Food resources and foraging habits of the common shrew,Sorex araneus: does winter food shortage explain Dehnel's phenomenon?

It is widely assumed that winter is a critical time for homeotherms because of decreased ambient temperatures coupled with reduced food supply. Shrews are excellent models for investigating overwintering strategies, not only because of their particularly small size, high energy requirements relative to their size and short fasting endurance, but also the dramatic reduction in body size (Dehnel's phenomenon) exhibited by soricine shrews in northern temperate winters. The cause of Dehnel's phenomenon is poorly understood but food supply is implicated. To test the hypothesis that winter at higher latitudes is a period of food shortage for small homeotherms, we compared feeding habits of common shrews, Sorex araneus, and abundance and biomass of their prey in winters and summers in northeastern Poland using scat analysis combined with pitfall and ground core sampling for invertebrates. Ground‐surface activity and numbers of invertebrates in pitfall traps were greatly reduced in winter but, contrary to prediction, no significant differences between winter and summer were found in total numbers and biomass of prey invertebrates in ground core samples. However, certain prey types changed seasonally with respect to numbers, biomass and distribution in the soil profile, which was reflected in shrews’ food composition and foraging behaviour. Dehnel's phenomenon appears not to be caused by reduction in total prey numbers and biomass, at least in our study area. Smaller body mass coupled with lowering of absolute food requirements may have important survival value in winter with its reduced numbers of certain major prey coupled with increased difficulty of locating and extracting invertebrates within the soil profile resulting in higher energetic costs of foraging.     

Prey size, prey nutrition, and food handling by shrews of different body sizes

We tested some predictions relating metabolic constraints offoraging behavior and prey selection by comparing food handlingand utilization in four sympatric shrew species: Sorex minutus(mean body mass = 3.0 g), S. araneus (8.0 g), Neomys anomalus(10.0 g), and N. fodiens (14.4 g). Live fly larvae, mealwormlarvae, and aquatic arthropods were offered to shrews as smallprey (body mass <0.1 g). Live earthworms, snails, and smallfish were offered as large prey (>0.3 g). The larvae werethe high-nutrition food (>8 kJ/g), and the other prey werethe low-nutrition food (<4 kJ/g). The smallest shrew, S.minutus, utilized (ate + hoarded) <30% of offered food,and the other species utilized >48% of food. The largerthe shrew, the more prey it ate per capita. However, highlyenergetic insect larvae composed 75% of food utilized by S.minutus and only >40% of the food utilized by the other species. Thus, inverse relationships appeared between shrewbody mass and mass-specific food mass utilization and betweenshrew body mass and mass-specific food energy utilization:the largest shrew, N. fodiens, utilized the least food massand the least energy quantity per 1 g of its body mass. Also,the proportion of food hoarded by shrews decreased with increase in size of shrew. With the exception of S. araneus, the sizeof prey hoarded by the shrews was significantly larger thanthe size of prey eaten. Tiny S. minutus hoarded and ate smallerprey items than the other shrews, and large N. fodiens hoardedlarger prey than the other shrews.     

The kestrel (Falco tinnunculus L.) in Berlin: investigation of breeding biology and feeding ecology

Approximately 200–250 pairs of kestrels (Falco tinnunculus) breed in Berlin, preferentially in nest boxes. From 2002 to 2004, ten monitoring sites (breeding sites) characterised by different housing structure, land utilisation, vegetation cover and degree of building density were studied in Berlin: four in the city centre, three in a mixed zone and three in the outskirts. All pairs bred in nest boxes, so the reproductive success could easily be determined. Pellets, and feathers of bird prey species, were collected during the breeding seasons, and the food spectrum was determined based on these remains. There was no significant difference in the reproductive success of the kestrels between the three zones. Data on the number of fledged young indicated a sufficient food supply. In total, 9 species of mice and shrews, 23 bird species and 31 beetle species were identified as prey of kestrels. Urban kestrels specialise in hunting birds if mice and shrews are not readily available, with the house sparrow (Passer domesticus) as the favoured prey bird. Of note are anthropogenic food items, such as cutlet bones, that were found only in the city centre. This shows that the kestrel can adapt itself to humans with regard to its diet. There was no urban gradient with regard to reproductive success, but there was with the composition of food, such as the domination of bird prey in the city centre. The number of individual items of bird prey decreased from the centre to the outskirts. In conclusion, the results show that the kestrel is an opportunistic species which survives well anywhere in the city of Berlin.

