Mean Prey Size Research Articles

Bioenergetic modelling of a marine top predator's responses to changes in prey structure.

Determining how animals allocate energy, and how external factors influence this allocation, is crucial to understand species' life history requirements and response to disturbance. This response is driven in part by individuals' energy balance, prey characteristics, foraging behaviour and energy required for essential functions. We developed a bioenergetic model to estimate minimum foraging success rate (FSR), that is, the lowest possible prey capture rate for individuals to obtain the minimum energy intake needed to meet daily metabolic requirements, for female sperm whale (Physeter macrocephalus). The model was based on whales' theoretical energetic requirements using foraging and prey characteristics from animal-borne tags and stomach contents, respectively. We used this model to simulate two prey structure change scenarios: (1) decrease in mean prey size, thus lower prey energy content and (2) decrease in prey size variability, reducing the variability in prey energy content. We estimate the whales need minimum of ~14% FSR to meet their energetic requirements, and energy intake is more sensitive to energy content changes than a decrease in energy variability. To estimate vulnerability to prey structure changes, we evaluated the compensation level required to meet bioenergetic demands. Considering a minimum 14% FSR, whales would need to increase energy intake by 21% (5-35%) and 49% (27-67%) to compensate for a 15% and 30% decrease in energy content, respectively. For a 30% and 50% decrease in energy variability, whales would need to increase energy intake by 13% (0-23%) and 24% (10-35%) to meet energetic demands, respectively. Our model demonstrates how foraging and prey characteristics can be used to estimate impact of changing prey structure in top predator energetics, which can help inform bottom-up effects on marine ecosystems. We showed the importance of considering different FSR in bioenergetics models, as it can have decisive implications on estimates of energy acquired and affect the conclusions about top predator's vulnerability to possible environmental fluctuations.

Feeding by larval fish: how taxonomy, body length, mouth size, and behaviour contribute to differences among individuals and species from a coastal ecosystem

AbstractData on individual stomach contents were used to describe length-dependent differences in feeding success of larvae of 11 species of fish found in coastal Newfoundland, Canada. Copepods dominated the diet with a gradual shift from nauplii to copepodites in all species. Differences in feeding success in both prey number and gut fullness among individual larvae was linked to increasing individual diet diversity in all taxa, although there was a weak decline in mean prey size. Maxilla and body length, within and among taxa, have a dominant positive influence on the potential feeding success of larval fish. In addition to differences in average stomach weight, the variability in number of prey per stomach among individuals indicates that each species perceives their prey environment in different ways. Taxonomic proximity had limited effect on differences in feeding success among taxa. The results suggest that behavioural differences among individuals and taxa, that likely reflect swimming capacity and/or prey perception/capture ability, are likely to be important elements contributing to feeding success. Body and mouth size may represent key characteristics that should be considered in evaluating differences in feeding success among species as well as among individuals within and among cohorts.

Diet of the Golden Eagle during the Breeding Season in Northwestern Chihuahua, Mexico

The diet of the Golden Eagle (Aquila chrysaetos) was studied in 4 territories in northwestern Chihuahua, México, during the breeding season. Prey remains and pellets were collected from 4 nests during 2014, 2015, and 2016. We identified 12 vertebrate species. Black-tailed jackrabbits (Lepus californicus) were the most important prey remains in terms of frequency (72%) and ingested biomass (86%). We estimated mean prey size to be 1291 g (SE = 3364), indicating that Golden Eagles prey on medium-sized animals (e.g., lagomorphs). The main threats to Golden Eagles are connected to habitat loss and habitat fragmentation, but other factors also affect eagles. We hope our information will help to establish better species management programs.

Inter-individual variation in provisioning rate, prey size and number, and links to total prey biomass delivered to nestlings in the Collared Flycatcher (Ficedula albicollis)

