Prey Eaten Research Articles

Early prey intake of a short-finned pilot whale (Globicephala macrorhynchus Gray, 1846, Cetacea: Delphinidae) in the Canary Islands.

This study reveals early prey eating by a short-finned pilot whale (Globicephala macrorhynchus Gray, 1846, Cetacea: Delphinidae) in the Canary Islands. Stomach contents, trophic markers, skin isotopic ratios of nitrogen (δ15N:15N/14N) and carbon (δ13C:13C/12C), and fatty acid profiles of the blubber of a short-finned pilot whale of 213 cm size euthanized in free-ranging conditions were analyzed. A total of 15 species of oegopsid squids, mostly diel vertical mesopelagic migrant species of the families Enoploteuthidae, Ommastrephidae, and Histioteuthidae, as well as mother's milk, were identified in the stomach contents. Asperoteuthis acanthoderma (Lu, 1977, Cephalopoda: Chiroteuthidae) was found as first time in this area, suggesting the possibility of its presence on both sides of the subtropical Atlantic, extending its current known distribution. The δ15N value (11.55‰) was higher than expected based on the size range of squid ingested, but lower than that of adult pilot whales, suggesting that mother's milk intake has a significant effect on these values in calves. Similarly, the δ13C values (-17.99‰) were shifted to those of adult pilot whales rather than the ingested squids, also due to the ingestion of high-fat breast milk. The fatty acid (FA) composition of blubber showed a clear stratification. Long-chain polyunsaturated fatty acids (LC-PUFA) were mainly present in the inner layer, while most relevant ≤C20 monounsaturated fatty acids (MUFA) were more abundant in the outer layer.

Feeding behavior of Gammarus aequicauda in the presence of two prey species of Artemia sp. and Baeotendipes noctivagus.

Gammarus aequicauda is the most abundant amphipod species in the Crimean hypersaline lakes, and is predatory upon other invertebrate species, suppressing their populations. The authors studied a time balance during the feeding of G. aequicauda in the presence of two prey species. The different variants of prey composition were: (1)two Artemia sp., (2)two chironomid larvae, and (3)one Artemia sp. and one chironomid larva. The duration of the experiments differed and continued until both prey were consumed. The experiments were carried out in vessels with and without bottom sediments. The result showed that in vessels with and without sediments in the case of both species of prey, the time to the capture of the first prey did not depend on whether Artemia sp. or chironomid larva was the first prey; the time differences are statistically insignificant. The duration of eating prey in all experimental variants depended on the kind of prey, and those differences were highly significant. The average hourly consumption rate was dependent on prey composition and order of prey capture and eating. The maximum consumption for two prey was when both were brine shrimp. In vessels without sediments and containing both Artemia sp. and chironomid larva the consumption rate was higher if the first caught prey was Artemia sp. rather than сhironomid larva. The order of prey eating did not influence a ration size in vessels with bottom sediments. When both prey were chironomid larvae, the total diet did not practically differ in vessels with or without sediments.

Five ixodid tick species including two morphotypes of Rhipicephalus turanicus on nestlings of Eurasian eagle owl (Bubo bubo) from south-eastern Bulgaria

