2 ind. l–1), O. longicauda is capable of removing >60% of the picoplankton standing stock from the water column daily. While generally much less abundant, during occasional peaks of 1 ind. l–1, O. fusiformis can remove an almost equivalent amount (> 50%). Nevertheless, the consistently higher abundances of O. longicauda make this species a more significant link between picoplankton production and higher-level consumers (chaetognaths and fishes) in this coastal tropical embayment. KEY WORDS: Appendicularian · Grazing · Clearance rates · Oikopleura spp. Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 294. Online publication date: June 09, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Ecological and Ethological Perspectives in Larval Fish Feeding

ABSTRACT Low survival rates during larval stages constitute a major bottleneck in the successful culture of many marine and some freshwater fish. The availability of live food is recognized as a critical factor influencing larval survival. Live food is still superior to the best larval diets in terms of larval survival and growth. This paper reviews important ecological and ethological aspects of feeding, from hatching tothe weaning stage, and relates them to problems in larval culture. In general, freshwater fish larvae are easier to raise than marine fish larvae, because at hatching they are larger and endowed with more yolk reserves, are less sensitive to starvation, and canbe weaned to artificial diets sooner. The feeding behavior of the larvae can be analyzed in terms of the sequential components of predation: search, encounter, pursuit, attack, capture, and ingestion. The searching efficiency and encounter rates of the visual predator are influenced by prey parameters such as body size, conspicuousness, and evasiveness. Turbidity of the water and light intensity also affect prey detection. To changing prey densities, the larvae show typical Type II functional responses, which are influenced by prey handling time, which in turn is largely a function of prey size. Knowledge of larval functional responses is helpful in providing the right concentrations oflive food for larval culture. The larvae are initially gape-limited and exhibit prey size selectivity but gradually widen their prey size range as they grow. An aquacultural application of this is the commonly employed feeding protocol, prey size sequencing, in which progressively larger live food items are offered as the larvae grow. A thorough knowledge of the feeding behavior is also essential in the formulation of acceptable larval diets.

Rush and grab strategies in foraging marine endotherms: the case for haste in penguins

The speed at which air-breathing marine predators that forage by diving should swim is likely to depend on a variety of factors that differ substantially from those relevant in animals for which access to oxygen is unlimited. We used loggers attached to free-living penguins to examine the speed at which three species swam during periods searching for prey and compared this to their speeds during actual prey pursuit. All penguin species appeared to travel at similar speeds around 2m/s during normal commuting between the surface and feeding depths, which accords closely with minimum costs of transport. However, Adélie penguins, Pygoscelis adeliae, slowed down to feed, Magellanic penguins, Spheniscus magellanicus, speeded up and king penguins, Aptenodytes patagonicus, travelled at a variety of speeds, although mean speed did not change from normal commuting. Since energy expenditure, and therefore oxygen usage, in swimming animals increases with the cube of the speed, we hypothesized that prey escape speed (a function of prey size) and prey density would prove critical in determining optimum pursuit speeds in predators. Simple models of this type help explain why it is that some penguin species apparently benefit by increasing speed to capture prey while others benefit by decreasing speed.

Size-dependent predation in piscivores: interactions between predator foraging and prey avoidance abilities

Body size is known to play a crucial role in predator-prey interactions. For a given predator size, it has been suggested that prey mortality should be a dome-shaped function dependent on prey body size. In this study, we experimentally tested (i) the suggested mechanisms responsible for the dome-shaped prey vulnerability function and (ii) whether a prey refuge affected the form of this function. As prey, we used young-of-the-year Eurasian perch (Perca fluviatilis), and as predator, larger Eurasian perch. The prey mortality as a function of prey size was dome shaped for large and medium predators but decreased monotonically with prey size for small predators. Capture success of predators decreased monotonically with increasing prey size and was lower for small predators. In refuge trials, the mortality of prey declined monotonically with prey size for all predator sizes. Refuge use of prey increased with the sizes of both prey and predator. Our results suggest that the hypothesized dome-shaped relationship on prey vulnerability can be altered by the presence of an absolute prey refuge. Our results further suggest that the ability to perform more flexible foraging behaviors is of increasing importance when prey size increases.

