The role of thermoregulation in lizard biology: Predatory efficiency in a temperate diurnal basker

Summary Decreasing levels of simulated solar radiation have the following effects in the diurnal basking lizard Lacerta vivipara: (i) increase in time spent basking with a consequent decrease in time available for foraging (ii) decrease in speed of movement whilst foraging (iii) decrease in total foraging distance, and hence contact with potential prey (iv) decrease in searching efficiency in an experimental arena. Complete absence of simulated solar radiation accentuates these effects, and reduces the proportion of faster-moving prey in the diet. Time taken to swallow prey (handling time) increases exponentially with decreasing body temperature. It is concluded from these results that maintaining relatively high activity temperatures (30–36°C) is adaptive for the species because the loss of potential foraging time caused by lengthy periods of basking is offset by the following advantages: increased contact with and capture of prey, increased efficiency of prey handling, and availability of a wider range of prey types.     

Parent-offspring aggression in moorhens

Summary Colonial orb-weaving spiders from Mexico were studied to test predictions of risksensitive foraging theory: 1. group foraging increases prey capture/individual, and reduces prey variance; 2. spiders should be expected to exhibit risk-averse behavior (forage in groups) when the average level of prey exceeds individual needs, and exhibit risk-prone behavior (forage solitarily) when prey are searce. Laboratory and field studies show that group foraging increases capture efficiency and reduces variability in prey captured per spider. In desert/mesquite grassland habitat, where prey availability is low, M. atascadero forage solitarily in most cases. In tropical rainforest/agriculture sites, M. increassata forage in large colonies of thousands of webs. In intermediate habitats, M. spinipes forages solitarily or in groups, depending on prey availability. Over a range of sites with varying levels of prey, M. spinipes shifts from a risk-prone to a risk-averse group foraging strategy as prey increases.Group foraging behavior observed in colonial Metepeira fits the predictions of risk-sensitive foraging models. These findings explain why spiders tend to group webs together only in areas of superabundant prey. The role of risk-sensitivity in the evolution of coloniality in spiders is discussed.     

Central place food caching: a field experiment with red-backed shrikes (Lanius collurio L.)

Summary Red-backed shrikes (Lanius collurio L.) stored experimentally presented mice (Mus musculus L) by reimpaling them on thorns of sloe bushes (Prunus spinosa L.) in the vicinity of the nest. Large mice (20 g) were stored further away from the nest than small mice (4 g), while the smallest mice (1 g) were transported directly to the nest and cut up there. Large prey required more round trips to deliver than smaller prey (4 g). Time to immobilize, load and deliver the prey and the proportion of undigestable tissue increased with prey size. Birds used stored prey as an alternative to hunting in other patches as expected from patch use models. The birds maintained a high rate of food delivery to the young by using stored mice during periods when their foraging success of natural prey was low. Several aspects of the shrike's food storing behaviour are in qualitative accord with suggestions derived from models assuming maximization of energy delivery rate.     

Forager specialization and the control of nest repair in Polybia occidentalis Olivier (Hymenoptera : Vespidae)

We measured patterns of individual forager specialization and colony-wide rates of material input during periods of response to experimental nest damage and during control periods in three colonies of the tropical social wasp Polybia occidentalis.

Movement patterns of the seastar Heliaster helianthus in central Chile: relationship with environmental conditions and prey availability

The rates and patterns of feeding and displacement of predators constitute two of the most important plastic behavioral responses that allow individuals to respond quickly to changes in abundance of their prey, predation risks and to rapid alterations in environmental conditions. In this study, we quantified seasonal and spatial variation in displacement (net changes in location in 12 or 24 h periods) and prey consumed of marked individuals of the keystone seastar Heliaster helianthus at six sites spanning 600 km along the coast of north-central Chile. We evaluated the hypotheses that: (1) at sites with low availability (cover) of the main prey, the mussel Perumytilus purpuratus, Heliaster displays larger displacements and consumes a greater proportion of other prey (e.g. mobile species) than at sites with high mussel cover, (2) daily displacements will be correlated with sea surface temperature (SST) and (3) increased wave action will reduce seastar daily displacement. Our results show that Heliaster displacement is higher at sites with lower availability of P. purpuratus; and at these sites, a larger proportion of Heliaster individuals are observed feeding, mostly on other prey (e.g. limpets), which could offset the higher costs associated with increased movement. In addition, wave forces affected the activity of Heliaster negatively. Contrary to our expectations, the daily displacements did not show any relationship with SST measured on the day or the previous days of the surveys, despite the fact that average displacement was generally higher in summer than in winter months. Future studies should examine Heliaster movement during single foraging excursions and determine whether these responses affect the growth and reproductive output of individuals. Such information is vital to understand how changes in prey abundance and environmental conditions alter the behavior and energy budget of this predator and its ability to control prey populations.     