BackgroundBirds are major hosts for many tick species (Acari: Ixodidae, Argasidae), and their role is especially important in transporting ticks over large distances along their seasonal migratory routes. Accordingly, most studies across Europe focus on the importance of avian hosts in tick dispersal, and less emphasis is laid on resident birds and their role in supporting tick life cycles. Eurasian eagle owls (Bubo bubo) exemplify the latter, but all the few studies on their tick infestation were carried out in Western Europe and even those did not involve a large sample size and did not assess infestation prevalence in natural habitats.MethodsIn this study, 320 ixodid ticks were collected from nestlings of Eurasian eagle owls during the period 2018–2020 in Bulgaria in south-eastern Europe. These ticks were analysed morphologically, and selected specimens molecularly based on cytochrome c oxidase subunit I (cox1) gene. The effects of environmental and habitat-related conditions and of the species of prey eaten by eagle owls on tick infestation were also evaluated.ResultsThe majority of ticks were identified as adults of Rhipicephalus turanicus (n = 296). In addition, 15 Hyalomma marginatum (three males, 11 nymphs and a larva), one female of Haemaphysalis erinacei and of Ha. punctata, and a nymph of Ixodes ricinus were found. Among R. turanicus, two distinct morphotypes were observed, but they do not form a monophyletic clade in the phylogenetic tree based on the mitochondrial gene cox1. We found a positive correlation between the total number of ticks on nestlings from a particular nest and the number of medium-sized to large prey mammals brought to the nestling owls. Also, the most important predictor for tick abundance was the effect of the extent of arable land (negative), while forests and grasslands contributed less, with no effect observed in case of urbanized areas and watercourses.ConclusionsThe intensity of tick infestation can be high on nestling Eurasian eagle owls (mean intensity 16.59 ticks/nestling). In this study, five different tick species were recorded, among which R. turanicus dominated. Two male morphotypes of this tick species were found, but their morphological differences were not reflected by genetic diversity or phylogenetic clustering. The most important factor determining tick abundance was the land-use structure.Graphical

Diets of mammalian carnivores in the deserts of north-eastern South Australia

We studied diets of feral cats (Felis catus), dingoes (Canis familiaris) and red foxes (Vulpes vulpes) in desert environments in north-eastern South Australia by analysing prey remains in opportunistically-collected scats. Four major landscapes were sampled (Simpson Desert, Sturt Stony Desert, Strzelecki Desert – Cooper Creek and Diamantina River) which yielded 238 cat scats, 298 dingo scats and ten fox scats. There was some overlap in prey eaten by cats and dingoes, but their diets were significantly different because cats typically ate small prey such as small mammals, small lizards and birds, while dingoes ate larger prey like domestic cattle, kangaroos and large reptiles. The few fox scats collected suggested fox diets were more similar to cat than dingo diets. Scat composition also differed significantly between landscapes irrespective of predator, with landscapes differing both in diversity and relative abundances of prey consumed. We detected several species in scats that are threatened with extinction either nationally (dusky hopping mouse, Notomys fuscus, and crest-tailed mulgara, Dasycercus cristicauda) or regionally (desert mouse, Pseudomys desertor, and long-haired rat, Rattus villosissimus), adding valuable knowledge to the distribution of these mammals and demonstrating the value of predator scat analyses in mammal surveys of Australian deserts.

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

We examined the influence of prey density and fish size on prey consumption in common sole (Solea solea L.) foraging on buried ragworm Alitta virens (Sars) (formerly known as Nereis virens (Sars)). The tested prey densities of 0.8, 2.2, 4.3 and 6.5 individuals dm−2 were exposed to common soles of either 100 g or 300 g. At each prey density common sole foraged for 48 h. At both common sole classes studied, a positive correlation between prey consumption and prey density was observed (P < 0.001). Relationships however differed between 100 and 300 g common sole. In 300 g common sole the relationship between prey consumption and prey density was linear (P < 0.001), whereas in 100 g common sole the relationship between prey density and prey eaten was polynomial (P = 0.018). Small common sole reached satiety prey consumption rates at nearly every prey density while large common sole did not reach satiation rates even at highest prey densities. The data suggest that in nature, polychaetes such as A. virens may contribute to the diet of small common sole even when they are only moderately abundant. In contrast, polychaetes may not be an ideal prey for larger common sole as indicated by the absence of satiety regardless of prey density.

