The following notes are based upon many different litters, and give
some idea of the sequence of events in their early development.
Newborn: The skin is pinkish gray dorsally and pink ventrally. In
profile, sparse and exceedingly fine hairs less than 1 mm. in length
are discernible. The vibrissae are approximately 2 mm. long. The
skin is thin and partly transparent, much wrinkled, with some deeper
folds, notably one between the knee and the heel. The young lie on
their sides making violent convulsive respiratory movements. When
not attached to the female's teats, they may make faint squeaking
sounds.
One day old: Little changed in appearance or behavior except that
the dorsal surface has become darker because of growth of hair.
Two days old: Covering of fine brown hair readily discernible on
dorsal surface; lower incisors protruding about .5 mm. from the gum;
upper incisors have barely pierced the gum.
Four days old: Pale brown hair averaging about 1 mm. in length over
the dorsal surface gives the young a sleek, seallike appearance. The
young have gained greatly in muscular co-ordination. Part of the
time they may still lie on their sides, but they are able also to
gain an upright sprawling posture. In crawling, they are unsteady
and often topple over on their sides after taking a few halting
steps. They make frequent jerky lateral flexions of the body,
probably to search for a teat. Their eyes and ears still are sealed
shut.
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.