BackgroundIn bird species where offspring growth and survival rely on parents’ food provisioning, parents can maximise their fitness by increasing the quantity and/or the quality of preys delivered to their offspring. Many studies have focused on inter-individual variation in feeding rate, yet this measure may not accurately reflect the total amount of food (i.e. energy) provided by parents if there is large variation in the quantity and quality of preys at each feeding. Here, we explored the relative role of individual (sex, age, body condition), breeding (hatching date, brood size) and environmental (temperature) factors on feeding rate, prey number, size and quality, and their contribution to total prey biomass delivered to the nestlings of 164 Collared Flycatcher (Ficedula albicollis) parents in 98 nests.ResultsPreys delivered to the nest were mainly larvae (53.6%) and flying insects (45.6%). Feeding rate increased with brood size and age, and was higher in males than females. Mean prey number decreased, but mean prey size increased, as the season progressed and parents feeding their brood with primary larvae brought more preys per visit. Relationships between feeding rate, mean prey number and size remained when taking into account the provisioning quality: parents brought either a large number of small prey or a small number of larger items, and the force of the trade-offs between feeding rate and mean prey number and size depended on the quality of the provisioning of the parents. Whatever the percentage of larvae among preys in the provisioning, the variance in total prey biomass was foremost explained by feeding rate (65.1% to 76.6%) compared to mean prey number (16.4% to 26%) and prey size (2.7% to 4%).ConclusionsOur study shows that variation in feeding rate, prey number, size, but not quality (i.e. percentage of larvae), were influenced by individual factors (sex and age) and breeding decisions (brood size and timing of breeding) and that, whatever the provisioning strategy adopted, feeding rate was the best proxy of the total biomass delivered to the nestlings.

DNA metabarcoding reveals that African leopard diet varies between habitats

AbstractUnderstanding carnivores’ diet is key to understanding their adaptability in a rapidly changing world. However, studying diet of large carnivores is difficult due to their elusive nature. In this study, we performed DNA metabarcoding analyses of 82 putative leopard scats collected from two distinct, but connected, habitat types (rainforest and grassland) in the Udzungwa Mountains, Tanzania. Two mitochondrial markers were used to identify predator and prey. Metabarcoding confirmed that 60 of the collected scats (73%) originated from leopards, and nineteen mammalian prey DNA sequences were identified to species. Using prey size correction factors for leopards, and covariates on habitat type and prey ecology, we investigated whether differences in leopard dietary composition were detectable between habitats. We found that leopards in grassland consumed a larger mean prey size compared with leopards in rainforest. Small prey (<19 kg) constituted >70% of the biomass consumed by leopards in rainforest, while large prey (≥80 kg) were only eaten in grassland. Arboreal species constituted 50% of the biomass consumed by rainforest leopards. Our results highlight the importance of arboreal species in their diet. From a management perspective, we suggest continued protection of all prey species in the protected areas to prevent human–wildlife conflicts.

Feeding patterns of transforming and juvenile myctophids that migrate into neustonic layers

Adult and juvenile myctophids feed at night in the epipelagic zone (<200 m) and are more dispersed in the mesopelagic zone (200-1000 m) during the daylight hours. In contrast, larvae inhabit the upper 200 m throughout a 24 h period and have daylight feeding patterns. Transforming stages occur both at the surface and in the mesopelagic zone and show less-defined feeding patterns. In this study, we analysed the trophic ecology of transforming and juvenile stages of 4 myctophids that occupy the neustonic layers (first 0-20 cm of the water column) during their nightly vertical migrations: Dasyscopelus asper, Gonichthys cocco, Myctophum affine, and M. nitidulum. Day and night neuston samples were collected across the equatorial and tropical Atlantic in April 2015. Transforming and juvenile stages occurred at night in the neuston, where they fed, but were absent from this layer during the day. The highest prey ingestion was observed between 01:00 and 04:00 h (UTC). Feeding incidence and the number of prey ingested increased from transformation stages to juvenile stages. Although the maximum prey size increased with fish body length, there was no significant increasing trend in mean prey sizes, but a great variability in the sizes of consumed prey. Diets of the 4 species mainly comprised a variety of copepod genera, usually dominated by Oncaea species. There was no evidence of resource partitioning among the 4 myctophid species. Estimates of daily feeding rations, based on the relationship between carbon content per gut and carbon content of fish body, throughout the night feeding period, showed that these species in these early stages ingested between 0.43 and 5.78% of their body carbon weight daily. We suggest that the occurrence and feeding of these early stages in the neuston may contribute to reducing trophic competition between migrating myctophids by space segregation.