Prey selection in <Emphasis Type="Italic">Octopus rubescens</Emphasis>: possible roles of energy budgeting and prey nutritional composition

This study explores the relationship between energy budgeting and prey choice of Octopus rubescens. Seventeen male Octopus rubescens were collected between June 2006 and August 2007 from Admiralty Bay, Washington. Prey choices made by individuals in the laboratory deviated widely from those expected from a simple optimal foraging model. O. rubescens chose the crab Hemigrapsus nudus over the clam Nuttallia obscurata as prey by a ratio of 3:1, even though prey energy content and handling times suggested that this octopus could obtain 10 times more energy intake per unit time when choosing the latter compared to the former prey species. Octopus energy budgets were similar when consuming either of the prey species except for lipid extraction efficiency that was significantly higher in octopuses consuming H. nudus. This suggests that lipid digestibility may play an important role in the prey choice of O. rubescens.     

Diet of young Atlantic bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) in eastern and western Atlantic foraging grounds

Atlantic bluefin tuna (Thunnus thynnus) are highly migratory predators whose abundance, distribution, and somatic condition have changed over the past decades. Prey community composition and abundance have also varied in several foraging grounds. To better understand underlying food webs and regional energy sources, we performed stomach content and stable isotope analyses on mainly juvenile (60–150 cm curved fork length) bluefin tuna captured in foraging grounds in the western (Mid-Atlantic Bight) and eastern (Bay of Biscay) Atlantic Ocean. In the Mid-Atlantic Bight, bluefin tuna diet was mainly sand lance (Ammodytes spp., 29% prey weight), consistent with historic findings. In the Bay of Biscay, krill (Meganyctiphanes norvegica) and anchovy (Engraulis encrasicolus) made up 39% prey weight, with relative consumption of each reflecting annual changes in prey abundance. Consumption of anchovies apparently declined after the local collapse of this prey resource. In both regions, stable isotope analysis results showed that juvenile bluefin tuna fed at a lower trophic position than indicated by stomach content analysis. In the Mid-Atlantic Bight, stable isotope analyses suggested that >30% of the diet was prey from lower trophic levels that composed <10% of the prey weights based upon traditional stomach content analyses. Trophic position was similar to juvenile fish sampled in the NW Atlantic but lower than juveniles sampled in the Mediterranean Sea in previous studies. Our findings indicate that juvenile bluefin tuna targeted a relatively small range of prey species and regional foraging patterns remained consistent over time in the Mid-Atlantic Bight but changed in relation to local prey availability in the Bay of Biscay.     

Foraging tactics of an ambush predator: the effects of substrate attributes on prey availability and predator feeding success

The foraging sites selected by an ambush forager can strongly affect its feeding opportunities. Foraging cane toads (Rhinella marina) typically select open areas, often under artificial lights that attract insects. We conducted experimental trials in the field, using rubber mats placed under lights, to explore the influence of substrate color and rugosity on prey availability (numbers, sizes, and types of insects) and toad foraging success. A mat's color (black vs. white) and rugosity (smooth vs. rough) did not influence the numbers, sizes, or kinds of insects that were attracted to it, but toads actively preferred to feed on rugose white mats (50% of prey-capture events, vs. a null of 25%). White backgrounds provided better visual contrast of the (mostly dark) insects, and manipulations of prey color in the laboratory showed that contrast was critical in toad foraging success. Insects landing on rugose backgrounds were slower to leave, again increasing capture opportunities for toads. Thus, cane toads actively select backgrounds that maximize prey-capture opportunities, a bias driven by the ways that substrate attributes influence ease of prey detection and capture rather than by absolute prey densities.     

Feeding rate and duration of daily activity of Astropecten aranciacus (Echinodermata: Asteroidea) in relation to prey density

In a feeding experiment, 16 individuals of Astropecten aranciacus were kept in two enclosures on the sea floor, and were provided with a surplus of food. Feeding at a daily rate of 1.3% of their dry tissue weight, their food intake was four times higher than that of free-living sea stars. The periods of daily activity were shorter than in control individuals kept in enclosures without additional food.     

