Quimby (_loc. cit._) records an average dimension of 4.7 mm (4.5-5.0) for the length of the hind foot in three newly born _Z. hudsonius_. The hind foot grew rapidly in length and by the fourth week had increased 5.6 times in its length and averaged 26.3 mm. Growth was much less rapid from the fourth to the thirteenth week when the hind foot averaged 27.7 mm, only five per cent more than in mice four weeks old. Assuming the average length of the hind foot of the adults to be 29.0 mm, the hind foot in individuals 13 weeks old is 96 per cent of the adult size.
According to Quimby (_loc. cit._), the pinna of the ear at birth is small and folded over the external auditory meatus. The length of the ear increases proportionately more (29 per cent) than any other external dimension after the first four weeks of growth.
If the average length of the ear (measured from the crown) of adults is 14.7 mm, the animals from Michigan in age groups 1 and 2 are 91.8 per cent and 96.5 per cent as large as adults.
TABLE 1.--Average Dimensions (in Millimeters) for Specimens of Z. h. hudsonius of Various Ages (Specimens from Michigan).
==============+=======+=======+=======+=======+=======+======= Age groups | 1 | 2 | 3 | 4 | 5 | 6 --------------+-------+-------+-------+-------+-------+------- No. examined | 4 | 13 | 33 | 12 | 3 | 3 --------------+-------+-------+-------+-------+-------+------- Body | 70.0 | 74.8 | 80.9 | 83.7 | 89.0 | 83.6 --------------+-------+-------+-------+-------+-------+------- Tail | 113.8 | 118.5 | 122.9 | 125.0 | 125.0 | 118.3 --------------+-------+-------+-------+-------+-------+------- Hind foot | 28.8 | 28.6 | 28.9 | 29.1 | 28.9 | 29.3 --------------+-------+-------+-------+-------+-------+------- Ear | 13.5 | 14.2 | 14.7 | 14.8 | 15.0 | 14.3 --------------+-------+-------+-------+-------+-------+-------
From these data, concerning growth of external parts, it seems that: growth is most rapid during the four weeks following parturition; specimens from Michigan, assigned to age groups 1 and 2 on the basis of tooth wear, are less fully developed and probably younger than mice from Minnesota, with a known age of 13 weeks; individuals with sufficient wear on the teeth to be placed in age group 3, if they were obtained in the late fall, may be young from the first litters of the year or, if they were obtained in early spring, may be at least one year old; individuals in age groups 4, 5, and 6 are at least one year old.