NAVIGATOR SHREW. Neosorex palustris navigator (Baird)

Field characters.—Size about that of House Mouse; total length 6 to 6-1/2 inches (150-165 mm.), tail about 3 inches (75 mm.) long. Snout pointed; fore and hind feet of about same size and structure; ear inconspicuous. Pelage short, fine in texture; hind toes fringed with short close-set hairs (pl. 20a). Coloration black or hoary black above, often with a distinct sheen; whitish on under surface.

Occurrence.—Common in Canadian Zone and parts of Hudsonian Zone, on both slopes of Sierra Nevada. Recorded from Merced Grove Big Trees (5500 feet altitude) and Chinquapin, eastward to Mono Lake Post Office and Walker Lake. Highest station, 10,350 feet altitude at Vogelsang Lake. Lives in and near swift-flowing streams. Solitary.

The Navigator Shrew is larger than any of the other shrews in the Yosemite section and is more strictly an inhabitant of aquatic situations. We did not find even one of the animals that was more than four feet from running or standing water, and most of our specimens were taken immediately at the water's edge.

In structure the Navigator Shrew exhibits marked adaptations for existence in and near streams. The feet are large (pl. 20a); the toes of the hind foot are obliquely placed and margined with close-set fringes of hairs which serve like webbing to increase the surface of the foot. Furthermore, the pelage is of a rather distinctive type, like that found in aquatic or semi-aquatic animals; it does not soak up water, but holds air within its surface. An animal swimming beneath the water presents a shining silvery appearance because of this 'envelope' of air.

This is the species which fishermen, patrolling the banks of Sierran trout streams, often see swimming in the water. The fact that this shrew is active during the daytime is thus attested.

The breeding season of this shrew occupies the summer months. In 1915, suckling females were captured at Merced Grove on June 11, in Indian Cañon on June 20, and at Porcupine Flat on June 26 (this latter individual contained 6 small embryos). A female containing 7 embryos nearly large enough to be born was taken on June 23, 1915, in East Fork of Indian Cañon. Since none of the individuals collected in the fall months was sufficiently small to be classed on superficial inspection as young-of-the-year, the adult size must be attained rapidly.

At Mono Lake Post Office two specimens of Navigator Shrew were collected, on June 30 and July 2, 1916. These were taken in grass along a stream through a poplar grove, while in the sagebrush not over 100 feet away, specimens of the Great Basin Pocket Mouse were captured. The capture of this species of shrew at such a low station on the east side of the central Sierras was unusual and also furnished a striking example of how species of animals with totally different habitat preferences may occur in close proximity because of the juxtaposition of their respective niches.