Testing predictions of foraging theory for a sit-and-wait forager, Anolis gingivinus

Research in foraging theory has been dominated by studies of active foragers choosing among patches and among prey within a patch. Studies of central-place foraging have mainly focused on loading decisions of an animal provisioning a central place. The problem faced by a sit-and-wait forager that encounters prey at a distance has received little attention. In this study we tested foraging theory predictions for such foragers, Anolis gingivinus females in the West Indies island of AnguiOa. We presented lizards with antHon larvae at various distances. Experiment 1 showed that an individual's probability of pursuing prey decreases with the prey's distance and is best described by a sigmoidal function (which may be as steep as a step function). This function's inflection point defines a cutoff distance. Experiment 3 tested how cutoff distance changes as a function of prey size. Cutoff distances were greater for larger prey, as predicted for an energy-maximizing forager. Experiments 2 and 4 tested how cutoff distance changes as a function of prey abundance. As predicted, cutoff rii*nnrr+ were greater at a site where prey abundance was lower. Furthermore, cutoff distances decreased immediately following prey augmentation and returned to previous values within one day of ending augmentation. Thus, moles' foraging behavior is a dynamic process, consistent with the qualitative predictions of foraging theory. We attribute the success of this study in supporting fundamental foraging theory predictions to the lizards exhibiting natural behavior under field conditions and to particular advantages of studying sit-and-wait foragers.

Virtual reality of planktivores:a fish's perspective of prey size selection

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 140:271-283 (1996) - doi:10.3354/meps140271 Virtual reality of planktivores: a fish's perspective of prey size selection Luo J, Brandt SB, Klebasko MJ Traditionally, field studies of size selective predation by visual-feeding planktivorous fish compare the size distribution of prey in stomachs to that of zooplankton measured in the field. However, the size frequency of prey perceived by a fish may differ from that measured by researchers using integrative plankton nets. In this study, a mechanistic spatial foraging model was developed to test the hypothesis that size-selective predation by visual-feeding planktivorous fish can arise simply by random encounters with prey. Our model was based on movement of a single predator and a size structured prey population distributed in 3-dimensional space. The model assumed that a predator's encounters with prey was a function of prey size and predator swimming speed. Upon encounter, the predator either selected prey by random choice or the largest apparent size. The size frequency and distance distribution of prey encountered, selected, and captured were estimated using Monte Carlo techniques. The model performance was initially evaluated with hypothetical prey size frequencies, and different light attenuation coefficients, capture efficiencies, visual distances and predator swimming speeds. Simulation results showed that the size frequency of prey encountered randomly by a predator is very different from the size frequency of the ambient prey in the environment. The size frequency of prey selected by random choice only differed from that selected by apparent size choice when visual distance was greater than reactive distance. Prey distance distribution, which was defined as frequency of prey encountered at different distances, showed that as prey density increased predators selected prey at closer distances by the largest apparent size choice than by random choice. We also tested the model specifically for the bay anchovy Anchoa mitchilli. Ambient zooplankton size frequencies found in mid-Chesapeake Bay were used to predict the size frequencies of zooplankton consumed by fish. Predicted prey size frequency in the diet matched the size frequency of zooplankton found in bay anchovy stomachs. We conclude that prey size selection by fish can be described mechanistically by differential random encounter from a fish's perspective, and that behavioral choice plays a minor role in prey size selection. Spatial foraging model · Prey size selection · Visual-feeding planktivore Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 140. Publication date: September 12, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Two Unusual Foraging Behaviors of Great Blue Herons in Lincoln County, Oregon

