Part 3

Although the amount of growing tissue of plants probably is at least as important to voles as the total amount of vegetation, some correlation seemed to exist between the density of grassy vegetation and the density of populations of voles. A mixed stand of grasses, with an obvious weedy component, can support a larger population of voles than can either a nearly pure stand of grass or the typical early seral stages dominated by weeds. Probably the more or less continual supply of young plants provided preferred food easily available to voles. A more homogeneous vegetation would tend to pass through the young and tender stage as a unit, thus causing a feast to be followed by a relative famine.

POPULATION STRUCTURE

During the period of study the percentage of males in most of my samples was less than 50 per cent (Fig. 2). Only once, in June, 1952, did the mean percentage of males in samples from three areas (House Field, Quarry Field, Fitch traps) exceed that level and then it was only 50.1 per cent. On several occasions, however, the percentage of males in a sample from a single area was slightly above 50 per cent. The highest percentage of males recorded was 56.69 per cent, in a sample taken from the Quarry Field population in June, 1952. In the samples taken in April, 1952, the mean percentage of males was 39.67 per cent, the lowest mean recorded. The low point for one sample was 28.02 per cent in August, 1952, from Quarry Field. The mean percentage of males in all samples taken was 45.02 +- 2.72 per cent. Percentages observed would occur in random samples taken from a population with 50 per cent males less than one per cent of the time. Exactly 50 per cent of the young in the 65 litters examined were classified as males but the sample was small and the sexing of newborn individuals was difficult.

The extent to which sex ratios in samples were affected by trapping procedure was not determined. A possibility considered was that the greater wandering tendency of males (Blair, 1940:154; Hamilton, 1937c:261; Townsend, 1935:98) impaired the formation of trap habits (Chitty and Kempson, 1949:536) on their part and thus unbalanced the sex ratios of the samples. If this were the explanation, the apparent sex ratio on larger areas would more nearly approximate the true ratio, and the frequency of capture of females would exceed that of males. The evidence is somewhat equivocal. In the populations described here the mean number of captures per individual per month was 2.31 for females, which was significantly greater (at the one per cent level) than the 2.20 captures per individual per month which was the mean number for males. This difference supports the idea that differences in habits between the sexes result in distorted sex ratios in samples obtained by live-trapping. Mean percentages of males did not, however, differ significantly between the House Field-Quarry Field samples and the samples from the Fitch trapping area, nearly five times as large.

Three age classes, juvenal, subadult and adult, were separated on the basis of condition of pelage. The percentage of adults in populations varied seasonally (Fig. 2). January, February and March were the months when the adult fraction of the population was highest and October and November were low points, with May and June showing percentages almost as low. The only marked variation in this seasonal pattern occurred in July and August, 1952, when the percentage of adults rose sharply. This was due to a depression in the reproductive rate during the dry summer of 1952, which is discussed later in this report. Juveniles made up only a small fraction of the population from December through March and a relatively large fraction in the October-November and May-June periods (Fig. 2). Again, July and August of 1952 were exceptions to the pattern as the percentages of juveniles in these months fell to midwinter levels. As expected, the curve of the percentages of subadults in the population followed that of the juveniles and preceded that of the adults. The mean percentages for the thirty month period for which data were available were: adults, 77.72 +- 4.48 per cent; subadults, 14.06 +- 3.14 per cent; and juveniles, 8.22 +- 2.62 per cent. Seasonal and yearly changes in the population structure occurred, with notable variation in the ratio of breeding females to the entire population, as discussed in this report under the heading of reproduction.

Since some of the juveniles did not move enough to be readily trapped, the real percentage of juveniles in the population was probably far greater than that shown by trapping data. I tried, therefore, to estimate the number of juveniles on the study plot each month by multiplying the number of lactating females by the mean litter size. As expected, the results were consistently higher than the estimate based on trapping data. The discrepancy was largest in April, May, June and October. During the winter there was no important difference between the two estimates. Even when the discrepancy was greatest, the estimated weight of the juveniles missed by trapping was not large enough to modify the picture of habitat utilization in any important way. I chose, therefore, to count only those juveniles actually trapped. Although probably consistently too low, such a figure seemed more reliable than an estimate made on any other basis.

