White-toothed shrews (Crocidurinae)
Class: Mammalia
Order: Insectivora
Family: Soricidae
Subfamily: Crocidurinae
Thumbnail description
Small mouse-like mammals with long pointed snouts, white teeth, short legs, and small eyes
Size
1.3–5.3 in (3.5–13.5 cm); 0.06–1.5 oz (2–40 g)
Number of genera, species
11 genera; 212 species
Habitat
Typically in moist habitats that are rich in invertebrate prey; can also be found in habitats ranging from arid regions to tropical forests
Conservation status
Critically Endangered: 16 species; Endangered: 16 species; Vulnerable: 36 species; Lower Risk/Near Threatened: 7 species; Data Deficient 6 species
Distribution
Throughout the Old World, found in many African countries, much of continental Europe and southeastern Asia
Evolution and systematics
Among the most ancient of all living animals, shrews have remained almost unchanged over the past 45 million years. This ancient family diverged from other insectivores before the Eocene period, with modern genera (subfamilies Crocidurinae and Soricinae) first appearing in the Miocene in Europe. Soricids later migrated to Africa and North America. The taxonomic status of modern shrews (Soricidae) is currently undergoing a significant overhaul due to considerable recent taxonomic research on the genetics, morphology, ecology, and behavior of shrews. At present, the shrew family is divided into two living subfamilies, the Crocidurinae and the Soricinae. The members of the subfamily Crocidurinae, white-toothed shrews, are further subdivided into 11 genera (Crocidura, Diplomesodon, Feroculus, Myosorex, Paracrocidura, Ruwenzorisorex, Scutisorex, Solisorex, Suncus, Surdisorex, and Sylvisorex). Current generic boundaries are based on very few characters. The delimitation of some genera of the subfamily Crocidurinae is still the focus of considerable genetic and morphologic research.
The genetic distances between different genera of Crocidinurae based on allozyme variation generally support a sub-division of the living species into these genera. Those species of Crocidurinae that have been studied to date show considerable chromosomal and genetic differentiation. Karyotypic and protein electrophoresis studies have been very helpful in identifying many sibling and cryptic species that were difficult to recognize by conventional methods.
Crocidurinae includes the genus Crocidura, which has 151 species—the largest number of living species of any placental mammal genus.
Physical characteristics
Shrews are the smallest of the insectivores. All shrews have short legs, five claws on each foot, short dense fur, small external ears, an elongated, pointed snout with long tactile hairs (vibrissae), and most have relatively long tails. All have extremely small eyes (often hidden in the fur) and relatively poor eyesight, but the sense of smell is keen, as suggested by their long, mobile snouts. The external ears are reduced in some species and usually hidden in their fur. Hearing is acute. The fur is short, dense, and usually some shade of brown or gray. The skull is long and narrow and has no zygomatic arch. The shrew has one of the most primitive brains of all placental mammals; the brain is small and smooth, dominated by large olfactory bulbs. The dentition is unlike any other family. The very large upper and lower incisors slant forward and meet like forceps. The external genitals of some species are enclosed in a fold of skin. Some species have a venomous saliva. Shrews have skin glands and genital or marking glands that secrete a substance with an unpleasant musky odor. The foot is not specialized, except in some aquatic species.
Members of the white-toothed shrew (Crocidurinae) subfamily are initially distinguished from their red-toothed (Soricinae) counterparts by the color of the enamel of the tips of their incisors. Red-toothed shrews have red teeth due to red pigmentations, and white-toothed shrews lack pigmentation. The two exceptions are Chimarrogale and Nectogale, two white-toothed aquatic species placed in the red-toothed subfamily.
Crocidurinae are also characterized by the retention of primitive dental characteristics. Modern forms differ from those of the later Miocene and from one another by the loss of one, two, or three upper and lower antemolars; reduction in the talonid of the lower third molar and greater emargination of the posterior basal outline of the upper premolar and upper first molar. The first set of teeth is shed in the embryonic stage, so that the teeth at birth are the permanent set. Crocidurinines have 26–32 teeth, normally six on each side of the lower jaw.
Differences in physiology, brain anatomy, and morphology are also diagnostic for this subfamily, including the articulation of the mandibular condyle and the position of the mental foramen. The two bones of the lower leg are fused. The cerebrum is less highly developed in the white-toothed shrews than the red-toothed shrews.