COMPARATIVE DIET ANALYSIS OF BARN OWL (TYTO ALBA) AND LONG- EARED OWL (ASIO OTUS) IN A HABITAT ISLAND IN NORTH-CENTRAL TEXAS

AbstractPredator-prey interactions often define the coexistence of species in space and time. In this research, we are filling a regional gap in knowledge of diet composition of the barn owl (Tyto alba) and the long-eared owl (Asio otus). The study was conducted at the Lewisville Lake Environmental Learning Area (LLELA), a patch of tallgrass prairie and post oak forest surrounded by areas with large anthropogenic modifications in north-central Texas. We examined the remains found in owl pellets collected September 2016 to March 2017. Small mammals, mainly rodents, constituted 100% of the T. alba and 98.5% of the A. otus diet. The hispid cotton rat (Sigmodon hispidus) was the most common prey detected (68.2% for T. alba and 89.0% for A. otus). Diet niche breadth of T. alba was broader than that of A. otus (0.47 and 0.06, respectively), whereas mean prey size was smaller in the T. alba diet. Diet overlap between these owls was high (0.96), likely because they hunt similarly at night, on the wing, and the prey species have limited diversity. Most rodents eaten were juveniles and the sex ratio of prey was even in both owls’ diets. We highlight the value of conducting studies in such a unique and relatively pristine environment in the face of multiple threats. Additionally, although the occurrence of A. otus is rare and infrequent in the study area, we were still able to detect a difference in diet between the ubiquitous T. alba and A. otus.

Foraging consistency of coral reef fishes across environmental gradients in the central Pacific.

We take advantage of a natural gradient of human exploitation and oceanic primary production across five central Pacific coral reefs to examine foraging patterns in common coral reef fishes. Using stomach content and stable isotope (δ15N and δ13C) analyses, we examined consistency across islands in estimated foraging patterns. Surprisingly, species within the piscivore-invertivore group exhibited the clearest pattern of foraging consistency across all five islands despite there being a considerable difference in mean body mass (14g-1.4kg) and prey size (0.03-3.8g). In contrast, the diets and isotopic values of the grazer-detritivores varied considerably and exhibited no consistent patterns across islands. When examining foraging patterns across environmental contexts, we found that δ15N values of species of piscivore-invertivore and planktivore closely tracked gradients in oceanic primary production; again, no comparable patterns existed for the grazer-detritivores. The inter-island consistency in foraging patterns within the species of piscivore-invertivore and planktivore and the lack of consistency among species of grazer-detritivores suggests a linkage to different sources of primary production among reef fish functional groups. Our findings suggest that piscivore-invertivores and planktivores are likely linked to well-mixed and isotopically constrained allochthonous oceanic primary production, while grazer-detritivores are likely linked to sources of benthic primary production and autochthonous recycling. Further, our findings suggest that species of piscivore-invertivore, independent of body size, converge toward consuming low trophic level prey, with a hypothesized result of reducing the number of steps between trophic levels and increasing the trophic efficiency at a community level.

What are jaguars eating in a half-empty forest? Insights from diet in an overhunted Caatinga reserve

Persistence of top predators in protected areas requires healthy populations of prey species. Jaguars (Panthera onca) are top predators in the Caatinga dry forest, a xeric domain in northeastern Brazil. Poaching is a threat to populations of jaguar prey in the Caatinga, but it is still widely popular among locals. Here, we investigated molecularly identified jaguar scats to assess prey composition in a protected area where large prey has been heavily depleted or driven extinct by poaching. We also make direct comparisons between trophic niche width and mean prey size through a literature review. We show that over 90% of the diet of jaguars was comprised of prey under 5 kg, mainly armadillos. Furthermore, we found that the values of trophic niche width (2.21) and mean prey size (5.23 kg) in our study area are among the lowest ever described for jaguars in the literature. Our results demonstrated that jaguars are able to shift their diet to small prey when larger quarry is scarce. However, subsisting in such a stressful trophic position may lead to decreased levels of recruitment and low emigration rate. Breeding females would have difficulty raising cubs without abundant large prey. If jaguars are to persist in the Caatinga, effective actions to reduce poaching inside protected areas and corridors must be implemented. One of the most important jaguar populations in this domain inhabits our study site, and prey conservation is paramount for long-term persistence of this top predator in the Caatinga.

Influence of prey size on reproduction among populations of Diamond-backed Watersnakes (Nerodia rhombifer)

Mean prey size often varies across landscapes, resulting in predator populations having differing access to energetic resources. Shifts in resource quality are likely to cause differences in energy allocation of reproduction. Thus, additional energy intake may lead to (i) increased offspring size, (ii) increased numbers of offspring, (iii) increased relative proportion of energy allocated to reproduction, (iv) increased absolute amount of energy allocated to reproduction or (v) increased energy allocated to growth of some other nonreproductive function, or (vi) some combination of the above outcomes. To test this hypothesis, we evaluated the reproductive allocation patterns of four populations of Diamond-backed Watersnakes (Nerodia rhombifer (Hallowell, 1852)) that differ in their mean prey size. Snakes at large prey sites produced longer, heavier babies compared with snakes from small prey sites. Statistical interactions among sites confounded our ability to compare differences in litter size, litter mass, and relative clutch mass. We suggest that increased prey size results in populations shifting reproductive allocation to increase offspring size. Effects of prey size on litter size, litter mass, and relative clutch mass remain unclear.

