MOUNTAIN WEASEL. Mustela arizonensis (Mearns)⁷

Field characters.—Body about as long as that of California Ground Squirrel, but much more slender (fig. 9b); tail about half length of head and body. Head and body 8-1/3—10-1/2 inches (211-269 mm.), tail 5-1/4—6-1/3 inches (132-160 mm.), ear 4/5—1 inch (21-26 mm.), weight 7-1/2—12-1/4 ounces (212-345 grams); among adults, males are larger than females. Coloration in summer uniform brown above, under surface rich creamy yellow; in winter, solidly white above and below; end of tail black at all seasons.

Occurrence.—Moderately common in Transition, Canadian, and Hudsonian zones on both slopes of Sierra Nevada. Recorded from Merced Grove Big Trees and Chinquapin eastward to Walker Lake and Mono Lake Post Office. Common on floor of Yosemite Valley. Lives around rock piles and old logs and under buildings. Solitary.

---

⁷Another species, the California or Yellow-checked Weasel, with whitish patches on the nose and cheeks, common in the Lower and Upper Sonoran zones of southern and central California, probably occurs in the lowland and foothill districts of the Yosemite region, though we obtained no specimens. We were told of a weasel having been seen at Snelling; presumably it was of this lowland species, Mustela xanthogenys Gray.

---

The weasel is a fearless animal, and active at all seasons of the year. Visitors to the Yosemite region therefore frequently see it, sometimes at very close range, both in the Valley and in the higher mountains.

The weasel is one of the most bloodthirsty of all of its tribe; the wild birds and mammals know this as thoroughly as do naturalists, for the presence of a weasel in any locality is immediately announced by cries of alarm from the native denizens. The weasel's body is extremely slender (fig. 9b); so small is the girth that it can easily make its way into the retreat of a ground squirrel or even into the burrow of a pocket gopher; and it readily enters the nests of those rodents which live among rocks or in hollow logs or trees. Furthermore, the weasel is an adept climber and can run up or down the trunks of the smaller trees as readily almost as a tree squirrel. By reason of its structure and capabilities, it is therefore able to prey upon a much larger variety of animals than any other species of carnivore.

Fig. 9. (a) Sierra Least Weasel; Vogelsang Lake, August 31, 19-15. See p. 89. (b) Mountain Weasel; Ten Lakes, October 8, 1915. See p. 86. (c) Pacific Mink; Merced Lake, August 23, 1915. See p. 89. All photographed from freshly taken animals; reproduced about 1/3 natural size.

The body coloration of the weasel is unique among our predatory mammals. It changes abruptly with the seasons, being solidly white in the winter months and brown and yellow in the summer season. The weasel is thus able to hunt the year round, well concealed in its protective coloration be the season that of blanketing snow or of brown logs on the bare ground.

In the summer months we found weasels at practically all of our camps in the territory from the 4000-foot contour up to the head of Lyell Cañon, at 9800 feet altitude. In Yosemite Valley, in both the winter and summer months, weasels are observed commonly. On December 20, 1914, a 'white' weasel was reported near Sentinel Hotel. December 9, 1914, tracks were seen in the snow on the Yosemite Falls Trail, and December 23, the same year, tracks were seen on the Vernal and Nevada Falls trail. In the latter case runways crossed the trail in many places, but these did not extend very far out on the unbroken snow. The weasels were then evidently living among the rocks which bordered the trails, for the short runs often led into holes about 1-1/2 inches in diameter burrowed in the snow covering the rocks and adjacent bushes and small trees.

During the rather brief stops which our party made at the various camps occupied in the Yosemite country, we saw many of these animals. At Chinquapin, on June 19, 1915, one of our party came upon a weasel in a small pile of old logs near a clearing. The weasel disappeared. The observer waited ten minutes and then went cautiously around to the other side of the pile where he found the animal peeking out at him curiously. When we stopped near the Tuolumne Meadows camp of the Sierra Club in late July of 1915, one weasel was shot right in camp as it made its appearance under a log beside a small rocky eminence. Another individual was seen close by, at the base of the same rock heap, where it was traveling in long bounds along the boulders. These two individuals caused particular concern to a number of White-crowned Sparrows which had their broods in the near vicinity; the birds evinced their anxiety over the presence of the enemy with many sharp notes of alarm. In Yosemite Valley on June 25, 1920, a Mountain Weasel was discovered through the excited calling of a pair of Spurred Towhees in a cascara thicket. This weasel took refuge from our pursuit up in an apple tree; there he dodged about among the branches and repeatedly looked down at us, monkey fashion. The black-appearing head, big round ears, and beady eyes had a strikingly alert expression.

In Yosemite Valley domestic cats were kept by the local residents until about 1908 when they were banished by order of the park authorities. The following year mice swarmed; then weasels began to be noted and they have been observed there in numbers ever since. We were told by Mr. C. W. Baker that on July 25, 1915, there was a brood of young to be seen playing about an occupied tent. The same informant stated that weasels were common about the horse barns and that they came out and watched like cats when bales and sacks were moved about and mice were likely to appear. Twice, we were told, weasels in the Valley had been seen carrying pocket gophers. At Tuolumne Meadows a packer told us that he saw one kill a 'picket-pin' (Belding Ground Squirrel); the weasel had the squirrel by the back of the neck.

At Walker Lake on September 12, 1915, a Red Squirrel was caught in one of the traps in a setting placed between the butts of two logs. Later, a Mountain Weasel happened along and nearly consumed the squirrel before it in turn was caught in another of the traps in the same set. On the same day the greater portion of another weasel caught elsewhere had been eaten, but there was nothing to indicate the identity of the animal which had attacked the victim of the trap. It is evident from the first mentioned case and from other trapping experiences not specifically cited that weasels will eat dead flesh, even when not fresh.

A Mountain Weasel three-fourths grown, living in a den under a willow clump at the edge of the lake, was taken at Mono Lake Post Office on June 30, 1916. There were many droppings at the entrance to the den.