Figure 1. A masked shrew, Sorex cinereus
The shrew is a small, mouse-sized mammal with an elongated snout, a dense fur of uniform color, small eyes, and five clawed toes on each foot (Fig. 1). Its skull, compared to that of rodents, is long, narrow, and lacks the zygomatic arch on the lateral side characteristic of rodents. The teeth are small, sharp, and commonly dark-tipped. Pigmentation on the tips of the teeth is caused by deposition of iron in the outer enamel. This deposition may increase the teeth’s resistance to wear, an obvious advantage for permanent teeth that do not continue to grow in response to wear. The house shrew (Suncus murinus) lacks the pigmented teeth. Shrew feces are often corkscrew-shaped, and some shrews (for example, the desert shrew [Notiosorex crawfordi]) use regular defecation stations. Albino shrews occur occasionally. Shrews are similar to mice except that mice have four toes on their front feet, larger eyes, bicolored fur, and lack an elongated snout. Moles also are similar to shrews, but are usually larger and have enlarged front feet. Both shrews and moles are insectivores, whereas mice are rodents.
Worldwide, over 250 species of shrews are recognized, with over 30 species recognized in the United States, the US Territories, and Canada (Table 1). Specific identification of shrews may be difficult. Taxonomists are still refining the phylogenetic relationships between populations of shrews. Consult a regional reference book on mammals, or seek assistance from a qualified mammalogist.
Shrews are among the world’s smallest mammals. The pigmy shrew (Sorex hoyi) is the smallest North American mammal. It can weigh as little as 0.1 ounce (2 g). Because of their small size, shrews have a proportionally high surface-to-volume ratio and lose body heat rapidly. Thus, to maintain a constant body temperature, they have a high metabolic rate and need to consume food as often as every 3 to 4 hours. Some shrews will consume three times their body weight in food over a 24-hour period.
Shrews usually do not live longer than 1 to 2 years, but they have 1 to 3 litters per year with 2 to 10 young per litter. Specific demographic features vary with the species. The gestation period is approximately 21 days. Shrews have an acute sense of touch, hearing, and smell, with vision playing a relatively minor role. Some species of shrews use a series of high-pitched squeaks for echolocation, much as bats do. However, shrews probably use echolocation more for investigating their habitat than for searching out food. Glands located on the hindquarters of shrews have a pungent odor and probably function as sexual attractants. Blarina brevicauda, and presumably B. carolinensis and B. hylophaga (the short-tailed shrews), have toxic venom in their saliva that may help them subdues small prey.
Some shrews are mostly nocturnal; others are active throughout the day and night. They frequently use the tunnels made by voles and moles. During periods of occasional abundance, shrews may have a strong, although temporary, negative impact on mouse or insect populations. Many predators kill shrews, but few actually eat them. Owls in particular consume large numbers of shrews.
Some shrews exhibit territorial behavior. Depending on the species and the habitat, shrews range in density from 2 to 70 individuals per acre (1 to 30/ hectare) in North America.
Shrews are broadly distributed throughout the world and North America. For specific range information, refer to one of the many references available on mammal distribution for your region. Publications by Burt and Grossenheider (1976), Hall (1981), and Junge and Hoffmann (1981) are particularly helpful.
Shrews vary widely in habitat preferences throughout North America. Shrews exist in practically all terrestrial habitats, from montane or boreal regions to arid areas. The northern water shrew (Sorex palustris) prefers marshy or semiaquatic areas. Regional reference books will help identify specific habitats. A word of caution is in order, however. Distribution studies based on the results of snap-trapping research have a pronounced tendency to understate the abundance of shrews. Studies using pit traps are more successful in assessing the presence or absence of shrews in a particular location.
Shrews are in the taxonomic order Insectivora. As the name implies, insects make up a large portion of the typical shrew diet. Food habit studies have revealed that shrews eat beetles, grasshoppers, butterfly and moth larvae, ichneumonid wasps, crickets, spiders, snails, earthworms, slugs, centipedes, and millipedes. Shrews also eat small birds, mice, small snakes, and even other shrews when the opportunity presents itself. Seeds, roots, and other vegetable matter are also eaten by some species of shrews.
Robert H. Schmidt. Assistant Professor. Department of Fisheries and Wildlife. Utah State University. Logan, Utah 84322-5210