The water rat is mainly nocturnal but not uncommonly is seen in the daytime. Twice I cornered a water rat away from its burrow and each time it escaped by swimming. The rats swam with great speed but with much splashing. One dived under the surface of a small pool and disappeared. Stones along the bank were pulled out until the rat was captured in a small chamber at the end of a burrow. The burrow entered the bank at the base of a large stone six inches beneath the surface. This burrow resembled a miniature muskrat burrow and apparently had been dug when the rat was under water.
Like Phenacomys, the water rat constructs sub-snow nests on the surface of the ground. These are recognizable by their large size and by piles of ovoid droppings a quarter of an inch in length. These nests are loosely built and fall apart soon after the snow melts.
The water rat is sometimes a pest to the mammal collector for they spring mouse traps set for other mammals without becoming caught. At times the greater part of an entire trap line was thus rendered ineffective by these mammals. Fully adult animals are uncommon in collections for a rat trap or steel trap is needed to take them and these items, when packed on back up mountains to water-rat habitat, are usually set for still larger animals. [Racey] ([Racey] and [Cowan], 1935: H27) recounting his difficulty in securing specimens when no suitable traps were at hand, writes: "Killed one with my hands, shot another, and a third was stunned by a mouse trap."
[Taylor] and [Shaw] (1927: 76) list food eaten by the water rat on Mount Rainier as the avalanche lily, dogtooth violet, Ligusticum purpureum, Valeriana sitchensis, Polygonum bistortoides, Petasites frigida, Phyllodoce empetriformis, Potentilla flabellifolia, Aster sp., grass, wild clover, conifer seeds, two kinds of blueberry (Vacinnium) and Xerophyllum tenax. [Racey] and [Cowan] (1935) list foods eaten in the Cascades of southern British Columbia as Lupinus polyphyllus, Senecio balsamitae, Pedicularis bractiosa and Arnica alpinus.
A female from Dewey Lake, Yakima County, contained 4 embryos on September 1, 1940. One from Tye, King County, had 2 embryos on September 8, 1940.
Microtus richardsoni arvicoloides ([Rhoads])
Aulacomys arvicoloides [Rhoads], Amer. Nat., 28:182, February, 1894.
Microtus richardsoni arvicoloides [Bailey], N. Amer. Fauna, 17:62, June 6, 1900.
Type.—Obtained at Lake Keechelus, Kittitas County, Washington, by A. Rupert in September, 1893. [Rhoads] gives the altitude as 8,000 ft. This apparently is an error, for the elevation of the lake is 2,458 ft. and the summit of Snoqualmie Pass, to the west, is 3,100 ft. Probably 3,000 ft. was intended; type in Philadelphia Academy of Natural Sciences.