Heavy Metal Concentrations in Free-Living Southern Caracaras (Caracara plancus) in the Northeast Region of Brazil

Background: With the continuously increasing release of heavy metals in the environment, mostly from anthropogenic sources, there is a need to find ways of evaluating and managing the issuance of these contaminants and correct its damages. The birds being at the top of some food chains reflect the presence of metals in the environment, keeping this in perspective, raptors have been successfully used for heavy metals biomonitoring studies in the past. The purpose of this study was to identify and quantify cadmium (Cd), mercury (Hg), lead (Pb), copper (Cu), and chromium (Cr) in the feathers and livers of free-living southern caracaras, live and dead, from Recife, Pernambuco state, Northeast of Brazil.Material, Methods & Results: Sixty-two feathers from live and dead southern caracaras and 21 livers from dead southern caracaras were analyzed by atomic absorption spectrometry for Hg and by inductively coupled plasma optical emission spectrometry for Cd, Cr, Pb, and Cu. Concentrations of Cr and Cu elements were detected in all feather and liver samples analyzed from live and dead caracaras. There was no difference in the concentration of metals between feathers of dead and live caracaras: Pb (P = 0.3576), Cd (P = 0.0792), Cr (P = 0.5475), and Cu (P = 0.3603), but significant variation was observed for Hg (P = 0.0459). The highest concentrations of Pb (P < 0.0001) and Cr (P < 0.0001) were found in the feathers than in the liver. On the other hand, the highest concentrations of Cu were found in liver samples (P = 0.0011). No significant variation in the concentrations of Cd (P = 0.7770) and Hg (P = 0.3669) was found between feathers and liver samples.Discussion: Chromium, as well as Hg, and Cu have a high affinity for keratin, which may explain the higher concentrations of Cr in caracaras feathers in this research. Lead was detected in all liver samples analyzed. Elevated levels were also found in the feathers of dead (95.2%) and live (75.6%) caracaras. The presence of Pb may be due to external contamination by pollutants, such as fossil fuels. Higher concentrations of Hg were observed in dead caracaras feathers, this concentrations change during molting but are not affected by external contamination. Metals, such as Hg, and Cd, accumulate in organisms from different trophic levels, which may indicate that this contamination comes mainly from feed. Pearson’s coefficient here showed no correlation between metals from dead caracaras feathers and livers, although a few previous studies have shown a correlation between metal concentrations from tissues and feathers. This pattern can be attributed to the different time of exposure of feathers to metals in relation to livers. The concentrations of metals in hepatic tissues reflect the levels of elements in the diet of these birds, characterizing acutely the contamination of the ecosystem. On the other hand, feathers represent the exposure in the time of molting, when the artery supplies metals, thus representing a chronic exposure. In feathers from live birds, the Pearson’s correlation analysis showed a high to moderate correlation between metals, which may suggest that they come from a similar source, but there is no way to specify the origin since the metals surveyed could be present in the soil, water, or in the prey eaten by these animals. For the purpose of environmental analyses, it is required to determine the exact source of contamination. Considering that it is possible to confirm the presence of these heavy metals in southern caracaras, these birds may be important environmental bioindicators. The development of systematic research on animals and the environment is essential for monitoring the levels of metal pollutants and evaluating their impact in order to guide measures to protect fauna and human health.

Large batoid fishes frequently consume stingrays despite skeletal damage.

The shapes of vertebrate teeth are often used as hallmarks of diet. Here, however, we demonstrate evidence of frequent piscivory by cartilaginous fishes with pebble-like teeth that are typically associated with durophagy, the eating of hard-shelled prey. High-resolution micro-computed tomography observation of a jaw specimen from one batoid species and visual investigation of those of two additional species reveal large numbers of embedded stingray spines, arguing that stingray predation of a scale rivalling that of the largest carnivorous sharks may not be uncommon for large, predatory batoids with rounded, non-cutting dentition. Our observations demonstrate that tooth morphology is not always a reliable indicator of diet and that stingray spines are not as potent a deterrent to predation as normally believed. In addition, we show that several spines in close contact with the jaw skeleton of a wedgefish (Rhynchobatus) have become encased in a disorganized mineralized tissue with a distinctive ultrastructure, the first natural and unequivocal evidence of a callus-building response in the tessellated cartilage unique to elasmobranch skeletons. Our findings reveal sampling and analysis biases in vertebrate ecology, especially with regard to the role of large, predatory species, while also illustrating that large body size may provide an escape from anatomical constraints on diet (e.g. gape size, specialist dentition). Our observations inform our concepts of skeletal biology and evolution in showing that tessellated cartilage—an ancient alternative to bone—is incapable of foreign tissue resorption or of restoring damaged skeletal tissue to its original state, and attest to the value of museum and skeletal specimens as records of important aspects of animal life history.

