No individual vole was recaptured with sufficient regularity, from
birth to maturity, to provide a complete growth curve. The curve in
Fig. 7 is a composite based on all available records of voles that
were recorded as making growth in length and were recaptured before
they were fully grown, so that growth rates could be computed. The
figure shows that growth is extremely rapid for the first three
weeks, and thereafter slows gradually but steadily, until in
individuals of adult size, the increment per day is much less than
that in the small young.
Since rate of growth changes rapidly, with a slowing trend, only
those young voles that were recaptured within a few weeks showed the
approximate growth rate for any specific portion of the ontogenetic
curve. Table 5 summarizes the records of 98 such young sorted into
size groups representative of several stages in development. The
slowing trend of growth in voles that are nearing subadult size is
well shown by these records. Throughout the greater part of the
growth curve no difference could be found in rate between the sexes.
It is only after sexual maturity has been attained and growth has
become relatively slow that males become noticeably larger than
females. This tendency for continued growth in the adult males
results in a much more marked disparity in size between the sexes in
the oldest voles, as evident in Fig. 2.
[Illustration: FIG. 2. Size distribution of prairie voles
in a year-around sample, including all the measurements of voles
taken over a three-year period. Young are not represented in
their actual ratio to the total population in this sample,
because they are less attracted to the bait, and range less
widely than adults. The higher ratios of males than of females
in the three largest size groups is well shown, as is the higher
ratio of females among those voles of small adult size.]
Table 5. Average Growth (in Over-all Length) in Young Voles of
Several Sizes. ([M] = Male; [F] = Female)
==================+==============+===========+===========================
Average lengths | Average | Average |
in mm. at | length, | increment | Total, and
beginning and | in days, | per day | number of each
end of growth | of growth | in mm. | sex in sample
period | periods | |
------------------+--------------+-----------+---------------------------
97.0 to 126.6 | in 16.8 | 1.76 | 5 (1 [M], 4 [F] [F])
103.3 to 127.3 | in 14.9 | 1.61 | 9 (3 [M] [M], 6 [F] [F])
107.5 to 123.4 | in 11.0 | 1.44 | 8 (5 [M] [M], 3 [F] [F])
114.0 to 132.3 | in 17.5 | 1.05 | 6 (5 [M] [M], 1 [F])
118.5 to 136.0 | in 19.7 | .88 | 6 (3 [M] [M], 3 [F] [F])
122.1 to 135.8 | in 16.2 | .85 | 15 (5 [M] [M], 10 [F] [F])
129.3 to 145.5 | in 22.8 | .71 | 4 (all [M] [M])
130.6 to 146.1 | in 19.8 | .78 | 12 (all [F] [F])
139.8 to 147.5 | in 29.5 | .26 | 10 (all [M] [M])
141.2 to 148.8 | in 26.2 | .29 | 23 (all [F] [F])
------------------+--------------+-----------+---------------------------
[Illustration: FIG. 3. Changing numbers and composition
(according to size of individual) in a population of voles on
an area of approximately one half an acre that was intensively
sampled with live-traps over periods of months. The population
as a whole and the ratio of young to adults tended to be higher
in spring and summer, but with little regularity from one year
to the next. Weather was far more important than season in
determining the population trend. Many of the voles recorded
on the half-acre area ranged more or less beyond its boundaries.]
[Illustration: FIG. 4. Weight in free-living prairie voles
in a year-around sample from juveniles to large adults (grouped
in length-classes of 6 mm. range, separate for each sex). In each
sample mean, standard error, standard deviation, and extremes are
shown. Note that mean weight is proportional to length, that in
each size class females average heavier (because of pregnancy
in some) and have a much wider range of variation in weight.]
Martin (1956:389) stated that growth in young prairie voles was, in
general, most rapid in the period April-May-June and least rapid in
mid-winter. However, his data were based entirely on weights. The
high incidence of pregnancy in the larger young females in spring
and early summer may have caused the trend. Measurements taken by me
of lengths do not bear out the idea of more rapid growth in the
spring and summer, but, indeed, show the opposite. In most
instances, voles of comparable sizes made significantly more rapid
growth in the colder half of the year (mid-October to mid-March)
than in the warmer half. Dividing the young voles in eight size
groups and separating each group into comparable summer and winter
samples, I found more rapid average growth in the summer sample in
only two instances. These deviations from the general trend
probably resulted from inadequately small sizes of some samples. On
the average, the growth rate in summer was 92 per cent of that in
winter.
[Illustration: FIG. 5. Over-all length plotted against
weight in young prairie voles, from newborn to the minimum
size at breeding maturity. The range of variation increases
as development proceeds, especially after the age of weaning
is attained.]