Great blue herons (Ardea herodias) rarely kleptoparasitize other birds (Willard 1977; Kushlan 1978; Brockman and Barnard 1979; Bayer 1985) or prey upon birds (Packard 1943; Palmer 1962; Olson and Johnson 1971; Bayer 1979; Chapman and Forbes 1984; Hancock and Kushlan 1984; Evens and Page 1986). In this note, kleptoparasitism of a double-crested cormorant (Phalacrocorax auritus) and the eating and stalking of red phalaropes (P1alaropus fulicaria) by great blue herons are described. At about 1525 hr PST on 9 December 1990 at Eckman Lake, a freshwater impoundment adjacent to Alsea Bay in Lincoln County on the Oregon central coast, I saw an immature double-crested cormorant dive and then surface holding an orangish, cylindrical fish about 10 cm long. For about 10 sec, the cormorant swam and turned its head from side to side while holding the fish crosswise in its upward-pointed bill. A great blue heron flew to the cormorant from a nearby Douglas-fir (Pseudotsuga menziesii) forested area. Splashing water obscured the ensuing 5 sec struggle. The cormorant then dived and surfaced about 15 m away, while the heron stood in 0.5 m deep water with the large fish in its bill. Kleptoparasitism is a function of prey size and handling time (Kushlan 1978; Brockman and Barnard 1979; Quinney et al 1981; Bayer 1985.) The conspicuous handling of a large prey item by this immature cormorant may have resulted from its lack of experience. At 1700 hr PST on 13 January 1991, I saw a great blue heron attempting to swallow a dead red phalarope along the Newport bayfront at Yaquina Bay, Lincoln County. The phalarope was swallowed head first after four unsuccessful attempts within 2-3 min of my initial observation. Between attempts, the heron used its bill to hold the phalarope crosswise around the middle of the body, shaking it at times. After the body-was swallowed, the wrist of one wing protruded from the heron's mouth for about 60 sec before swallowing was complete. A lump in the heron's neck was visible afterwards for approximately 5 min. Over the next 3 min, the same great blue heron stalked 15 to 20 phalaropes foraging within 10 to 20 m of its original position. First, the heron walked slowly with extended neck towards a small group of phalaropes, which swam about 5 m away. Maintaining an upright posture, the heron then walked quickly towards the phalaropes, which again flew about 5 m and landed on the water. Both of these stalking methods were tried unsuccessfully about five times on different groups of three to five phalaropes, but the heron caught no phalaropes during this observation time. For the next approximately 3 min, the heron attempted to approach the phalaropes from a board floating near the phalaropes. After climbing onto the board, the heron stood motionless, facing a phalarope group. Although the phalaropes swam within about 2 m, the heron did not strike. On two occasions when phalaropes approached, the heron raised and held its wings over the water for about 5 sec. Both times, the phalaropes swam away. After flying back from the board to the original mudflat location, the heron continued stalking phalaropes for approximately 3 min. It turned towards phalaropes standing on the mud or swimming in water a few cm deep, and with hunched neck, flexed legs, and forward-pointed bill, it ran quickly towards them into the shallow water. The phalaropes dispersed 2 to 3 m by flying or swimming. The heron returned to the mud and then to the floating board, watching but not stalking phalaropes from both points for 1 to 2 min. Approximately 12 min after my initial observation, the heron was flushed from the area by human activity. Red phalaropes occur sporadically during migration or after winter storms in coastal Lincoln County. Although no other dead phalaropes were seen in a visual search of the area, the heron may have scavenged a dead bird, as described by Kubisz (1989). Red phalaropes are not among the birds documented as great blue heron prey items (Palmer 1962; Brockman and Barnard 1979; Hancock and Kushlan 1984), although predation on a Wilson's phalarope was described by Packard (1943).