Why are shrews so small? The costs and benefits of small size in northern temperateSorex species in the context of foraging habits and prey supply

This paper reviews the ecological advantages and disadvantages of very small body size inSorex Linnaeus, 1758 shrews living at high latitudes with cold winters. It examines the feeding and foraging habits of small and large shrews in the context of prey supply, location of winter prey sources, territory requirements, habitat exploitation and inter-specific competition. Data on feeding habits and prey availability show that the major costs of small size are a reduction in food niche breadth and prey biomass resulting from restrictions on the type and size of prey eaten, and large territory requirements. Major benefits of small size are the ability to subsist on small, numerous and accessible arthropods with high encounter rates, enabling coexistence with larger congeners and exploitation of low-productivity habitats less suitable for larger earthworm-eating species. Small size, coupled with low per capita food intake, is shown to be of special adaptive value in cold winters when food supply is restricted mostly to small arthropods, and earthworms are few.     

Do Tengmalm’s Owls alter parental feeding effort under varying conditions of main prey availability?

We studied the diet composition and behavioural responses to variable food conditions in Tengmalm’s Owls (Aegolius funereus). The abundance of main prey (voles and mice) of owls was higher in the Ore Mountains, Czech Republic, than in the Kauhava region, Finland. We monitored nests continuously by a camera system to estimate the feeding frequency and to identify prey items provided to nestlings. We recorded 990 prey deliveries at six nests in the Ore Mountains and 1,679 prey deliveries at nine nests in the Kauhava region. Mice (Apodemus) and voles (Microtus and Clethrionomys) were the main foods of owls in the Ore Mountains, whereas voles (Clethrionomys and Microtus) and shrews (Sorex) were the main foods in the Kauhava region. In consequence, on average smaller prey items were brought to nestlings at the Finnish site. However, both absolute and relative (per one nestling) feeding frequency was higher in the Kauhava region, and the biomass available to individual nestlings did not differ between the two areas. Moreover, the Finnish and Czech pairs produced about the same number of fledglings. Our results suggest that male owls are able to maintain the amount of food required for chicks by switching to alternative prey, and to increase their prey delivery rates under conditions of reduced abundance of main food.     

Distribution and coexistence of shrews in patchy landscapes: A field test of multiple hypotheses

Despite the important role of shrews (Soricomorpha: Soricidae) in the functioning of ecosystems, as predators and prey, the effects of habitat loss and fragmentation on this guild of mammals are still unclear. We studied the distribution of 5 species (the greater white toothed shrew Crocidura leucodon; the lesser white toothed shrew Crocidura suaveolens; the pigmy shrew Sorex minutus; the Appennine shrew Sorex samniticus and the Etruscan shrew Suncus etruscus) in a fragmented landscape in central Italy.Shrews were trapped with pitfall traps made from plastic water bottles, the number of traps increased with patch size. A total of 170 individuals, of 5 species of shrews were captured. Shrews were widely distributed in our study area, however patch occupancy was determined mainly by vegetation and geometrical characteristics of the patches. Our data supports the hypotheses that patterns of habitat selection and the dynamics of seasonal abundance (habitat and temporal partitioning between similarly sized species) reduce competitive pressure, thus allowing coexistence of shrews in relatively species-rich assemblages, for such small amounts of habitat. The most important outcome of our results is the crucial role played by vegetation structure in determining distribution patterns. These results strongly suggest that measurements of the vegetation structure of habitat patches should always be included as explanatory variables when studying the distribution of shrews in fragmented landscapes.     