Factors Affecting Foraging Microhabitat Selection by Wading Birds at an Artificial Weir

Abstract. To better understand how wading birds select among microhabitats that show spatial heterogeneity, a concrete weir located at the terminus of the Little Arkansas River in Wichita, Kansas, USA, was divided into patches based on water depth. Black-crowned Night-Herons (Nycticorax nycticorax; n = 396), Great Egrets (Ardea alba; n = 54), Snowy Egrets (Egretta thula; n = 36), and Great Blue Herons (Adrea herodias; n = 30) were observed for 60 hr from 10 June-29 July 2015. The total number of individuals was recorded, as was species-specific patch use, feeding activity, and aggression. Only time of day and water depth predicted the number of birds at the weir (R 2 = 0.52). Wading bird species differed in capture efficiency, mean prey size, and patch use. Great Blue Herons and Black-crowned Night-Herons mainly captured larger fish below the weir, Snowy Egrets captured mostly smaller fish along the weir edges, and Great Egrets captured both large and small fish. Mean fish length differed by patch and ran...

Sex and Age-Specific Variation In Provisioning By A Long Distance Neotropical Migrant, the Wood Thrush (Hylocichla mustelina)

ABSTRACT Parental investment is a unique balance between optimizing reproductive success and survival, yet how individuals vary their effort with respect to age and the contributions of their partner remains poorly studied. Here, we examine the provisioning of nestlings by Wood Thrushes (Hylocichla mustelina) at 38 nests in southern Indiana during the breeding season of 2013 to address the influence of adult age and sex on reproductive investment. We used video cameras to quantify an adjusted provisioning rate (provisioning rate = adult visits/hr/nestling × mean prey size × mean number of prey/visit) by individuals in both age-assortative (same age) and disassortative (different age) pairs (n = 33 pairs), hereafter referred to as pair type. Disassortative pairs delivered less prey than expected given nestling age, and sexes differed in their respective provisioning effort. In contrast, age-assortative pairs consistently provisioned more, and provisioning effort was comparable between the sexes regardless ...

Watch Out for Your Neighbor: Climbing onto Shrubs Is Related to Risk of Cannibalism in the Scorpion Buthus cf. occitanus.

The distribution and behavior of foraging animals usually imply a balance between resource availability and predation risk. In some predators such as scorpions, cannibalism constitutes an important mortality factor determining their ecology and behavior. Climbing on vegetation by scorpions has been related both to prey availability and to predation (cannibalism) risk. We tested different hypotheses proposed to explain climbing on vegetation by scorpions. We analyzed shrub climbing in Buthus cf. occitanus with regard to the following: a) better suitability of prey size for scorpions foraging on shrubs than on the ground, b) selection of shrub species with higher prey load, c) seasonal variations in prey availability on shrubs, and d) whether or not cannibalism risk on the ground increases the frequency of shrub climbing. Prey availability on shrubs was compared by estimating prey abundance in sticky traps placed in shrubs. A prey sample from shrubs was measured to compare prey size. Scorpions were sampled in six plots (50 m x 10 m) to estimate the proportion of individuals climbing on shrubs. Size difference and distance between individuals and their closest scorpion neighbor were measured to assess cannibalism risk. The results showed that mean prey size was two-fold larger on the ground. Selection of particular shrub species was not related to prey availability. Seasonal variations in the number of scorpions on shrubs were related to the number of active scorpions, but not with fluctuations in prey availability. Size differences between a scorpion and its nearest neighbor were positively related with a higher probability for a scorpion to climb onto a shrub when at a disadvantage, but distance was not significantly related. These results do not support hypotheses explaining shrub climbing based on resource availability. By contrast, our results provide evidence that shrub climbing is related to cannibalism risk.

