Table 4. Mean Sizes and Ages of Voles Molting from Subadult to Adult Pelage
| Weight | Body length minus tail | Condylo-basilar length | Age | |
| Six males | 32.67 gms. | 106.16 mm. | 23.78 mm. | 9.67 wks. |
| (30-36) | (96-116) | (23.2-24.4) | (8-12) | |
| Four females | 29.0 gms. | 100.25 mm. | 23.45 mm. | 10.5 wks. |
| (28-30) | (98-102) | (23.5-23.8) | (8-12) | |
| Ten voles | 31.2 gms. | 103.8 mm. | 23.73 mm. | 10.0 wks. |
| (28-36) | (96-116) | (23.2-24.4) | (8-12) |
The mean age of the ten voles molting was ten weeks (8-12). Six males averaged 9.67 weeks, almost a week younger than four females, who averaged 10.5 weeks. The difference in age at time of molting between the sexes was not significant. Differences between the sexes in other characteristics to be described also lacked significance. Mean weights at the time of molting were: males, 32.67 gms. (30-36); females, 29.0 gms. (28-30); and all individuals, 31.2 gms. (28-36). Because a piece of the tail of each vole had been removed in marking, the total length of the voles could not be determined. Body length, excluding tail, was used. Howell (1924:986) found this measurement subject to less individual variation than total length and thought body length was probably a better indicator of age. Mean body length at the time of molting was 103.8 mm. (96-116). Males averaged longer than females and were also more variable. The mean body length of males was 106.16 mm. (96-116) and that of females was 100.25 mm. (98-102).
Of the subadults showing no signs of molting, none was above the mean age of molting. Twenty-five per cent of them were longer and heavier than the mean length and weight of those that were molting. Of the 20 adults in the series, one was below the mean weight of molting and one was shorter than the mean length of molting.
When Howell (op. cit.:1014) studied skulls of Microtus montanus he found that the condylobasilar length was the most satisfactory means for arranging his series of specimens according to their age. When the skulls of my series were arranged according to their age (as determined from trapping records) the graph of the condylobasilar lengths showed a clear, though not perfect, relationship to age ([Fig. 13]). No separation of sexes was made because the sample did not permit it. In [Fig. 13] graphs of weight, as determined in the field, and of length (excluding tail) also were included because they are the most easily measured characters of live voles. The graphs indicate individual variation in these characters which limits their usefulness in determining age.
Fig. 13. Graphs of the condylobasilar lengths, body lengths and weights of a series of voles of known age. Within each age group, the youngest vole is on the left in the graphs.
When other cranial measurements, and ratios of pairs of measurements, were plotted in the same order, individual variation obscured some of the variation due to age and the curves resembled those of weight and length of body rather than that of condylobasilar length. When the cranial measurements were averaged for the age groups the curves showed a relationship to age but the relationship of mean measurements is of little use in determining the age of individual specimens. The data described above indicated that a study of the relationship of the condylobasilar length and age in a large sample might provide useful information.
Anyone who has examined mammalian skulls knows of many other characters which vary with age but which are difficult to measure and describe with precision. [Figs 14] and [15] are drawings of skulls of voles of known age. The most obvious change, related to aging, evident in the dorsal view of the skulls ([Fig. 14]) is the increasing prominence and closer approximation of the temporal ridges in older specimens. The lambdoidal ridge is also more prominent in older voles, and their skulls have a generally rougher and more angular appearance. The individual variation evident in these ridges is probably due to variations in the development of the muscles operating the jaws (Howell, 1924:1003). There is an increased flattening of the roof of the skull of older voles.