Maximization of foraging efficiency and resource defense by group retrieval in the ant Formica schaufussi

Workers of the ant Formica schaufussi forage as individuals and cooperate in groups to retrieve arthropod prey. In 2 sample years, group-transported prey were on average 6.8 and 4.7 times heavier than individually retrieved items, and the average loading ratios of groups were greater than the loading ratios of single foragers. Retrieval group size was adjusted to prey size, and prey transport velocity for individuals and groups tended to decrease with increasing prey weight. The efficiency of individual and group retrieval, estimated from calculations of the prey delivery rate to the nest (PDR) achieved by each foraging mode, varied as a function of prey size. Individual retrieval maximized PDR at a prey weight of 19.5 mg, and group transport maximized PDR at 190 mg. Although the PDR maxima of an individual in a group and a solitary forager were approximately equal, depending on prey size, group transport may maximize foraging efficiency. Group transport also decreased interference competition from sympatric ant species. Group-transported prey having a greater likelihood of successful retrieval were within the size range of prey that maximized foraging efficiency. Transport group size appeared to be more important in prey defense than in increasing prey transport velocity, suggesting an important role of group size in competitive ability.

The influence of prey size and abundance, and individual phenotype on prey choice by the three‐spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three‐spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit (p) and handling (h) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E/(p+h) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E/(p+h) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.

Stimulus control of predatory behavior in the brown tree snake (Boiga irregularis): III. Mandibular protractions as a function of prey size.

Brown tree snakes (Boiga irregularis, n=23) were observed swallowing rodent meals of varying mass, expressed as percentage of snake body weight. The number of mandibular protractions (MPs) as a measure of swallowing effort was recorded. The relation between MPs and meal size was linear, but the slope was significantly shallower than that obtained in earlier studies with nonvipers. Thus, although B. irregularis is not a viper, its swallowing ability is quite comparable with that seen in wide-headed, stout-bodied vipers. It is proposed that the triangular-shaped head of B. irregularis facilitates the ingestion of large prey

Stimulus control of predatory behavior in the brown tree snake (Boiga irregularis): III. Mandibular protractions as a function of prey size.

Stimulus control of predatory behavior in the brown tree snake (Boiga irregularis): III. Mandibular protractions as a function of prey size.

Predation by Didinium Nasutum: Effects of Predator and Prey Size

Experimental manipulation of the size of Didinium nasutum, a protozoan predator, feeding on large and small Paramecium species was used to examine the effects of predator and prey size on predator behavior and capture success. Didinum size is a function of prey size; large and small Didinium were obtained by conditioning predators on large or small prey species prior to experimentation. The predatory behavior of large and small predators feeding on large and small prey was monitored through an entire division cycle. Large Didinium had shorter handling times and encounter times on both sizes of prey than small Didinium. Large Didinium made more captures per division on either size of prey. Large prey required longer handling times, longer engulfing times, and were encountered more frequently than small prey for both sizes of predator. First captures were more difficult than subsequent captures for all predators. Small predators were more successful initially in capturing small prey than were large predators; first captures by large predators on small prey were particularly difficult. Encounter times, the number of encounters per capture, capture times, and handling times varied with the size of the predator and also varied within the division cycle of the predator. The effect of time within the division cycle may be due to increasing size as prey are captured and/or to adaptations to the specific prey species following initial captures. Didinium may adapt to new prey species through morphological changes in the oral apparatus, chemoreceptors, or extrusive organelles, in addition to shifts in size. That each size of Didinium had more successes per encounter on the size of prey to which it had been conditioned suggests that the size shift improves that aspect of the predatory behavior. However, other aspects (such as handling time and time to each encounter) show a clear advantage to large size. Large size also improves survival ability of Didinium under conditions of starvation (Hewett 1987). For large predators to maintain a large size on small prey would require a substantial increase in the division time. Selection forces favoring fast division rates, may impose restraints, making the size of Didinium on any prey species a compromise.

Risk of predation and the feeding behavior of juvenile coho salmon (Oncorhynchus kisutch)

During their first 1–2 years of life, juvenile coho salmon (Oncorhynchus kisutch) are stream-dwelling, and feed upon drifting invertebrates. They move upstream from a holding position to intercept individual prey items; the distance moved (attack distance) is an increasing, but decelerating, function of prey size. Since the fish are presumably more visible to predators during such feeding excursions, prey size and risk are associated variables.