From feces to data: A metabarcoding method for analyzing consumed and available prey in a bird‐insect food web

Diets play a key role in understanding trophic interactions. Knowing the actual structure of food webs contributes greatly to our understanding of biodiversity and ecosystem functioning. The research of prey preferences of different predators requires knowledge not only of the prey consumed, but also of what is available. In this study, we applied DNA metabarcoding to analyze the diet of 4 bird species (willow tits Poecile montanus, Siberian tits Poecile cinctus, great tits Parus major and blue tits Cyanistes caeruleus) by using the feces of nestlings. The availability of their assumed prey (Lepidoptera) was determined from feces of larvae (frass) collected from the main foraging habitat, birch (Betula spp.) canopy. We identified 53 prey species from the nestling feces, of which 11 (21%) were also detected from the frass samples (eight lepidopterans). Approximately 80% of identified prey species in the nestling feces represented lepidopterans, which is in line with the earlier studies on the parids' diet. A subsequent laboratory experiment showed a threshold for fecal sample size and the barcoding success, suggesting that the smallest frass samples do not contain enough larval DNA to be detected by high‐throughput sequencing. To summarize, we apply metabarcoding for the first time in a combined approach to identify available prey (through frass) and consumed prey (via nestling feces), expanding the scope and precision for future dietary studies on insectivorous birds.     

First results on the feeding ecology of sympatric shrews (Insectivora: Soricidae) in the Tai National Park,Ivory Coast

The feeding ecology of a multi-species community of shrews inhabiting secondary forest and cacao-coffee plantations in the Tai National Park (Ivory Coast) was investigated. A total of 553 shrews were captured and 194 alimentary tracts were examined. Ten species were found, includingSylvisorex megalura and nine species ofCrocidura, forming a series with respect to body size. New ecological data on these little known African species are presented. All species of shrews ate a wide diversity of arthropods, with Coleoptera, Araneae, Formicidae and Diplopoda making the largest dietary contributions. Lumbricidae were eaten by two species.C. obscurior had an exceptionally long intestine for its size but there was no evidence of dietary specialisation in this or other shrew species. All species investigated ate predominantly small prey and there was no correlation between size of prey items consumed and body mass of shrew species. There was little evidence of resource partitioning amongst the shrews, despite differences in body size.     

Do predators limit the abundance of alternative prey? Experiments with vole‐eating avian and mammalian predators

Predation has been invoked as a factor synchronizing the population oscillations of sympatric prey species, either because predators kill prey unselectively (the Shared Predation Hypothesis; hereafter SPH), or because predators switch to alternative prey after a density decline in their main prey (the Alternative Prey Hypothesis; APH). A basic assumption of the APH is that the impact of predators on alternative prey depends more on the density of main prey than on the predator/alternative prey ratio. Both SPH and APH assume that the impact of predators on alternative prey is at least periodically strong enough to depress prey populations. To examine these assumptions, we utilized data from replicated field experiments in large areas where we reduced the breeding densities of avian predators during three years and the numbers of least weasels (Mustela nivalis) in two years when vole populations declined. In addition, we reduced the breeding densities of avian predators in two years when vole populations were high. The reduction of least weasels increased the abundance of their alternative prey, small birds breeding on the ground, but did not affect the abundance of common shrews (Sorex araneus). In years when vole populations declined, the reduction of avian predators increased the abundance of their alternative prey, common shrews and small birds. Therefore, vole‐eating predators do at least periodically depress the abundance of their alternative prey. At high vole densities, the reduction of avian predators did not increase the abundance of common shrews, although the ratio of avian predators to alternative prey was similar to years when vole populations declined, which supported APH. In contrast, the abundance of small birds increased after the reduction of avian predators also at high vole densities, which supported SPH. The manipulations had no obvious effect on the number of game birds, which are only occasionally killed by these small‐sized predators. We conclude that in communities where most predators are small or specialize on a single prey type, the synchronizing impact of predation is restricted to a few similar‐sized species.     

The neurobiology and behavior of the American water shrew (Sorex palustris)

American water shrews (Sorex palustris) are aggressive predators that dive into streams and ponds to find prey at night. They do not use eyesight for capturing fish or for discriminating shapes. Instead they make use of vibrissae to detect and attack water movements generated by active prey and to detect the form of stationary prey. Tactile investigations are supplemented with underwater sniffing. This remarkable behavior consists of exhalation of air bubbles that spread onto objects and are then re-inhaled. Recordings for ultrasound both above and below water provide no evidence for echolocation or sonar, and presentation of electric fields and anatomical investigations provide no evidence for electroreception. Counts of myelinated fibers show by far the largest volume of sensory information comes from the trigeminal nerve compared to optic and cochlear nerves. This is in turn reflected in the organization of the water shrew’s neocortex, which contains two large somatosensory areas and much smaller visual and auditory areas. The shrew’s small brain with few cortical areas may allow exceptional speed in processing sensory information and producing motor output. Water shrews can accurately attack the source of a water disturbance in only 50 ms, perhaps outpacing any other mammalian predator.     