Predicting the diet of coastal fishes at a continental scale based on taxonomy and body size

Predicting diet of animals in ecological communities is necessary for a better understanding of trophic links and piecing together food webs to inform ecosystem-based management. A dietary model, Consume, was recently developed to predict detailed dietary information for fishes on the basis of fish identity and size. This model was field-tested over a continental scale, predicting community-level consumption for other temperate marine fish communities that differed in species composition and size structure. Using local stomach contents data to field-test predictions, accurate performance of the model was found across 14 locations around southern Australia. Prey type and mean prey size were predicted for fishes at new locations with high accuracy (mean percentage overlap between predicted and actual prey types=73%; r2 between predicted and observed mean prey size=89%) when trained with stomach contents data from subsets of sampled fishes at all locations. Model accuracy dropped, but was still respectable, when using training data only from one location (prey type accuracy=67%; mean prey size r2=56%). Prey type was more accurately predicted on the basis of consumer body size than species identity, while consumer family identity and size were needed for accurate prediction of mean prey size. The most important factors were evaluated by leaving out predictors (species, genus and family identity; size of consumer; habitat, location, ecoregion and biogeographic province). Exclusion of geographical location information resulted in little loss in accuracy. Our results highlight the need for consideration of consumer body size in trophic models, rather than binning species into functional groups solely on the basis of taxonomy. Application of Consume to situations where no dietary information exists, but at least fish family identity and size structure are known, will provide a novel mechanism for testing important ecological hypotheses and assessing trophic consequences of anthropogenically-induced changes in community structure.

Contributions of body size, habitat and taxonomy to predictions of temperate Australian fish diets

Using k- nearest neighbour procedures to predict prey type and linear models to predict mean prey size, we developed a 2-step dietary model based on the stomach contents of fish of known species, size and location from Western Port, Victoria (Australia). The model, nicknamed ‘Consume’, was used to assess the relative extent to which fish diet varied with body size, species identity, season, and location. Both prey type (mean overlap between predicted and actual prey types = 77%) and mean prey size (r 2 between predicted and observed mean prey size = 93%) were predicted with reasonable accuracy when species identity and length of consumer fish were known. The most important predictor for prey type was the size of the individual consumer, while the most important predictor for mean prey size was the consumer’s taxonomic identity. Predictors were individually removed from both k -nearest neighbour and linear models to assess their relative contributions to the model. Little loss of accuracy (1%) was evident when family rather than species identity was used for both prey type and mean prey size. Environmental information associated with the time and location of fish sampling (habitat, site and season) contributed only marginally to predictions of prey type. Use of the Consume model will allow for an improved understanding of community-level trophic pathways through the integration of prey type and size predictions for consumer fishes.

Trophic links and the relationship between predator and prey body sizes in food webs

The relationship between predator and prey body sizes is an important property of food webs with potential implications for community dynamics and ecosystem functioning. To shed more light on this issue I here analyze the relationships between prey size, predator size and trophic position of consumers, using body size estimates of 697 species in 52 food webs. First I show that the relationship between predator and prey body sizes across many systems can be different from, and potentially obscure the true relationship within systems. More specifically, when data from all webs are aggregated average prey size is positively correlated to predator size with a regression slope less than unity, suggesting that predators become less similar in size to their average prey the larger the predator is, and consequently that the relative size difference between a predator and its prey should increase with the trophic position of the consumer. However, despite this I find the predator-prey body mass ratio to be negatively correlated to the trophic position of the consumer within many webs. The reason for this is that the across-webs pattern is not representative for the within-web relationship. Second, I show that the pattern observed is not compatible with a simple null-model for the distribution of trophic links between predators and prey. The observed relationship between predator size and mean prey size is for most webs significantly steeper than that predicted by the cascade model. Furthermore, the observed relationship also deviates significantly (but less so) from an ecologically more realistic model for the distribution of trophic links (the niche model). The results contradict the traditional Eltonian paradigm that predator-prey body mass ratios do not vary consistently across trophic levels. It is concluded that more studies are needed to establish the generality of the results and explore its dynamical implications.

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

AbstractA complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (On= 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (meanML= 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected inδ15N andδ13C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, −17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, −17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.

