Females in confinement were attentive to young, and, soon after
parturition, licked them clean and huddled over them protectively.
Ordinarily, the newborn young soon attached to a teat, and spent a
large part of its time attached during its early development.
Females found in live-traps with their litters of young less than a
day old, often had some or all of the young clinging to their teats.
Females with newborn litters, when released from live-traps, always
left without attempting to retrieve any young that were unattached.
Such young usually were permanently deserted, but in some instances
disappeared within an hour or less, perhaps rescued by the female
returning for them.
Females with newborn young were made far more aggressive than most
other voles by their tendency to protect their young from possible
danger. In captivity such females usually took the offensive in
attacking or rebuffing any other voles confined with them.
Post-partum females obviously in oestrus were prevented from being
fully receptive by their hostility toward males whose presence might
endanger the young. Such a female has been seen to turn on a
pursuing male and attack him viciously, several times within a few
minutes, before copulation occurred. In captivity, at least, such
attacks would soon discourage a male so that unless he was
exceptionally active sexually, mating was prevented.
Cannibalism, involving destruction of the newborn, is probably an
important factor in the population dynamics of the prairie vole.
Only a small percentage of the young known to have been born on an
area ever survived to be live-trapped; this small percentage was
indirect evidence of decimating losses in the young. Under
unfavorable conditions each of several females killed and ate her
own litter, but the degree of provocation varied greatly among
individuals. Females that gave birth to young in live-traps
occasionally ate one or more of their newborn young, as evidenced by
discarded remnants. Perhaps other instances passed unnoticed because
no remnants were found. That need for food or moisture as well as
psychological stress often motivated such cannibalism was suggested
by the fact that surviving litter mates might be accepted and cared
for by a female that had already eaten one or more of her young.
Although cannibalism is most likely to occur within a few hours
after birth of the young, they may be killed and eaten at any stage
of development. One female that had probably eaten one or more of
her litter, soon after parturition, nursed the two survivors. When
these were two weeks old, all were "pastured out" in a wire mesh
cage in tall brome grass. When the supply of grass had become scarce
(though some was still available), the female killed and partly ate
both her remaining young.
One female was captured with three young attached that were several
days old. The young were detached from the female's teats with great
difficulty. When these young were returned to the female a few
minutes later, after they had been measured, weighed and marked, she
attacked them viciously, and within a few seconds had killed all of
them by biting their heads. In this instance the dead young were not
eaten, although they were temporarily left with the female.
Females with young have ample cause for their circumspective
demeanor toward adult males, which are especially inclined to eat
the newborn. A male engaged in sexual pursuit has been observed to
grasp a young dangling behind the female, pull it from her teat, and
pausing momentarily, nibble its head off, before continuing to
follow the female. Like the genitalia of the post-partum female, the
newborn young seem to have an odor that attracts and excites the
male.
To a lesser degree, adult females also display marked interest in
the newborn young of other individuals, which is liable to result in
cannibalism. The incidence of cannibalism is affected by the
condition, collectively, of the population of voles, and the
availability of nutritious food and moisture. In periods of summer
drought the grass becomes coarse and fibrous, and its protein
content declines. Under such conditions many voles appear to be
undernourished, and some are actually emaciated. Dehydration may be
an important factor at times when dew is unavailable for drinking
and the green vegetation remaining is exceptionally low in moisture
content. Voles caught at such times and brought to the laboratory,
drank avidly, and gained several grams soon after being offered
water or succulence. Cannibalism by adults on newborn young in times
of drought may be motivated by the acute need for moisture and
nutritious food. In times of drought the birth rate is at low ebb.
Adult males have never been observed to display paternal solicitude
toward young, but some individuals, kept with females and their
litters, did not molest the young and were accepted by the females
as members of the family group.
Other things being equal, cannibalism involving the young might be
expected to be greater at times of high population density. Then,
young left in the nest by a female in the course of her foraging
would more often encounter adults and partly grown young, both those
that lived in the same burrow system and exploring intruders from
other areas.
The eyes open at an age of nine or ten days. Then the young enter
upon an exploratory period, when each wanders out of the nest,
emerges from the burrow, and wanders through the adjacent surface
runways in frequent short forays, sometimes following the female and
sometimes alone. Such forays usually cover only a few inches at
first, but as the young vole grows, becomes familiar with its
surroundings, and takes more plant food, its sphere of activity
gradually widens, and family ties are dissolved. Voles reared to an
age of three weeks in the laboratory and then released, survived
just as well if the female was not released with them demonstrating
that they were fully capable of shifting for themselves at this age.
In confinement, however, young voles of greater age continued to
suckle and remained closely associated with the female. Females in
confinement evinced much uneasiness because of their inability to
evade the young when the latter were old enough to walk. The young
then followed the female continually and suckled whenever she
stopped or even while she moved about, unless she paused to remove
them from her teats, but they would not remain detached for more
than a few seconds. When a young followed the female away from the
nest and then attached to a teat, the female after pulling the young
from her teat, would usually carry it, grasped between her incisors,
back to the nest and deposit it there. On one occasion a young vole
caught in a live-trap was partly plucked and eventually killed by
the female on the outside trying to pull it through the wire mesh.
On several occasions, young were successfully transferred from the
mother to another lactating female in confinement, which accepted
them as part of her own litter. Young, up to the time of weaning,
appeared not to differentiate between the mother and other adult
voles. They would follow any larger individual indiscriminately, and
would huddle against it or nuzzle its undersurface searching for a
teat.