Epigeal movement of the smoky shrewSorex fumeus following precipitation in ridgetop and streamside habitats

Epigeal movement of smoky shrewsSorex fumeus (Miller, 1895) following precipitation was examined in two habitats with different moisture conditions. Shrews and invertebrates were collected in pitfall traps over several consecutive nights each month from August to November 1996 and from March to August 1997. Capture rates of smoky shrews significantly increased following rainfall at dry ridgetop sites (p=0.001) but not at moist streamside sites (p=0.335). In mesic environments, favorable conditions on the forest floor not associated with precipitation may increase movements of shrews. Available invertebrate biomass did not increase significantly with rainfall in either habitat type (p=0.121 and 0.368). Increased surface activity by smoky shrews after rain events is probably related more to their ecophysiology than to increased prey availability.     

A review of the status of the New Zealand flatworm in the UK

The indigenous terrestrial planarian fauna (three species) of the UK is outnumbered by introduced exotics, of which there are at least nine species. The New Zealand flatworm, Arthurdendyus triangularis, is one of the most widespread and apparent of these non-indigenous earthworm predators, particularly in Northern Ireland and central Scotland. Despite its having been in the UK for at least 35 yr, our knowledge of the biology and ecology of this species remains somewhat limited. In Scotland, A. triangularis occurs predominantly in botanical and domestic gardens, and is not generally considered to be a problem on agricultural land. The situation in Northern Ireland is different; although predominantly found in domestic gardens, it appears to have colonised grass leys many localities although the impact on earthworm populations remains ambiguous. In England, records are increasing, predominantly from northern regions. Studies have indicated that earthworm species vary in terms of their vulnerability to predation by A. triangularis; surface-active and anecic species are considered to be most at risk. However, A. triangulatus and earthworm populations are known to coexist, apparently in a state of dynamic equilibrium, in a number of localities. Naturally-occurring planarian populations are often severely constrained by food supply, but individuals survive lengthy periods of starvation. Such behaviour may preclude the natural recolonisation of habitats by prey species. Very much higher rates of population growth can be achieved where food is not limited, and where favourable conditions result from horticultural practices. The mobility of the prey species may determine the dispersal strategy of the flatworm predator, and a propensity not to move away from centres of prey density might account for the relatively low rate of colonisation of agricultural land by A. triangulatus. Additionally, reliance on protected refuges, may explain the present, somewhat limited distribution in the UK, particularly in southern England. It is difficult to estimate the potential distribution of A. triangulatus, because of the lack of fundamental knowledge of its ecoclimatic requirements. A better understanding is needed of the precise habitat requirements (and constraints) of A. triangulatus, particularly where this species appears to have adapted to rural conditions.     

Trophic relationships of common frog (Rana temporaria) and pigmy shrew (Sorex minutus) in upland Co. Antrim, Northern Ireland

The common frog and the pigmy shrew are common vertebrate predators in the uplands of Co. Antrim, Northern Ireland. This report concerns their habitat preference and trophic biology which are poorly documented at higher altitudes. Frogs preferred damp, grassy, reclaimed habitats while pigmy shrews appeared to favour moorland dominated by Calluna vulgaris. Both frogs and shrews were absent from pitfall trap collections from December to February and numbers of both were low until late spring. This corresponded to the period of lowest invertebrate abundance. Examination of stomach contents suggests that frogs are less selective feeders and utilize a greater diversity of prey items than pigmy shrews. Although sample size is low early in the year, diversity of the diet of pigmy shrews apparently increased from the first to the second quarter and again in the third quarter of the year. This led to an increase in food niche overlap with frogs in late summer but at a time of year when food supply was greatest. It is unlikely, therefore, that dietary overlap constitutes a basis for interspecific competition between frogs and shrews. This is likely to be ameliorated by habitat and behavioural differences between the two predators.     