Seasonal variation in the diet of Spoonbill chicks in the Wadden Sea: a stable isotopes approach

We used stable isotope tracers in the growing primary feathers of Eurasian Spoonbill chicks (Platalea leucorodia leucorodia) to study seasonal variation in their diet on one of the Frisian islands, Schiermonnikoog, The Netherlands. Using growing individual primaries as natural samplers over time, samples were taken along the length of primary feathers to estimate both within- and between-individual variation in diet. Absolute isotopic ratios of feather material ranged from −26.2 to −14.7 ‰ for carbon (δ13C) and from 13.0 to 18.7 ‰ for nitrogen (δ15N). The variation in δ13C values suggests the use of a variety of feeding habitats, ranging from freshwater to marine. Across the breeding season, there was a shift from predominantly freshwater prey early on to a more marine diet later in the season. Surprisingly, this shift did not occur within the growth trajectory of early born chicks which instead showed the opposite, but it did occur within individual chicks born later in the season. Stable isotope Bayesian mixing-model (SIAR) outcomes demonstrated that the freshwater/brackish prey had the highest isotopic contribution “(51 %; 95 % confidence interval 39–63 %) to the diet early in the breeding season, whereas marine prey contributed most (78 %; 66–89 %) to the diet later. That chicks fed with either freshwater or marine food items had similar body condition indices suggests that the eating of marine prey did not come at a major cost for growing Spoonbill chicks.

Piscivore Responses to Enhancement of the Channelized Kissimmee River, Florida, U.S.A.

AbstractEvaluation of the success of ecosystem restoration projects requires identification of appropriate ecological metrics. Comparison of reconstructed food webs (or subsets thereof) from restored and non‐restored habitats may be a valuable tool to evaluate restoration success because food webs help identify critical predator–prey relationships, keystone species, relative importance of direct and indirect trophic interactions, and other aspects of ecological function. We compared the diets of apex predatory fishes collected from enhanced and non‐enhanced portions of the channelized Kissimmee River, Florida, USA to determine whether food web structure responded to experimental hydrologic manipulations. Diets were reconstructed for black crappie (Pomoxis nigromaculatus), bowfin (Amia calva), chain pickerel (Esox niger), Florida gar (Lepisosteus platyrhincus), largemouth bass (Micropterus salmoides), and warmouth (Lepomis gulosus) collected from enhanced and non‐enhanced portions of the Kissimmee River. Prey eaten by apex predatory fishes in the enhanced portion of the Kissimmee River were quantitatively and qualitatively different from prey eaten in non‐enhanced portions of the river. Predators in the enhanced portion of the river had fewer empty stomachs, more prey items per individual, more prey types per individual, more fish prey per individual, greater overall richness of prey, and a multivariate suite of prey distinct from predators in non‐enhanced portions of the river. Results from hydrologic manipulations suggest that large‐scale restoration of hydrologic linkages between the main channel and floodplain habitats will positively affect food web structure and ecosystem function in the Kissimmee River.

