As compared with other mammals, voles are tolerant and somewhat
social. That individuals are not mutually exclusive (territorially)
in areas occupied was demonstrated on many occasions when more than
one individual was caught simultaneously in the same live-trap.
Injury of a vole by a trap-mate was a rare occurrence.
Multiple captures often involved a female in oestrus and one or more males,
or a female and her young, but other instances involved various
combinations of sex and age groups. As many as five adults have been
caught in a trap simultaneously at times when the population density
was high. At such times, the meadow habitat is crossed by a maze of
interconnecting surface runways and one runway may be traced
continuously for 100 yards or more. Because each individual vole
normally confines its activity to a small area, only a fraction of
an acre, it is evident that individuals living at different places
overlap in their home ranges, and also in the trailways followed in
foraging. A high degree of tolerance is indicated. Where population
is so sparse that the systems of surface runways comprise separate
and isolated units, trapping experience has shown that one such
system may harbor several or many individuals.
As direct observations on voles under natural conditions are rarely
feasible, because of the animals' timidity, their utilization of
concealing cover, and tendency to crepuscular habits, best evidence
of social habits and underground life is based upon behavior of
captive individuals. Many voles were kept in confinement for varying
lengths of times, either singly or in association with others. Under
such conditions there was sometimes sporadic fighting, but it was
mainly defensive and serious injuries were rare. Two or more voles
caught at a given spot regardless of whether they were found in the
same trap simultaneously, or trapped separately within a short time,
usually were completely tolerant of each other. When at rest in
their container, such voles would huddle together in a corner or in
a nest, if materials were provided, so that collectively they
presented the minimum exposed surface. The intimacy and lack of
antagonism displayed on such occasions, suggested that the voles
were accustomed to living together amicably in the same nest
chamber. In live-trapping, "double" captures in a single trap often
involved the same two individuals. Such trap-mates were often male
and female, and in many instances the female was not in breeding
condition. That the voles are not monogamous in habits was
demonstrated when the same female was often trapped in association
with either of two males. Other trap associates taken together
repeatedly often were two males, or two females. Voles that are nest
mates or "neighbors" may tend to move about together in their
foraging, or one confined in a trap may attract the other
sufficiently to cause it to force an entrance by lifting the heavy
door of a trap.
When a new vole, caught at a different location, is added to a
container in which one or more are already confined, there is mutual
circumspection between the original occupants and the newcomer. At
first, each vole is intimidated by movements of the other, and as a
result, the original occupants huddle in their established corner
while the newcomer cowers in the most remote part of the container.
Gradually the voles become less timid and one may approach another
slowly and cautiously, to sniff at it. The vole approached may react
with a show of hostility which is largely defensive. In the
characteristic posture of threat for defense, the vole crouches, or
rears back on its haunches, with snout elevated and incisors
prominently displayed. If the warning posture is unheeded, or if the
vole is made unusually aggressive by having young to defend, or for
some other reason, it attacks with a sudden forward lunge, striking
the adversary simultaneously with both forefeet and with the
incisors. The lunge is so rapid that when I have observed it, I have
been unable to discern whether the attacker bit its opponent. The
attack serves to force back the other animal, throwing it off
balance and intimidating it. The attacked animal may dodge nimbly to
avoid the lunge, but whether or not it is actually struck, it
usually retreats, avoiding or postponing further hostilities. Voles
that have been kept in containers for periods of hours or days tend
to be more hostile and aggressive toward a newcomer than are those
newly introduced. After series of meetings resulting from the
exploratory behavior of the newcomer and the curiosity or normal
activity of those longer confined, hostility gradually subsides.
Within a few hours a newcomer is usually accepted, and thenceforth
he huddles with other members of the group when at rest, and
hostility is rarely evident.
This ready acceptance on short acquaintance of strange voles into
the family or social group suggests that lack of territoriality
extends even to the use of the nest burrows, and that groups of
voles may share the same nest, huddling together and deriving mutual
benefit from the association, such as warmth in cold weather.
Schmidt (1931: 113), studying this vole in Clark County, Wisconsin,
noted its colonial habits. He found isolated small mounds that were
riddled with burrows, and little sign in intervening areas. At one
mound he trapped two adult males, one adult female, and two young;
at another mound, two adult males, two adult females, and four young
were trapped. My individuals that were released from live-traps were
on many occasions trailed by means of a stiff wire collar with spool
of thread attached, to holes that presumably were their home
burrows. Data obtained in this manner indicated that ordinarily
several or many individuals use the same burrow system. The
histories of individual voles on the study area at the Reservation
indicate shift of home base from time to time, usually for short
distances within the area already included in the home range, but
occasionally to new areas relatively remote from the original home
range.
Severe fighting between adult prairie voles occurs at times.
Occasionally, sharp squeaks accompanied by brisk rustling in the
grass suggesting pursuit or conflict, are heard in their habitat. An
unusually large adult male, long resident on a study area, suddenly
lost weight and deteriorated in condition over a period of several
days, then was found dead in a nest-box attached to a trap.
Dissection revealed numerous punctures in the skin and flesh of the
neck and back, probably made by the incisors of another vole.
Extensive hemorrhage and swelling had occurred, and obviously these
injuries were the cause of death.
Although it was not feasible to study the home life of the voles
underground, clues were gained from those uncovered in runways and
nests beneath large boards and strips of tarpaper, previously
distributed for this purpose. Nests were constructed by the voles
beneath several such pieces of tarpaper and runways appeared beneath
all the pieces that were placed in habitat favorable to the voles.
In summer, however, the high daytime temperatures beneath these
shelters made them uninhabitable to the voles, and they were used
mainly in spring. From February 15 to May 1, 1953, 14 voles were
caught 19 times beneath five of the tarpaper strips, and many other
voles that were seen beneath them escaped. Upon turning one of the
strips I often discovered voles in close proximity. Sometimes two or
more darted from the same nest. The disturbance of repeatedly
raising the strips and exposing the voles' shelters soon caused them
to desert the sites; consequently the information obtained by this
means was limited.