Artificial light affects the foraging behavior in greater white-toothed shrews (CROCIDURA RUSSULA)

Light pollution is one of the forms by which human-induced alterations are changing natural environments. Artificial light at night (ALAN) has been increasing over the past decades and it is already known that ALAN can have a major influence on the ecology, behavior, and physiology of different taxa. Nocturnal small mammals are particularly vulnerable, as ALAN can increase their predation risk while foraging. The aim of this study was to investigate foraging strategies under different light conditions in a nocturnal small insectivore, the greater white-toothed shrew (Crocidura russula). Compared with rodents, shrews have a higher metabolic rate and thus present a good model for a foraging study. In three laboratory experiments with wild-caught shrews we tested (i) food preference under dark conditions as well as the effect of different light conditions on (ii) foraging strategies and (iii) food choice. The results showed that shrews had a clear food preference under dark conditions. They also preferred to forage under dark over light conditions when presented with the same food in both conditions. However, when presented with the choice of foraging their preferred food under illuminated conditions or a lower food quality in the dark, the food preference of shrews overruled their preference for feeding in the dark. It seems that food preference, rather than risk perception, is the main driver determining the foraging strategy of the greater white-toothed shrews. This study suggests that ALAN does not necessarily prevent high-metabolic nocturnal insectivores from achieving their energetic needs, which might help explain their persistence in urban environments.     

Convergence and divergence in prey of sympatric canids and felids: opportunism or phylogenetic constraint?

Since the canids and felids diverged in the mid‐Eocene or earlier, each family has developed a suite of morphological and behavioural adaptations for obtaining and consuming prey. We here distinguish between prey taxa captured and eaten as a result of these phylogenetic adaptations, and those because they are fortuitously encountered, and argue that such supplementary prey, often opportunistically caught, create a buffer between sympatric, and potentially competitive, canids and felids and thus enhance coexistence. We base our analysis on dietary data derived from the stomach contents of four sympatric canid and felid species in the Free State Province, South Africa (canids: Cape fox Vulpes chama and black‐backed jackal Canis mesomelas; felids: African wild cat Felis silvestris lybica and caracal Caracal caracal), and from results of studies on these species elsewhere in southern Africa. The two canid species preyed heavily on invertebrates, and thus opportunistically, while the felids (especially the caracal) concentrated on mammals, prey they are phylogenetically adapted to capture. Only three species of mammalian prey are shared by the four species. The ratio of opportunistically‐to‐phylogenetically mediated prey taxa used (the O/P ratio) differ between the species, with the black‐backed jackal having the most opportunistically caught taxa in its diet, and the caracal the least. As predicted, a comparison of this data with those from dietary studies of the same species carried out elsewhere indicates that the number of opportunistically obtained prey taxa varies more than those resulting from phylogenetic adaptations. The largest canid had the widest food spectrum (35 prey taxa) while the smallest felid had the most restricted one (11 prey taxa). We argue that using the O/P distinction allows a better understanding of changes in food niche breadth of particular species, especially in xeric areas, and gives a better indication of possible exploitative competition for food by sympatric carnivores than when regarding all prey taxa as actively pursued. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83 , 527–538.     

The occurrence of the Pigmy shrew Sorex minutus on moorland, and the implications for its presence in Ireland

Throughout most of its range in western Europe, the Pigmy shrew Sorex minutus is sympatricwith the Common shrew Sorex araneus but greatly outnumbered by it. This makes it difficult to explain the presence of the Pigmy shrew, in the absence of its congener, in Ireland. A limited pitfall trapping exercise in England has shown that in moorland on deep peat, the Pigmy shrew may in fact outnumber the Common shrew. Reference to the literature on the ecology of the two shrews shows that this result is readily explained by existing knowledge of their diet; earthworms are a major prey for Common shrews and are absent from waterlogged, acidic, peats. Further, evidence on the nature of the possible land bridge from Scotland to Ireland suggests that it was at best low-lying, and probably partly flooded. By analogy with moorland, such a land bridge might have been suitable for Pigmy shrews but not for Common shrews.     