Foraging behaviour of sympatric Antarctic and subantarctic fur seals: does their contrasting duration of lactation make a difference?

The duration of periods spent ashore versus foraging at sea, diving behaviour, and diet of lactating female Antarctic (Arctocephalus gazella, AFS) and subantarctic (A. tropicalis, SFS) fur seals were compared at Iles Crozet, where both species coexist. The large disparity in lactation duration (SFS: 10 months, AFS: 4 months), even under local sympatry, has led to the expectation that AFS should exhibit higher foraging effort or efficiency per unit time than SFS to allow them to wean their pups in a shorter period of time. Previous evidence, however, has not supported these expectations. In this study, the distribution of foraging trip durations revealed two types of trips: overnight (OFT, <1 day) and long (LFT, >1 day), in common with other results from Macquarie Island. However, diving behaviour differed significantly between foraging trip types, with greater diving effort in OFTs than in LFTs, and diving behaviour differed between fur seal species. OFTs were more frequent in SFS (48%) than in AFS (28%). SFS performed longer LFTs and maternal attendances than AFS, but spent a smaller proportion of their foraging cycle at sea (66.2 vs. 77.5%, respectively). SFS dove deeper and for longer periods than AFS, in both OFTs and LFTs, although indices of diving effort were similar between species. Diel variation in diving behaviour was lower among SFS, which foraged at greater depths during most of the night time available than AFS. The diving behaviour of AFS suggests they followed the nychthemeral migration of their prey more closely. Concomitant with the differences in diving behaviour, AFS and SFS fed on the same prey species, but in different proportions of three myctophid fish (Gymnoscopelus fraseri, G. piabilis, and G. nicholsi) that represented most of their diet. The estimated size of the most important fish consumed did not vary significantly between fur seal species, suggesting that the difference in dive depth was mostly a result of changes in the relative abundance of these myctophids. The energy content of these fish at Iles Crozet may thus influence the amount and quality of milk delivered to pups of each fur seal species. These results contrast with those found at other sites where both species coexist, and revealed a scale of variation in foraging behaviour which did not affect their effort while at sea, but that may be a major determinant of foraging efficiency and, consequently, maternal investment.     

Linking marine predator diving behavior to local prey fields in contrasting habitats in a subarctic glacial fjord

Foraging theory predicts that animals will adjust their foraging behavior in order to maximize net energy intake and that trade-offs may exist that can influence their behavior. Although substantial advances have been made with respect to the foraging ecology of large marine predators, there is still a limited understanding of how predators respond to temporal and spatial variability in prey resources, primarily due to a lack of empirical studies that quantify foraging and diving behavior concurrently with characteristics of prey fields. Such information is important because changes in prey availability can influence the foraging success and ultimately fitness of marine predators. We assessed the diving behavior of juvenile female harbor seals (Phoca vitulina richardii) and prey fields near glacial ice and terrestrial haulout sites in Glacier Bay (58°40′N, ?136°05′W), Alaska. Harbor seals captured at glacial ice sites dived deeper, had longer dive durations, lower percent bottom time, and generally traveled further to forage. The increased diving effort for seals from the glacial ice site corresponded to lower prey densities and prey at deeper depths at the glacial ice site. In contrast, seals captured at terrestrial sites dived shallower, had shorter dive durations, higher percent bottom time, and traveled shorter distances to access foraging areas with much higher prey densities at shallower depths. The increased diving effort for seals from glacial ice sites suggests that the lower relative availability of prey may be offset by other factors, such as the stability of the glacial ice as a resting platform and as a refuge from predation. We provide evidence of differences in prey accessibility for seals associated with glacial ice and terrestrial habitats and suggest that seals may balance trade-offs between the costs and benefits of using these habitats.     

