The prairie vole is by far the most abundant mammal on the
University of Kansas Natural History Reservation and on grassland
areas throughout northeastern Kansas. This vole therefore affects
the vegetation, perhaps more than any other native vertebrate, and
it is an important food source for most of the vertebrate predators.
Since the Reservation was established, in 1948, more data have been
accumulated concerning this vole than for any other species of
animal there. From February, 1950, to February, 1954, a grid of
live-traps at 50-foot intervals was set for several days each month
in a three-acre field inhabited by voles, and the population of
marked individuals was studied throughout the four-year period. From
November, 1953, to June, 1956, a half-acre trap grid with 20-foot
interval was used on an area adjoining the three-acre field. Other
trap lines in somewhat different habitats were maintained for
shorter periods as a basis for comparison. By June, 1956, a total of
some 3550 voles had been caught and recorded 14,750 times in all.
The present report is a preliminary attempt to analyze, in part,
these extensive data, and is concerned with certain phases of the
species' reproduction and growth that have bearing on the observed
population changes from month to month and from year to year on the
Reservation.
Through the studies of Jameson (1947) and Martin (1956), both made
in the same general area as my own, and several earlier studies, the
life history and ecology of the prairie vole are already well known.
The present report, with much larger amounts of data, further
clarifies certain phases of the ecology; and by using types of data
not available to Jameson and Martin I have dealt with some topics
not included in their reports.
Previous studies of growth in _Microtus_ have been based almost
entirely on weights. However, the weight of an individual vole may
fluctuate widely over a short period, depending on pregnancy and
parturition, length of time in a trap without food, availability of
moisture, and other factors. In the course of my study, in 1954
and 1955, and parts of 1953 and 1956, measurements of total length, in
addition to weights, were recorded for most of the voles
live-trapped.
To test the accuracy of measurements, successive readings were
compared in individual voles that were already of large adult size
and that presumably either had stopped growing or were growing so
slowly that the gain was scarcely detectable in the relatively short
periods involved. For 200 such readings 33 per cent were just the
same as previous records for the same animals, 24 per cent deviated
by 1 mm., 22 per cent deviated by 2 mm., 15 per cent by 3 mm., 4.5
per cent by 4 mm., .5 per cent by 5 mm., 1 per cent by 6 mm., and .5
per cent by 7 mm. On the average, successive measurements varied by
1.43 mm., somewhat less than one per cent of the adult vole's total
length. Occasional errors of two to four per cent were easily
eliminated because for the voles used for growth records, series of
measurements were available, with clearly defined trends. The
occasional readings that deviated from the general trend for the
individual were discarded.
Measurements were recorded along with other data in the field at the
point of capture. Obtaining a reasonably accurate measurement on a
live and struggling vole required patience and practice. With the
thumb and forefinger of the left hand, I grasped the vole by loose
skin of the nape, and simultaneously grasped the tail at a point
approximately three-fourths of the distance to the tip. Then, with
gentle but steady pressure, I stretched the vole out to its full
length, meanwhile manipulating a millimeter ruler with the free
fingers, so that the vole was pressed against it, with the nose pad
at the end of the ruler.
The total length measurement is considered the best index to
over-all size. The relative tail-length varies slightly between
individuals, averaging approximately 22 per cent of the total
length. Individuals having broken tails, or having the distal parts
of their tails missing, were not included. The total length can be
measured with greater accuracy than can either the head-and-body
length or the tail-length separately.