Habitat occurrence and prey distribution of a multi-species community of shrews in the Siberian taiga

The habitat occurrence and invertebrate prey distribution of nine species of shrew in the mid-taiga of central Siberia were investigated. Species richness ranged from 4–9 shrews per habitat. Sorex araneus and S. caecutiens were numerically dominant in all seven habitats (44 and 36% of the total catch, respectively) while Sorex minutus, S. tundrensis, S. isodon , and S. roboratus each constituted 4–6% and Sorex minutissimus, S. daphaenodon , and Neomys fodiens were rare (< 1% each). There was no overall correlation between abundance of shrews and invertebrate prey, but flood-plain habitats supported the greatest abundance and species richness of shrews, and high density and biomass of prey. Oligochaete-eating shrews were twice as numerous here as in other habitats, coincident with high abundance of oligochaetes. The large, earthworm-feeding Sorex roboratus occurred only here. The more acid, typical taiga habitats had lower adundance and species richness of shrews. They had the lowest density and biomass of prey, particularly oligochaetes, and far fewer oligochaete-eating shrews. The relative paucity of shrews in bush-meadow habitats, despite abundant prey, implied that habitat structure influences shrew distribution. Differential numbers of certain species in the presence or absence of larger congeners also suggested that interspecific competitive effects influence habitat selection by shrews. The high species richness of shrews here in the mid-taiga may be accounted for by the heterogeneous nature of the constituent habitats which provide niches for small and large species of shrew with a range of feeding habits.     

Barrelettes without Barrels in the American Water Shrew

Water shrews (Sorex palustris) depend heavily on their elaborate whiskers to navigate their environment and locate prey. They have small eyes and ears with correspondingly small optic and auditory nerves. Previous investigations have shown that water shrew neocortex is dominated by large representations of the whiskers in primary and secondary somatosensory cortex (S1 and S2). Flattened sections of juvenile cortex processed for cytochrome oxidase revealed clear borders of the whisker pad representation in S1, but no cortical barrels. We were therefore surprised to discover prominent barrelettes in brainstem of juvenile water shrews in the present investigation. These distinctive modules were found in the principal trigeminal nucleus (PrV), and in two of the three spinal trigeminal subnuclei (interpolaris – SpVi and caudalis – SpVc). Analysis of the shrew''s whisker pad revealed the likely relationship between whiskers and barrelettes. Barrelettes persisted in adult water shrew PrV, but barrels were also absent from adult cortex. Thus in contrast to mice and rats, which have obvious barrels in primary somatosensory cortex and less clear barrelettes in the principal nucleus, water shrews have clear barrelettes in the brainstem and no barrels in the neocortex. These results highlight the diverse ways that similar mechanoreceptors can be represented in the central nervous systems of different species.     

Predator‐prey migration phenologies remain synchronised in a warming catchment

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Chemical cues from multiple predator-prey interactions induce changes in behavior and growth of anuran larvae

Chemical signals are used as information by prey to assess predation risk in their environment. To evaluate the effects of multiple predators on prey growth, mediated by a change in prey activity, I exposed small and large bullfrog (Rana catesbeiana) larvae (tadpoles) to chemical cues from different combinations of bluegill sunfish (Lepomis macrochirus) and larval dragonfly (Anax junius) predators. Water was regularly transferred from predation trials (outdoor experiment) to aquaria (indoor experiment) in which activity and growth of tadpoles was measured. The highest predation mortality of small bullfrog larvae in the outdoor experiment was due to Anax, and it was slightly lower in the presence of both predators, probably resulting from interactions between predators. There was almost no mortality of prey with bluegill. The activity and growth of small bullfrog larvae was highest in the absence of predators and lowest in the presence of Anax. In the presence of bluegill only, or with both predators, the activity and growth of small bullfrog tadpoles was intermediate. Predators did not affect large tadpole activity and growth. Regressing mortality of small bullfrog tadpoles against activity and growth of bullfrog tadpoles revealed a significant effect for small bullfrog larvae but a non-significant effect for large bullfrog larvae. This shows that the response of bullfrog tadpoles to predators is related to their own body size. The experiment demonstrates that chemical cues are released both as predator odor and as alarm substances and both have the potential to strongly alter the activity and growth of prey. Different mechanisms by which chemical cues may be transmitted to species interactions in the food web are discussed. Received: 28 June 1999 / Accepted: 15 November 1999