Age-related shifts in the diet composition of southern elephant seals expand overall foraging niche

Southern elephant seals are important apex predators in a highly variable and unpredictable marine environment. In the presence of resource limitation, foraging behaviours evolve to reduce intra-specific competition increasing a species’ overall probability of successful foraging. We examined the diet of 141 (aged 1–3 years) juvenile southern elephant seals to test the hypotheses that differences between ages, sexes and seasons in diet structure occur. We described prey species composition for common squid and fish species and the mean size of cephalopod prey items for these age groups. Three cephalopod species dominated the stomach samples, Alluroteuthis antarcticus, Histioteuthis eltaninae and Slosarczykovia circumantarcticus. We found age-related differences in both species composition and size of larger prey species that probably relate to ontogenetic changes in diving ability and haul-out behaviour and prey availability. These changes in foraging behaviour and diet are hypothesised to reduce intra-specific food competition concomitant with the increase in foraging niche of growing juveniles.     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

Prey chemical discrimination and strike-induced chemosensory searching in the lizardLiolaemus zapallarensis

Summary Experimental tests were conducted to determine whether the ambush foraging iguanian lizard,Liolaemus zapallarensis, was capable of discriminating prey chemicals from control substances and whether this lizard exhibits strike-induced chemosensory searching (SICS) or its components after biting prey. The two components of SICS are a poststrike elevation in tongue-flicking rate (PETF) and apparent searching movements for relocation of prey that has been bitten, but released or escaped.Liolaemus zapallarensis failed to discriminate prey chemicals from control substances, but exhibited significant PETF lasting one minute. SICS was absent inL. zapallarensis because no post-strike movements were observed. The absence of both prey chemical discrimination and SICS exhibited byL. zapallarensis is common to all the insectivorous iguanians and ambush foraging lizards studied to date. However,L. zapallarensis is the first insectivorous iguanian species shown to exhibit PETF. The results suggest thatL. zapallarensis does not use the tongue for detection, identification, or relocation of prey while foraging. The possibility does remain thatL. zapallarensis may be capable of chemically identifying prey once the prey stimuli reach the oral cavity.     

Individual specialization in the hunting wasp <Emphasis Type="Italic">Trypoxylon</Emphasis> (<Emphasis Type="Italic">Trypargilum</Emphasis>) <Emphasis Type="Italic">albonigrum</Emphasis> (Hymenoptera,Crabronidae)

Individual-level variation in resource use occurs in a broad array of vertebrate and invertebrate taxa and may have important ecological and evolutionary implications. In this study, we measured the degree of individual-level variation in prey preference of the hunting wasp Trypoxylon albonigrum, which inhabits the Atlantic Forest in southeastern Brazil. This wasp captures several orb-weaving spider genera to provision nests. Individuals consistently specialized on a narrow subset of the prey taxa consumed by the population, indicating the existence of significant individual-level variation in prey preferences. The population niche was broader in the wet season in terms of both prey size and taxa. In the case of prey size, the population niche expansion was achieved via increased individual niche breadths, whereas in the case of prey taxa, individual niches remained relatively constrained, and the population niche expanded via increased interindividual variation. The observed pattern suggests the possibility of functional trade-offs associated with the taxon of the consumed prey. The nature of the trade-offs remains unknown, but they are likely related to learning in searching and/or handling prey. We hypothesize that by specializing on specific prey taxa, individuals increase foraging efficiency, reducing foraging time and ultimately increasing reproductive success.     

Persistent bimodal activity patterns in wild and captive black-tailed godwit <Emphasis Type="Italic">Limosa limosa</Emphasis> under different environmental conditions

There is a large literature dealing with daily foraging routines of wild birds during the non-breeding season. While different laboratory studies have showed that some bird activity patterns are a persistent property of the circadian system, most of field studies preclude the potential role of an endogenous circadian rhythm in controlling bird’s foraging routines. In this study we compared the patterns of diurnal foraging activity and intake rates of migrating black-tailed godwits, Limosa limosa (radio-tagged and non-tagged individuals) at two stopover sites (habitats) with different environmental characteristics, aiming at identifying proximate factors of bird activity routines. To gain insights into the role of food availability in control of such foraging routines, we also estimated foraging activity patterns in captive godwits subjected to constant food availability. Captive and wild black-tailed godwits showed a persistent bimodal activity pattern through daylight period. Food availability had a significant effect on the intake rates, but had a subtler effect on foraging and intake rate rhythms. Temperature and wind speed (combined in a weather index) showed non-significant effects on both rhythms. Although we could not discard a role for natural diurnal changes in light intensity, an important timing cue, our findings support the idea that an endogenous circadian rhythm could be an important proximate factor regulating foraging activity and food items taken per unit time of wild black-tailed godwits during migration.     