Foraging and vulnerability traits modify predator–prey body mass allometry: freshwater macroinvertebrates as a case study

1. Predation is often size selective, but the role of other traits of the prey and predators in their interactions is little known. This hinders our understanding of the causal links between trophic interactions and the structure of animal communities. Better knowledge of trophic traits underlying predator-prey interactions is also needed to improve models attempting to predict food web structure and dynamics from known species traits. 2. We carried out laboratory experiments with common freshwater macroinvertebrate predators (diving beetles, dragonfly and damselfly larvae and water bugs) and their prey to assess how body size and traits related to foraging (microhabitat use, feeding mode and foraging mode) and to prey vulnerability (microhabitat use, activity and escape behaviour) affect predation strength. 3. The underlying predator-prey body mass allometry characterizing mean prey size and total predation pressure was modified by feeding mode of the predators (suctorial or chewing). Suctorial predators fed upon larger prey and had ˜3 times higher mass-specific predation rate than chewing predators of the same size and may thus have stronger effect on prey abundance. 4. Strength of individual trophic links, measured as mortality of the focal prey caused by the focal predator, was determined jointly by the predator and prey body mass and their foraging and vulnerability traits. In addition to the feeding mode, interactions between prey escape behaviour (slow or fast), prey activity (sedentary or active) and predator foraging mode (searching or ambush) strongly affected prey mortality. Searching predators was ineffective in capturing fast-escape prey in comparison with the remaining predator-prey combinations, while ambush predators caused higher mortality than searching predators and the difference was larger in active prey. 5. Our results imply that the inclusion of the commonly available qualitative data on foraging traits of predators and vulnerability traits of prey could substantially increase biological realism of food web descriptions.

Ontogenetic Dietary Shifts and Food Selection of Endemic <I>Squalius Carolitertii</I> (Actinopterygii: Cypriniformes: Cyprinidae) in River Tormes, Central Spain, in Summer

Background. The northern Iberian chub Squalius carolitertii (Doadrio, 1988) is a small endemic cyprinid inhabiting the rivers of the Iberian Peninsula. The knowledge of feeding patterns is essential to understand the ecological role of fish populations, helping to the development of conservation and management plans. The aim of the present study was to analyze the ontogenetic dietary shifts and food selection of S. carolitertii, contributing to knowledge of the feeding behaviour of this fish species. Materials and methods. Diet composition of S. carolitertii was compared to benthos and drift composition in a river of Central Spain (Ávila, River Tormes) using selectivity indices of Ivlev and Savage. The age of 57 S. carolitertii collected in August 2010 was determined by scale reading and by length frequency analyses (LFA) with the Petersen method. Maximum length of benthos, drift and prey invertebrates was measured for each item to establish whether prey-size selection depends upon the size-frequency distribution of available prey. Results. Detritus were found in 33 fish (57.9% of occurrence). Nymphs of Baetis spp. were the most abundant prey (46.6%) and were identified in the 49.1% of the stomachs. Moreover, Baetis spp. was selected positively from the benthos and drift by all age classes. Abundant potential prey items such as Epeorus spp. in the benthos and Simuliidae in the drift were negatively selected. Individuals without detritus in the gut contained more animal prey items than individuals with a dominance of detritus, and the frequency of occurrence of detritus decreased with the age. Mean prey size increased with fish size (r = 0.646, P < 0.001). Conclusion. Age-related diet shifts occur at three different levels: (1) frequency of occurrence of detritus decreases with fish age; (2) prey selection varied with fish age; and (3) mean prey size increased as fish size increased. The rejection of Epeorus spp. and Simuliidae suggests that other factors, apart of prey abundance, including site-specific prey accessibility, prey size, energetic selection criteria and prey preference of fishes play an important role in feeding behaviour of S. carolitertii. Prey-size selection is probably dependent on the size-frequency distribution of the available prey.

Body-Size Evolution on Islands: Are Adult Size Variations in Tiger Snakes a Nonadaptive Consequence of Selection on Birth Size?

Mean adult size has been used as the traditional measure of body size to explain trends of insular gigantism and dwarfism in a wide array of taxa. However, patterns of variation in body size at birth have received surprisingly little attention, leaving open the possibility that adult body-size differences are nonadaptive consequences of selection acting on neonate body size. Here I used an empirical and correlative approach to test this hypothesis in a mosaic of 12 island and mainland snake populations in Australia. Data collected on 597 adult and 1,084 neonate tiger snakes showed that (1) both adult and neonate mean body sizes varied strongly across populations; (2) prey diversity and size convincingly explained birth-size variations: birth size-notably, gape size-correlated with prey size; (3) neonate snout-vent length was significantly correlated with neonate gape size; and (4) neonate snout-vent length was significantly correlated with adult snout-vent length. Postnatal growth rates recorded under common-garden conditions differed across populations and were correlated with mean prey size. These data collectively suggest that (1) prey size is the main driver for the evolution of body size at birth in gape-limited predators, (2) adult size variations may reflect selective forces acting on earlier life stages, and (3) adult size variations may also reflect resource availability during ontogeny (notably, prey diversity).