There is considerable anatomical variation among genera of Crocidurinae. Suncus etruscus of southern Europe has a body mass of approximately 0.07 oz (2 g), making it one of the smallest mammals known. One of the largest true shrew representatives of the subfamily Crocidurinae is Suncus murinus, with a head and body length of 6 in (15 cm) and weight of 1.0 oz (30 g) for males and 0.7 oz (20 g) for females.
Myosorex is considered the most ancient of the living genera because it has the largest number of teeth. Suncus differ from Crocidura by retention of a fourth upper antemolar. Sylvisorex differ from both Suncus and Crocidura by a lack of tail bristles, and differ even more from Crocidura by the retention of a fourth upper antemolar. In most genera, the genital and urinary systems have a common opening though the skin. Myosorex has an independent urinary tract system.
There are also significant differences in brain development between genera within the Crocidurinae subfamily. Members of Crocidura show more brain development than members of the thick-tailed shrews of the genus Suncus.
Distribution
The subfamily Crocidurinae is Paleotropical in origin; the white-toothed shrews originated in the Old World tropics and radiated across Africa, Europe, and Asia. This is reflected in the pattern of present day Crocidurinae distribution. Some species of Crocidurinae live in arid regions and semideserts.
Habitat
Crocidurinae shrews usually inhabit damp and dry forests, grassland, cultivated areas, and occasionally human settlements and buildings. Some white-toothed shrews in the
Diplomesodon and Crocidura genera live in arid areas and semi-deserts. They are mainly terrestrial, but some take to water freely, and others burrow a little. They are usually abundant wherever there is sufficient ground vegetation to provide cover. They occur over a great altitudinal range and in many kinds of plant communities. Most species prefer moist habitats, but some species are also found in arid regions.
Behavior
Shrews are extremely active, nervous animals. Some are active day and night, others only at night. When frightened, their heart may beat at a rate of 1,200 times per minute. Shrews have been known to die from loud noises, even thunder. Experiments with oxygen consumption suggest that shrews have a higher metabolic rate than mice of similar size. They are active throughout the year, though some go into torpor.
Some genera of Crocidurinae are notably aggressive and voracious. Shrews have been known to fight to the death. It is widely believed that most of these cases occurred during famine, when shrews may have become cannibalistic. The chirping and buzzing vocalizations sometimes heard are thought to be aggressive signals, and shrews may emit a squeak when alarmed or threatened.
Most shrews are solitary, except when they pair off during breeding. Members of Suncus and Crocidura can be tolerant of other shrews of the opposite gender during mating. After a period of initial fighting, a pair may live together. Once the pair bond has formed, the parent shrews may share a nest. Pairs from some species, such as Crocidura russula, have been observed building nests together.
Shrews are often territorial and have relatively small home ranges (0.1–0.25 acres [0.04–0.1 ha]) that vary in size with seasons of the year and with the timing of the animal's mating season. Shrews mark their territories with their scent glands. Data on population densities are limited to a few species. The population densities of shrews varies considerably between species. The recorded population density of the lesser whitetailed shrew on Corsica is 1.9–5.3 animals per hectare.
White-toothed shrews can dig their own burrow but often use those of other animals such as woodmice or molerats. They can tunnel through loose humus and leaf mold and often are active under fallen trees and heaps of brushwood or stone.
Feeding ecology and diet
Shrews are primarily insectivorous and carnivorous, but will eat some plant materials such as seeds and nuts. Considering their high energy needs, it is likely that shrews need to adapt their diet according to food availability throughout the year. Their diets may include frogs, toads, and lizards. In captivity, many species will consume all but the skin, tail, and parts of the limbs of a small mammal; the brain is always eaten first. If no other food is available, some species resort to cannibalism.
The common belief that shrews cannot survive without eating every few hours is not a rule. In some Crocidurinae species, animals are able to lower their body temperatures in response to food scarcity. This is an example of food deprivation triggering torpor, a state of reduced activity level. This energy-saving state permits the animal to survive difficult conditions temporarily. For example, Savi's pygmy shrew (Suncus etruscus) loses 10–15% of its body weight by day, then increases food consumption at night to make up the loss. If food is scarce, the shrew may enter torpor.
