Voles less than 100 mm. in total length were seldom captured,
because those less than this size are dependent on the female, and
rarely venture far enough from the nest to be caught in a trap. A
further difficulty in obtaining growth records on the smallest young
is that of making accurate measurements. During their first few days
they partially retain the fetal posture, usually lying on one side,
with the head, body and tail flexed in an arc almost completed by
the tail approximating the muzzle. Straightening the animal by
stretching it and holding it with sufficient firmness to obtain a
measurement might have involved injury to it. Therefore, in most
instances the newborn voles examined were merely weighed or an
approximate measurement was estimated without stretching the young
to its full length.
Newborn voles were obtained when females that were caught in
live-traps produced their litters before they were found and
released. In some instances, females caught while in late pregnancy
were retained in the laboratory for a day or more until parturition
occurred. Many of the newborn voles were marked by toe-clipping,
according to the same system used for adults. Early growth was
measured in some instances by keeping the female with her litter in
confinement, measuring and weighing the young at intervals. In most
instances, the female was released at the point of capture
(presumably near her nest burrow) with the young clinging to her
teats. For the young so released, the incidence of recovery was
remarkably low, seeming to indicate that they were subject to
decimating losses. Perhaps such losses are normal, at least on the
study area where voles are live-trapped regularly. Holding of adults
and partly grown young in live-traps ordinarily has no harmful
effects on them, but the resultant separation of females from newly
born litters may often result in death of the young either from
hunger and exposure, or from attack by other voles and natural
enemies.
During the first ten days the increase in length from an original 47
mm. is from three to four mm. per day. Figs. 2, 5, and 8 show length
and weights of voles whose ages in days were definitely known
because they were born in the laboratory, or in a live-trap after
the female was caught there. Young voles marked at birth and
released with the female were rarely recovered in the period of
suckling, as they ordinarily remain in the nest burrow when the
female ventures out to forage. Litters retained in the laboratory
therefore have provided most of the records of growth in suckling
young. Growth varied greatly between litters. It was not clearly
correlated with size of female, size of young at birth, or number of
young in litter, but probably was influenced by attentiveness of the
female, her adjustment to captivity, and her productivity of milk.
Within each litter there were usually persistent differences in
development, but these were minor (except for those of occasional
runts) compared with the differences between litters. In several
litters of five young, one was usually smaller than the others at
birth and therefore could not compete successfully with its litter
mates, so that it never gained possession of a teat other than one
of the pectoral pair, and always succumbed within a few days, after
failing to gain weight as its litter mates did. The relatively few
voles marked at birth and recovered after developing under natural
conditions, did not deviate from the trend of those in confinement.