SHREWS. Genus Sorex²

Field characters.—Size varying, but always less than half that of House Mouse. Head and body not over 3 inches (75 mm.) long, tail 2 inches (50 mm.) or less. Snout long and pointed (pl. 20); eyes and ears inconspicuous; pelage short, dense and smooth-appearing. Forefeet like hind feet, not specialized for digging. Coloration uniform, brown above (varying in tone according to the species), lighter, sometimes whitish, on under surface.

Occurrence.—Common from upper margin of Upper Sonoran Zone up to upper edge of Hudsonian Zone; recorded from Dudley, on Smith Creek (east of Coulterville), eastward to vicinity of Mono Lake.² (See fig. 6.) Live chiefly in damp situations along stream banks or in meadows, but sometimes found in protected situations at considerable distances from water.

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²Five species of true shrews (Genus Sorex) occur in the Yosemite region. The general appearance and the habits are much the same in all of these, though but little information other than that gained by trapping is available regarding their life histories. The species, their ranges, and their chief characters are as follows:

DUSKY SHREW, Sorex obscurus obscurus Merriam, a wide-ranging species found both in the Rocky Mountains and in the Sierra Nevada south to Tulare County, is common in the Canadian and Hudsonian zones of the Yosemite region from Mono Meadow (near Glacier Point) and from East Fork of Indian Cañon eastward to Warren Fork of Leevining Creek and to Williams Butte. Extreme altitudes of capture were 6900 and 10,800 feet. It lives not only along streams and in marshy places but also about logs on the forest floor. Total length about 4 to 4-1/2 inches, tail 1-3/4 inches, hind foot 1/2 inch (12.2-13.0 mm.); coloration dull sepia brown above, ashy on under surface. (See pl. 20c.)

ADORNED SHREW, Sorex ornatus Merriam, is found in mountainous parts of southern California and on the lower west slope of the Sierra Nevada, from the Mexican boundary north to the Yosemite region. Locally it is common at El Portal, and one individual was taken at Dudley, 6 miles east of Coulterville. Lives both along streams and on hillsides covered with live oaks and brush. Total length 4 inches, tail 1-3/4 inches, hind foot about 1/2 inch (12-13.5 mm.). Coloration dull brown above, whitish beneath.

YOSEMITE SHREW, Sorex montereyensis mariposae Grinnell, lives in the Transition Zone and lower part of the Canadian Zone on the west flank of the Sierra Nevada. It was found from Sweetwater Creek and Merced Grove Big Trees eastward to East Fork of Indian Cañon and to Merced Lake; it is the only shrew recorded for the floor of Yosemite Valley. Extreme altitudes of occurrence, 3800 and 7500 feet. It inhabits almost exclusively damp places near streams. Total length 4-1/2 to 5 inches, tail 2 inches, hind foot somewhat more than 1/2 inch (14-15 mm.). Largest local shrew of the genus Sorex. Coloration mixed hair brown and drab gray above, drab gray below with a silvery sheen. (See pl. 20b.)

SIERRA NEVADA SHREW, Sorex vagrans amoenus Merriam, of wide distribution along the northern Sierra Nevada, was found by us only at Williams Butte and Mono Lake Post Office. Lives near streams or in meadows. Total length about 4 inches, tail usually less than 1-1/2 inches, hind foot about 1/2 inch (11.5-13 mm.). Pelage sooty brown, grizzled with lighter brown above; under surface huffy white. The relatively short tail as compared with the tail of other shrews is a fairly good distinguishing feature.

LYELL SHREW, Sorex lyelli Merriam, is a rare species, known at present only from the general neighborhood of the peak for which it is named. Single specimens were taken by our party at Vogelsang Lake, 10,350 feet altitude, September 1, 1915, head of Lyell Cañon (= slopes of Mount Lyell) at 9800 feet, July 24, 1915, and near Williams Butte, at 6900 feet, September 20, 1915. Inhabits moist situations, near streams, in grass or under willows. Total length about 4 inches, tail 1-1/2 inches or more, hind foot less than 1/2 inch (11-12 mm.). Light hair brown above, paler on under surface.

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Shrews are present in the Yosemite region in large numbers, yet because of their small size and secretive habits they are much less well known than are the majority of other small mammals, such as meadow mice. The shrews leave little or no visible evidence of their activity, and it takes much observation, and usually trapping, to demonstrate their presence. Shrews and moles have many features of structure and behavior in common and are classed together in an order known as Insectivora, a term which indicates their principal food. The two groups are quite distinct, however; the shrews exhibit none of the peculiar specializations for digging possessed by moles, being in general much like small mice.

Fig. 6. Cross-section of the Sierra Nevada through the Yosemite region showing general zonal and altitudinal distribution of the shrews (genera Sorex and Neosorex).

