In Zapus hudsonius, new hair appears simultaneously on the anterior dorsal surface of the nose and on the mid-dorsal surface between the scapulae. The molt proceeds anteriorly from the shoulders and posteriorly from the nose. At the same time that the head is covered, new hair appears on the sides of the body from the forelegs to the cheeks. New pelage then appears posteriorly, and molt continues as a wave from these points over the sides and back with the rump receiving new hair last (see [figs. 42 and 43]).
In Zapus princeps new hair appears first on the mid-dorsal surface between the scapulae. From this starting point molt progresses anteriorly, laterally, and posteriorly. Progress over the head is rapid; the head receives its new hair sooner than the caudal region. Molt moves progressively nearer to the base of the tail and progressively nearer to the mid-ventral surface. The rump is the last area to complete its molt (see [figs. 40 and 41]).
The progress of molt in Z. princeps might be likened to the flow of a drop of paint on the curved surface of a ball where the paint flows in all directions but is speeded at one point and slowed at the opposite by a slight tilting of the ball from the horizontal.
In the species Zapus trinotatus new hair appears simultaneously on the anterior, dorsal surface of the nose and on the mid-dorsal surface between the scapulae. In this respect the progress of molt of Z. trinotatus resembles that of Z. hudsonius. From these starting points molt progresses rapidly over the head, the molt moving anteriorly from the shoulders and posteriorly from the nose with the result that it covers the dorsal surface of the head; hair then appears on the cheeks and sides of the neck. The progress of molt on the remaining areas of the body is comparable to that of Z. princeps; molt progresses toward the tail and toward the mid-ventral line. The rump, as in Z. princeps, is the last area to complete its molt (see [figs. 38 and 39]).
Figs. 38-43. Diagrams showing differences in progress of molt in the three species of the genus Zapus. All approximately 1/2 natural size. Figs. 38, 40 and 42 lateral view. Figs. 39, 41 and 43 dorsal view.
Figs. 38 and 39. Zapus trinotatus.
Figs. 40 and 41. Zapus princeps.
Figs. 42 and 43. Zapus hudsonius.