Predation by domestic cats in rural areas of central Poland: an assessment based on two methods

AbstractStudies of cat trophic behaviour can be based on collections of the prey brought home or the prey eaten by cats (i.e. analyses of scat/gut contents). Both methods involve biases with respect to palatability, prey size and assessment of hunting rates. Furthermore, these methods are often used on different groups of cats (i.e. house‐based vs. feral), thus results are difficult to compare. In the present study, cats from the same area (rural areas in central Poland) were studied by both methods: prey brought home and prey eaten (scat and gut analyses). Both methods identified mammals as the most frequent prey (followed by birds). However, differences occurred in the percentages of the four main vertebrate groups brought home versus eaten by cats: reptiles tended to be brought home, whereas amphibians tended to be eaten. No such difference was found for birds and mammals. Second, the relative proportions of presumably more palatable and presumably less palatable prey differed. The relative proportions of mice and voles (the latter eaten more frequently) and the relative proportions of soricomorphs and rodents (the latter eaten more frequently) were different. Finally, small prey items (i.e. invertebrates) were recorded incompletely for the brought‐home method. Overall, the prey‐brought‐home method underrepresented small prey and underestimated the predation rate for cats, whereas the prey‐eaten method was less likely to record unpalatable prey. We thus recommend to combine these two methods to obtain fuller and truer assessment of cat predation.

What do declining woodland birds eat? A synthesis of dietary records

Ground-foraging insectivores are prominent among the 26 species considered ‘declining woodland birds’ in southern Australia but the mechanisms driving their declines remain elusive. Nutritional factors may be critical, with larger and more structurally complex woodlands supporting greater arthropod biomass, but these differences need not translate into more arthropods actually consumed by these insectivores. We synthesised existing dietary records of a subset of the 26 declining woodland birds—13 ground-foraging insectivorous passerines—to determine the range of arthropods consumed and to estimate the relative importance of each prey group for these birds. Declining insectivores consumed a wide array of arthropods, but diets were characteristically dominated by one or two prey groups: Coleoptera, Formicidae and Lepidoptera accounted for 58% of prey records. Coleoptera contributed the greatest proportion of records (27%) and was the dominant prey group in the diets of nine of the 13 birds. These popular prey groups likely represent core resources supporting populations of declining insectivores and measurement of their abundance may provide meaningful estimates of the availability of prey. We highlight the need to quantify the size-range and identity of those prey eaten by declining woodland birds, and propose that reliance on a small number of prey groups may underlie the sensitivity of ground-foraging insectivores to modification of habitat.

Fine‐scale spatial and temporal variations in diets of the pipefish Stigmatopora nigra within seagrass patches

Diets of the pipefish Stigmatopora nigra were analysed to determine if food availability was causing S. nigra to distribute according to habitat edge effects. Gut analysis found little difference in the diets of S. nigra at the edge and interior of seagrass patches, regardless of time of day or season. Fish diets did, however, vary with seagrass density: S. nigra in denser seagrass consumed more harpacticoid copepods and fewer planktonic copepods. The lack of difference in prey eaten by S. nigra at the edge and interior of patches suggests either that food was not determining S. nigra distribution patterns within patches or that differences in fish densities across patches meant that relative fish-prey densities were similar at edge and interior positions. Alternatively, any edge effects in diet might be masked by gradients in seagrass structure.

Some Observations on the Killing and Eating of Prey by two dasyurid Marsupials: The Mulgara, Dasycercus cristicauda, and the Tasmanian Devil, Sarcophilus harrisi.

Some Observations on the Killing and Eating of Prey by two dasyurid Marsupials: The Mulgara, Dasycercus cristicauda, and the Tasmanian Devil, Sarcophilus harrisi.