The shrews live and do most of their foraging above ground, yet they keep beneath cover of varying kinds such as is afforded by matted vegetation and prostrate logs. Sometimes when foraging they invade the runways and even the burrows of other mammals—meadow mice, for instance—but none of our local species of shrews are known to make runs of their own or to put up mounds as do moles. Most kinds of shrews regularly patrol the sides of streams where often there are little beaten paths close under the overhanging banks. The Dusky Shrew, and to a less degree, the Adorned Shrew, are to be found away from water, sometimes a hundred yards or more, on hill slopes covered with trees and rocks. But none is known to inhabit the dry foothill chaparral, or the sagebrush tracts.

The nose of a shrew (pl. 20) is long and slender and equipped with numerous sensory hairs or vibrissae. The snout with its equipment is in almost constant motion when the animal is active. The eyes, while discernible, are small and do not seem to be of much use to the animal. The external ear also is small though the sense of hearing of shrews is said to be acute. The body of the shrew is cylindrical as in the mole, but the forefeet are normal in appearance like the hind feet. The tail, though varying somewhat according to the species, usually occupies slightly less than half the total length; it is thinly haired and has a constriction or narrowing at the extreme base where it joins with the body. This latter feature is not possessed by any of the mice. The teeth of shrews are sharply pointed and serve well in holding and killing insects or tearing the flesh of other sorts of prey.

The shrews, though of small bodily size as individuals, constitute, by reason of their numbers and their great activity, an important biological group in the fauna of the Yosemite region. They are actually "micro-carnivores" and exhibit an even greater degree of voracity than do the larger and better known flesh-eating species such as weasels, martens, and wildcats. Shrews kept in captivity have been known to eat more than their own weight of flesh in twenty-four hours. If they do this while in confinement there is no reason to suppose that their capacity would be any less (but rather more) when they are traveling about with full freedom in the wild. It is a common experience among naturalists who are trapping to find each morning one or more of the specimens in the traps mutilated to a greater or less degree. Numerous cases of this kind came to notice while we were engaged in field work in the Yosemite region. A part at least of this work may fairly be attributed to shrews, though various species of small rodents are known to eat maimed, trapped, or dead individuals of their own kind. Where only a beginning on the feast has been made, it is usually the brain of the trapped animal that is eaten. But not infrequently the work is done so completely that only a few fragments remain—scarcely enough to identify the victim. Not only rodents but even trapped shrews suffer from attaeks of this sort. And this cannibalistic tendency has been reported by observers who have kept shews in captivity. When any of the large carnivorous mammals, such as a coyote or a fox, raids a trapped specimen, the trap and all frequently disappear; if carnivorous beetles go after such prey, they accomplish but little in a single night; but if the shrews find the victim, they are apt to make short work of it, and without disturbing the trap in any way.

Evidence of several kinds shows that shrews forage to some extent by day as well as during the hours of darkness. It is likely that they depend less upon sight in searching for prey than upon the senses of smell, touch, and hearing.

As an indication of the density of population among the small mammals in a favorable location, and also of the extent to which shrews (in this case the Dusky Shrew) 'police' the ground in search of food, a record of trapping near Porcupine Flat may be cited. A line of traps set in a small meadow there from June 27 to July 3, 1915, produced the following mammals: Meadow Mouse, 3; Allen Jumping Mouse, 3; Sierra Nevada Pocket Gopher, 4; Dusky Shrew, 6; total, 16, in six nights of trapping. And the traps were still catching specimens when the line was taken up. This particular meadow had a total area of about 7350 square feet—the size of a large city lot (50 x 147). The vegetation consisted of grasses, lupines, and a species of orchid. It is possible that so large a number of shrews did not live and forage exclusively within so limited a tract. Only two or three may have been resident in the meadow; the others may have wandered in from adjacent territory.

The bodies of shrews have a distinctive odor, similar to that possessed by moles. This odor is currently presumed to be disagreeable to the flesh eating birds and larger mammals, and so is of value to the shrews in saving them from attack. Examination of the stomach contents of hawks and owls elsewhere has shown that but few 'insectivores' are taken by predatory birds. As an exception, however, a Sparrow Hawk collected by us in Yosemite Valley on October 25, 1915, had the remains of a shrew in its stomach along with parts of a meadow mouse and some insects. Perhaps each individual carnivore has to make one trial in order to learn that a shrew is an undesirable article of food.

We learned nothing with regard to the breeding places of shrews. As to season of birth and the numbers of young in a litter, only the following records can be offered: (1) Sierra Nevada Shrew, Mono Lake Post Office, May 21, 1916: 6 embryos. (2) Yosemite Shrew, Chinquapin, June 13, 1915: 4 small embryos. (3) Dusky Shrew, Mount Hoffmann, June 27, 1915: 6 large embryos; Porcupine Flat, June 29, 1915: 4 large embryos; Tuolumne Meadows, July 7, 1915: 5 large embryos; Merced Lake, August 24, 1915: 2 embryos.