Foraging behaviour and brain morphology in recently emerged brook charr, Salvelinus fontinalis

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams are either active, feeding on insects from the upper portion of the water column away from the stream bank, or sedentary, feeding on crustaceans emerging from the hyporheic zone near the stream bank. We tested whether the frequency of movement displayed by individual brook charr searching for prey in the field was related to the relative volume of the telencephalon, a brain region involved with movement and space use in fishes. Movement of individuals searching for prey was quantified in the field, individuals were captured and volumes of the telencephalon and of the olfactory bulbs, a brain region neighbouring the telencephalon but not implicated in space use, were measured. Individuals with larger telencephalon volumes moved more frequently on average while searching for prey in the field than did individuals with smaller telencephalon volumes. The frequency of movement was unrelated to differences in the volume of the olfactory bulbs, suggesting that the relationship between telencephalon volume and movement was not a consequence of differences in overall brain size. Demonstrating a correlation between foraging behaviour and brain morphology for brook charr exhibiting different foraging tactics suggests that diversification in brain structure and function could be important aspects of the foraging specialization believed to occur during early stages in the evolution and development of resource polymorphisms.     

Diurnal and nocturnal feeding rate in Kentish plovers <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis> on an intertidal flat as recorded by telescopic video systems

To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements. An erratum to this article can be found at     

Quantitative analysis of foraging behaviour in a field population of the South American sun-star Heliaster helianthus

Foraging behaviour in the South American sunstar Heliaster helianthus (Lamarck) was investigated quantitatively on a subtropical rocky shore in central Peru. H. helianthus feeds mainly on two mussel species, Semimytilus algosus (Gould) and Perumytilus purpuratus (Lamarck). A sequence of foraging behaviour was described and observations were made of the timing of attacks on mussel beds by sun-stars. Although H. helianthus is capable of foraging out of water, an unusual trait for asteroids, the timing of such foraging appears to be well adjusted to avoid the risk of prolonged heat and desiccation. Foraging activity began 4 h 52 min before high tide and 3 h 19 min after high tide, with a mode between 2 h 30 min and 2 h before high tide. Fifty-two percent of all foraging activity began between 3 h 30 min and 2 h before high tide, while only 12.4% began after high tide. This suggests that H. helianthus mainly relies upon changes in the rate of tidal increase as a cue to begin foraging. Foraging activity ceased between 3 h 40 min before and 4 h 29 min after high tide, with 48% ending between 2 and 0.5 h before high tide. The duration of foraging ranged from 19 to 190 min, with values between 30 and 80 min accounting for 67.4% of all observations. The median duration was 62 min. No significant correlation was detected between the time when foraging activity commenced and its duration. The intensity of foraging activity varied on consecutive days, with a general pattern of decreasing intensity after a day of relatively high activity. Foraging location in relation to a mussel bed was analysed on a marked, 8 m stretch of rocky shore. The numbers of foraging H. helianthus observed on different sections of the shore were related neither to the width of the mussel zone nor to the vertical position of the lower edge of the mussel zone, indicating that sun-stars do not rely upon these factors to assess prey availability and that ideal free distribution with regard to prey abundance does not occur on the spatial scale examined.     

Sea otter foraging on deep-burrowing bivalves in a California coastal lagoon

Sea otter, Enhydra lutris, predation had no detectable effect on abundance and size distribution of deep-burrowing bivalve prey in the Elkhorn Slough, California, USA. Up to 23 otters were present for 6 mo of the study period (March 1984 through April 1985). This is in contrast to previous studies of sea otter predation, especially on the shallow-burrowing Pismo clam Tivela stultorum, which can be found along the wave-exposed coast near the slough. The deep-burrowing clams Tresus nuttallii and Saxidomus nuttalli made up 61% of the prey taken in the slough, and are more difficult for otters to excavate than Pismo clams. The occurrence of foraging otters was highest in an area where the two bivalve prey were extremely abundant (18 individuals m^–2). However, the otters did not selectively prey on the largest clams available within the study sight, but foraged preferentially in a patch of smaller individuals where bivalve burrow depth was restricted by the presence of a dense clay layer. This foraging strategy maximized the amount of prey biomass obtained per unit volume of sediment excavated. Our findings suggest that in soft-sediment habitats deep-burrowing bivalves may be more resistant to otter predation than shallower burrowers.