The Distribution and Trophodynamics of Demersal Fish from Spencer Gulf

This paper describes the composition, distribution and diets of demersal fishes collected from Spencer Gulf during a quantitative Gulf-wide trawl survey in February 2007. A total of 132 fish species from 65 families were collected from the 120 trawl shots. Degens leatherjacket, Thamnaconus degeni, was the most abundant species collected (627 ± 239 ha−1). This fish was also dominant in terms of biomass, and accounted for more than 40% of the total catch weight. Most other species (95%) had mean biomasses ranging from less than 1 to 515 g ha−1, and individually contributed less than 3% to the overall catch. Correlation analyses revealed a broad latitudinal gradient in fish abundance, biomass and richness. All three parameters generally increase towards the south of the Gulf, in association with increasing water depths and decreasing water temperatures. Multivariate cluster analyses confirmed the presence of a strong environmental gradient between the north and south of the Gulf, and highlight the presence of three distinct fish assemblages (north, central and south) that are closely allied with changes in depth. Small subsets of species with restricted distributions characterised each regional assemblage. Dietary examinations of the stomach contents of 871 fish representing 107 species were also undertaken during this study. Five feeding guilds were identified from cluster analysis of the stomach contents. These included groups of fish feeding primarily on echinoderms (2 spp), other fish (13 spp), molluscs (11 spp), crustaceans (52 spp) and worms (23 spp). Atrophic model was constructed to evaluate levels of food consumption for each fish, and to assess regional differences in the composition and total volume of prey eaten. Total food consumption was estimated to decrease progressively towards the top of the Gulf, with daily food consumption rates in the northern region (219.55 g ha−1 day−1) more than seven times lower than those in the south of the Gulf (1580.99 g ha−1 day−1). Crustaceans and annelids were the two most important prey items consumed by demersal fish in all three biomes of the Spencer Gulf (north, central, south), and comprised more than 30% and 18% of the total diet, respectively. Our model suggests that almost half a kilogram of crustaceans and worms are consumed daily per hectare in the south of the Gulf. All other prey groups (e.g. molluscs, bryozoans, echinoderms, sponges) are eaten in much smaller volumes, and are individually consumed at daily rates of less than 127 g ha−1 in each biome. Estimates of consumption were broadly consistent with other temperate Australian estuaries. However, as no seasonal replication of sampling was undertaken it remains unclear if observed spatial patterns and trends are temporally robust.

Nickel dynamics in the lakewater metal biomonitor Chaoborus

Nickel (Ni) is a widespread contaminant present at toxic concentrations in aquatic systems in the vicinity of some mining and smelting operations. However, its accumulation by aquatic animals has been little studied and there are few biomonitors for this metal. Recently, larvae of the aquatic insect Chaoborus were shown to be effective as biomonitors for Ni concentrations in lakewater. Since animals are more effective as biomonitors when we understand how they take up their contaminants (from water or from food) and the rate at which they exchange contaminants with their surroundings, we set out to measure these parameters for Chaoborus. To achieve these goals, we exposed the components of a laboratory food chain (green alga, cladoceran, Chaoborus) to realistic Ni concentrations. We found that the majority (≈65%) of the Ni taken up by Chaoborus flavicans comes from lakewater, with the remainder coming from its planktonic prey ( Daphnia magna). This result is consistent with the low mean efficiency (14%) with which C. flavicans assimilated Ni from its prey. To explain the low efficiency of Ni uptake from food we measured the subcellular distribution of Ni in prey, which predicted that the majority of the Ni in prey (≈55%) was available for assimilation by the predator. This potential Ni uptake efficiency was only reached in animals that ingested few prey, likely because their gut passage time was longer than those ingesting many prey. We also measured Ni uptake and loss by C. flavicans exposed to Ni in water then used these data to parameterize a mechanistic bioaccumulation model that allowed us to describe Ni exchange between this insect and water. Lastly, we used these model constants, along with field measurements of Ni in 10 Canadian lakes, to predict Ni concentrations in field populations of Chaoborus. Model predictions overestimated Ni concentrations in field populations by a factor of 4. We suggest that uncertainties in the rate constant for Ni uptake from water and a lack of measured Ni concentrations in the prey eaten by Chaoborus larvae in the field could explain this difference.