Some species of shrews have salivary glands that secrete a poisonous substance that usually immobilizes the prey. There are records of shrew bites to humans that caused great pain. There is some question over whether the saliva of Savi's pygmy shrew is poisonous. These diminutive shrews need to bite their prey only once to subdue them.
Reproductive biology
In the tropics, shrews form monogamous pairs and breed throughout the year, while those living in northern temperate zones usually breed from March to November. The known gestation periods are 17–28 days. There are one or several litters a year with 2–10 young in each litter. The young are born naked and blind in a nest of dried grass or leaves placed under a shelter or in a ground cavity. Weaning appears to occur at 2–4 weeks in most forms. Some zoologists estimate the life span in the wild to be 12–18 months, possibly longer. The oldest known Crocidurinae lived approximately four years.
Among Crocidurinae, the young are hairless for the first week and fully haired at 16 days. The auditory canals of Crocidurinae species open between the fifth and ninth days of life and the eyes open between the thirteenth and fifteenth days. Crocidurinae species begin to wean their young after 20–22 days. The young are practically adults and are sexually mature by 2–3 months.
Within the Crocidura, gestation is 27–31 days; litter size varies from one to 10. Larger litter sizes are associated with higher energy demands on the mother. The young weigh about 0.04 oz (1 g) at birth and are weaned at around 20 days. In Crocidura russula, young females can conceive at approximately 30 days of age.
Some Crocidurinae species and at least one Sorex species, display an unusual caravan behavior. The mother initally carries the young infants in her mouth. Starting on the sixth to tenth day, depending on the species, the mother and her young move in a caravan. At the slightest suspicious sound, one of the baby shrews grasps its mother's fur near the base of the tail. A second shrew grabs onto the first, a third onto the second and so on until the whole litter is lined up behind the female who then pulls her train of offspring.
Conservation status
The greatest threat for the Crocidurinae shrews is environmental disturbance and pollution. Loss of forest habitats has a considerable effect on forest-dwelling species.
Data on the conservation status of most Crocidurinae shrew species are limited due to a paucity of population research. The Crocidurinae shrews included in the 2002 IUCN Red List often belong to very rare and endemic species with restricted distribution and low population densities. Some of the threatened species are only known from the locality of their original description. There is an urgent need for considerable research on most shrew species.
Significance to humans
Shrews may be valuable to agricultural communities as a form of natural control of insect pests due to their consumption of large numbers of insects and insect larvae. They also play a significant role in increasing the rate of litter decay.
Shrews are frequently used for neurological research and in other areas of biomedicine and evolutionary biology. They can also serve as useful indicators of environmental contamination such as heavy metals. Shrews feeding on earthworms in areas with heavy metals in the soil can accumulate significant levels of heavy metals in their tissues.
Species accounts
Common European white-toothed shrewSavi's pygmy shrew
Forest musk shrew
Piebald shrew
Forest shrew
Armored shrew
Pearson's long-clawed shrew
Schouteden's shrew
Ruwenzori shrew
Kelaart's long-clawed shrew
Resources
Books:Corbet, G. B., and J. E. Hill. The Mammals of the Indomalayan Region: A Systematic Review. Oxford: Oxford University Press, 1992.
IUCN. Eurasian Insectivores and Tree Shrews—Status Survey and Conservation Action Plan. Gland, Switzerland: IUCN, 1995.
Merritt, J. F., G. L. Kirkland, and R. K. Roberts, eds. Advances in The Biology of Shrews. Pittsburgh: Carnegie Museum of Natural History, Special Publication 18, 1994.
Mitchell-Jones, A. J., et al. The Atlas of European Mammals. London: Poyser Natural History—Academic Press, 1999.
Wilson, D. E., and S. Ruff, eds. The Smithsonian Book of North American Mammals. Washington, DC, and London: Smithsonian Institution Press, 1999.
Wójcik, J. M., and M. Wolsan, eds. Evolution of Shrews. Bialowieza: Mammal Research Institute, Polish Academy of Sciences, 1998.
Organizations:IUCN Species Survival Commission, Insectivore Specialist Group, Dr. Werner Haberl, Chair. Hamburgerstrasse 11, Vienna, A-1050 Austria. Phone: +4315861094. Fax: +4315861094. E-mail: shrewbib@sorex.vienna.at Web site:
[Article by: Corliss Karasov]
