Feeding ecology and diet

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Shrews feed mainly on soil invertebrates and as a whole may be seen as opportunistic and generalized invertebrate feeders, although some partitioning may occur among syn-

A masked shrew (Sorex cinereus) foraging. (Photo by Lynn Stone. Bruce Coleman, Inc. Reproduced by permission.)

topical species. No extreme specialist occurs, even though some shrews are more or less aquatic, which also is reflected in their diet. The fossil species Arctosorexpolaris from the late Neogene of Ellesmere Island might be the only shrew specialized for frugivory.

Sorex shrews form a well-defined guild of insectivorous mammals with opportunistic feeding habits and largely overlapping resource requirements. In a guild, up to six species may occur together in the same time and place. Competition is likely to be common in such a case. Major differences in dietary composition and foraging mode reflect body size in shrews. Small species are often epigeal, feed mainly on arthropods, and have a relatively narrow niche breadth. Large species are usually hypogeal, and feed on earthworms and other soil-dwelling invertebrates. Body size thus plays an important role in ecological separation in multi-species assemblages of terrestrial soricine shrews. Assemblages of shrews in boreal forests show a shift from the dominance of small species in unproductive habitats to the dominance of large species in productive habitats.

Shrews have two features in common that influence their whole energetic design: a small body size and an insectivorous food habit. They do not hibernate in winter and, with few exceptions, they are not able to enter torpor. The soricines have very high basal rates of metabolism and they maintain a high and precisely regulated body temperature. The very high metabolic rates in shrews are markedly higher than would be expected in mammals of their body size (up to 315% of the expected value). Extremely high metabolic rates are characteristic especially for the genus Sorex. The high basal rates of metabolism were attributed to their origin in a temperate climate and to their large litter size. High energy costs of reproduction are apparently associated with high metabolic rates. The high metabolic rates of most soricines make them susceptible to food shortage and result in the requirement of a constant food supply. Starvation time for the genus Sorex varies between five and 10 hours.

Soricine shrews in northern temperate regions undergo a decline in body mass and body size during winter (known as the Dehnel's effect). This is interpreted as an adaptation permitting reduction in food requirements. Winter weight loss in Sorex usually amounts to 25-40%, and it is accompanied by a reduction in the size of the skull and most organs.

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