Diet of the rattlesnake Crotalus durissus in southeastern Brazil (Serpentes, Viperidae)

Gut contents of 633 live rattlesnakes from southeastern Brazil received at the Butantan Institute, SP, Brazil between 1993 and 1995 were studied. The snakes were weighed, measured and sexed. Two hundred and fifty‐nine rattlesnakes had stomach and/or intestinal contents. Prey size was estimated by comparison of prey items with specimens from museum collections. Rodents and small marsupials were the main prey eaten by the rattlesnakes, and only 1% of the items were found in the stomach, whereas 41% of the individuals in the sample had feces in the intestine. There was low correlation between size of snake and prey size. No seasonal difference in frequency was found between fed and not fed males, but the occurrence of fed females was significantly lower during summer than winter months (28.9% and 51.8%, respectively). Fed newborn rattlesnakes had the lowest frequency, and also fed on rodents. O conteúdo do trato digestivo de 633 cascavéis do sudeste brasileiro recebidas pelo Instituto Butantan, SP, Brasil entre 1993–1995 foram estudados. As serpentes foram pesadas, medidas e tiveram seu sexo determinado. Duzentas e cinqüenta e nove serpentes apresentaram conteúdo estomacal e/ou intestinal. O tamanho das presas foi estimado comparando‐os com espécimes de museu. Roedores e pequenos marsupiais foram as principais presas ingeridas pelas cascavéis, e somente 1% dos itens foram encontrados no estômago, entretanto 41% das cascavéis tinham fezes no trato intestinal. Houve pequena correlação entre o tamanho da serpente e da presa ingerida. Não foi observada diferença sazonal na freqüência de machos com e sem conteúdo, mas a ocorrência de fêmeas sem conteúdo foi significativamente menor nos meses de verão do que nos meses de inverno (28,9% e 51,8%, respectivamente). Cascavéis recém‐nascidas também se alimentam de roedores e têm a mais baixa taxa de animais com conteúdo.

ADJUSTING PREY HANDLING TIMES AND METHODS AFFECTS PROFITABILITY IN THE BROAD-HEADED SKINK (EUMECES LATICEPS)

Time spent pursuing, capturing, and subduing prey, and time spent preparing and swallowing it all affect profitability of a prey item (net energy gain/handling time). In the diet selection model of classical optimal foraging theory, handling time is constant for each prey type, but several factors affect it in some predators. We studied factors that might cause time spent to capture and handle prey by broad-headed skinks (Eumeces laticeps) to vary between and within prey types. Pursuit time did not vary with prey (cricket) size in experimental conditions. In three experiments using crickets of the same size, handling times for the first prey eaten were shorter when more prey were simultaneously present. Reduced handling time likely maximizes number of prey captured before the remaining prey escape. Preparation time (capture to initiation of swallowing) did not vary with prey number, but swallowing time decreased with number of prey present. This result suggests that lizards must prepare crickets thoroughly for digestion by bites puncturing the exoskeleton, which constrains them from decreasing preparation time. However, lizards can reduce handling time by swallowing more quickly, increasing the likelihood of capturing additional prey. Handling times were shortest for hungry skinks and longest just before satiation. Two factors may contribute to the increase in handling time with degree of satiation: (1) Motivation to capture prey declines as a predator becomes satiated, removing the reason for shortening handling times when groups of prey are present; (2) as the stomach and the esophagus are filled, mechanical constraints may slow swallowing. Preparation time, number of bites, swallowing time, and handling time increased with cricket size. Consequently, although it appears that the largest prey were the most profitable for sizes of crickets studied, this may not be true for other ranges of prey size. Preparation time was shorter for prey bitten on the head than on the side or back, but prey orientation during swallowing did not affect swallowing time. In summary, (1) when prey are present simultaneously, total energetic profit may be increased by reducing handling time, and (2) handling for specific prey types vary with satiation and handling method.

