Five days old: Young have changed but little in appearance since the preceding day, but they have become notably more active, with movements better co-ordinated. When placed on a level surface they can crawl briskly.
Eight days old: Young are able to stand erect, with bodies held clear of the ground, and they can even run, but the gait is slow and clumsy, and the forequarters and hind quarters are poorly co-ordinated, so that the voles tend to fall on their sides. The fur averages approximately 3 mm. in length.
Nine days old: At this stage all young have their eyes open or beginning to open.
Ten days old: All young of this age have their eyes open, but not to their fullest extent, and the eyes are still slitlike in appearance. The young have become rather gopherlike in appearance and gait. They walk briskly but unsteadily, with bodies held high off the ground. When handled, they struggle vigorously, and try to bite. These young are similar in size and appearance to the smallest voles caught in live-traps apart from their mothers.
Thirteen days old: Hair on back has grown to an average length of 8 mm. (shorter on ventral surface, head, and limbs).
Seventeen days old: The young have become alert, and almost as quick in their movements as adults. They have molariform teeth, and are taking plant food. When a family group was examined, the young instantly detached from the female's teats and scattered. The hair on the back averages 10 mm. long and the vibrissae average 20 mm. long.
There is intense competition among the young of a litter, especially if the litter has more than the average number of young. In litters with more than four young, there is competition for the inguinal teats, since, in most females at least, the pectoral teats seem to have an inadequate milk supply. As a result, it is doubtful whether more than four young to a litter are ever able to survive. From the time their eyes open, the young compete actively. When litters in confinement were fed with fresh greens, there was nearly always quarrelsome squeaking and scuffling, as the young competed for food. At such times, they have been seen to chase and attack each other.
GROWTH FROM WEANING TO MATURITY
No individual vole was recaptured with sufficient regularity, from birth to maturity, to provide a complete growth curve. The curve in [Fig. 7] is a composite based on all available records of voles that were recorded as making growth in length and were recaptured before they were fully grown, so that growth rates could be computed. The figure shows that growth is extremely rapid for the first three weeks, and thereafter slows gradually but steadily, until in individuals of adult size, the increment per day is much less than that in the small young.
Since rate of growth changes rapidly, with a slowing trend, only those young voles that were recaptured within a few weeks showed the approximate growth rate for any specific portion of the ontogenetic curve. [Table 5] summarizes the records of 98 such young sorted into size groups representative of several stages in development. The slowing trend of growth in voles that are nearing subadult size is well shown by these records. Throughout the greater part of the growth curve no difference could be found in rate between the sexes. It is only after sexual maturity has been attained and growth has become relatively slow that males become noticeably larger than females. This tendency for continued growth in the adult males results in a much more marked disparity in size between the sexes in the oldest voles, as evident in [Fig. 2].