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.
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.
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.