Effect of prey density, prey mobility and habitat structure on size selection and consumption of amphipods by a benthic feeding fish

To examine the interactive effects of prey density, prey mobility and habitat structure on prey selection by benthic feeding fish, feeding experiments were conducted with 95-105 mm (total length) pumpkinseed (Lepomis gibbosus), given four sizes ( 9 mm) of active or immobilized amphipods (Gammarus pseudolimnaeus) as prey. The experiments involved the assessment of prey size selectivity, total number of prey consumed and size-specific indicators of foraging efficiency such as capture success and handling time. Pumpkinseeds were size selective only at medium and high prey densities, preferring large Gammarus when selectivity occurred. The proportion of mobile and immobile prey consumed increased with prey size in unstructured habitat, whereas no selectivity occurred with mobile prey in structured habitat. Both habitat and prey density significantly affected prey consumption, with more prey eaten at higher densities and in unstructured habitat. Prey mobility did not affect the number of prey consumed when mobile or immobile prey were offered in separate trials, but the consumption rate of immobile prey increased in structured habitat when they were offered with mobile prey in the same trial. We conclude that prey density and habitat structure affect pumpkinseed size selectivity and consumption rates in a similar manner, whereas the effect of prey mobility on these factors is more dependent on the combination of density and habitat and on whether mobile and immobile prey are present at the same time.

DIETARY ANALYSIS FROM FECAL SAMPLES: HOW MANY SCATS ARE ENOUGH?

Diets of mammals are increasingly being inferred from identification of hard parts from prey eaten and recovered in fecal remains (scats). Frequencies with which particular prey species occur among collections of scats are easily compiled to describe the average diet, and can be used to compare diets between and within geographic regions, and across years and seasons. Important to these analyses is the question of statistical power. In other words, how many scats should be collected to compare the diet among and between species? We addressed this problem by using Monte Carlo simulations and frequency of occurrence methods to analytically determine the consequence of sample size on the dietary analysis of scats. We considered 2 questions. First, how is the statistical power affected by sample size? Second, what is the likelihood of not identifying a prey species? We randomly sampled predetermined numbers of scats (n ¼ 10‐200) from computer-generated populations of scats containing prey of known species and frequencies of occurrences. We also randomly sampled a large database of field-collected scats from Steller sea lions (Eumetopias jubatus). We then used standard contingency table tests such as chi-square and Fisher’s exact test to determine whether differences between our samples and populations were statistically significant. We found that a minimum size of 59 scats is necessary to identify principal prey remains occurring in .5% of scats. However, 94 samples are required when comparing diets to distinguish moderate effect sizes over time or between areas. These findings have significant implications for the interpretation of published dietary data, as well as for the design of future scat-based dietary studies for pinnipeds and other species.

Foraging behavior and activity budgets of sea stars on a subtidal sediment bottom community

We dived at regular intervals (8, 12 or 14 h) over periods of 9 to 23 days to quantify the allocation of time to different activities by large adults of the four common sea stars foraging in a sediment bottom community (8 to 11 in depth) in the northern Gulf of St. Lawrence. The relative time spent at different activities was similar for Leptasterias polaris and Asterias vulgaris, the most frequent activity being moving over the bottom (42–46% of the time), presumably in search of prey, secondly being stationary (26–27%), and thirdly digesting prey (17–20%). Capture of prey was less frequent, 13% of the time for L. polaris (mainly digging for bivalves) and 5% for A. vulgaris. Compared to L. polaris and A. vulgaris, Crossaster papposus spent more time being stationary and periods of moving were of shorter duration. Solaster endeca differed from the other sea stars because of the high proportion of time spent digesting prey (40%) and the small proportion of time spent moving (22%). Capture events were not observed for C. papposus and S. endeca, likely because they exclusively fed on epifaunal species, which were rapidly captured. For all four species, the most frequent changes between activities were between moving and being stationary. The number of days per prey eaten (length of a feeding bout) was about 11 for S. endeca, 6.6 for L. polaris, 3.1 for C. papposus and 2.8 for A. vulgaris. All sea stars appear to be energy maximizers that spend most of their time finding, capturing and digesting prey.