In North America this is the largest shrew in the genus Sorex. Fur (pelage) is a velvety dark-brown to black, only slightly paler on the ventral area than on the dorsal area; more brownish in summer than winter. The dark brown tail is uni-coloured. The pelage has a dual ability to repel water while trapping a layer of air. This acts as an insulation layer reducing heat loss by 50% while swimming (critical as most shrew species have a high metabolic rate and can stress from energy loss quickly). The skull is large with a ventrally curved rostrum (area where the snout extends from the top of the skull) and holds 32 teeth. As their name implies, Pacific Water Shrew are excellent swimmers. Air bubbles trapped beneath the unique fringe hairs of the front and hind feet provide enough buoyancy to enable them to run on the surface of the water for up to 5 seconds.