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.

A common European white-toothed shrew (Crocidura russula). (Photo by © Peter Baumann/Animals Animals. Reproduced by permission.)

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.

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