Aging Meadow Mice, Microtus californicus, by Observation of Molt Progression

A distinct molt pattern in small rodents has been recognized by investigators for a number of years. Osgood (1909) described three color phases of Peromyscus as being due to difference in age but he did not correlate the three phases with specific ages. Collins (1918) found that the progression of molt pattern was consistent in P. gambeli as the animal's pelage changed from juvenile to post juvenile and found the change to occur between the 6th and 8th weeks of age. He points out that the molt may be well under way before it is evident at the hair surface. Coventry (1937) compared average sizes with the three principal color phases of P. leucopus noveboracensis and P. maniculatus gracilis. Pournelle (1952) correlated the “first molt” of P. g. gossypinus with age, Hatt (1931) gave a rough age-molt correlation in a captured flying squirrel, and Evans and Holdenried (1943) report that a molt starts in Citellus beecheyi soon after the young emerge from their burrows. Storer, et al. (1944) from observations of P. maniculatus describes ten stages of post juvenile molt, the entire molt requiring from eight to thirty-five days.