A study of the age groups in each month's population revealed a differential survival based on the season of birth. Blair (1948:405) found that chances of survival in Microtus pennsylvanicus were approximately equal throughout the year. In the present populations of M. ochrogaster, however, voles born in October, November, December and January tended to live longer than those born in other months (Fig. 3). Presumably these animals, born in autumn and early winter, were more vigorous than their older competitors and were therefore better able to survive the shrinking habitat of winter. Their continued survival after large numbers of younger voles had been added to the population probably was permitted by the expanding habitat of spring and summer. The percentage of the population surviving the winter of 1951-1952 was approximately double the percentage surviving the winter of 1950-1951. This difference seemed to be due to the smaller population entering the winter of 1951-1952 rather than any major difference in the environmental resistance.

As a consequence of the differential survival, most of the breeding population in the spring was made up of animals born the previous October and November. Fig. 4 shows that in February, when the percentage of breeding females ordinarily began to rise, 51.6 per cent of the population was born in the previous October and November. Voles born in these two months continued to form a large part of the population through March (45.1 per cent), April (38.5 per cent), May (23.9 per cent), June (18.7 per cent) and July (16.2 per cent) (Fig. 4). These percentages suggest that the habitat conditions in October and November were probably important in determining the population level for at least the first half of the next year.

POPULATION DENSITY

Population densities were ascertained on the study areas by means of the live-trapping program. Blair (1948:396) stated that almost all small mammals old enough to leave the nest (except shrews and moles) are captured by live-trapping. My experience, and that of other workers on the Reservation, requires modification of such a statement. The distance between traps is an important factor in determining the efficiency of live-trapping. As mentioned earlier, when House Field and Quarry Field were trapped out at the conclusion of the live-trapping program no unmarked voles were taken. This showed that the 30 foot interval between traps was short enough to cover the area as far as Microtus was concerned. The fact that unmarked adults were caught almost entirely in marginal traps is additional evidence. On the other hand, the Fitch traps were 50 feet apart and voles seemed to have lived within the grid for several months before being captured. Fitch (1954:39) has shown that some kinds of small mammals are missed in a live-trapping program because of variation in bait acceptance, both seasonal and specific.

A few individuals, missed in a trapping period, were captured again in subsequent months. These voles were assumed to have been present during the month in which they were not caught. The area actually trapped each month was estimated by a modification of the method proposed by Stickel (1946:153). The average maximum move was calculated each month and a strip one half the average maximum move in width was added to each side of the study area actually covered by traps. The study plots were bounded in part by gravel roads and forest edge acting as barriers, and for these parts no marginal strip was added. Trap lines on the opposite sides of these roads rarely caught marked voles that had crossed in either direction. It is perhaps advisable to say here that the size of House Field and Quarry Field study plots (0.56 acres) was too small for best results in estimating population levels (Blair, 1941:149). In the computations of population levels the data for males and females were combined, because no significant difference between the average maximum move of the sexes was apparent.

Fluctuations of the populations were graphed in terms of individuals per acre (Fig. 5). The variation was great in the 30 month period for which data were available, and was both chronological and topographical. The lowest density recorded was 25.2 individuals per acre and the highest density was 145.8 individuals per acre. The weight varied from a low of 847 grams per acre to a high of 5275 grams per acre.

There are few records of density of M. ochrogaster in the literature. Brumwell (1951:213) found nine individuals per acre in a prairie on the Fort Leavenworth Military Reservation and Wooster (1939:515) reported 38.5 individuals per acre for M. o. haydeni in a mixed prairie in west-central Kansas. High densities for M. pennsylvanicus reported in the literature include 29.8 individuals per acre (Blair, 1948:404), 118 individuals per acre (Bole, 1939:69), 160-230 individuals per acre (Hamilton, 1937b:781) and 67 individuals per acre (Townsend, 1935:97).

Because the study period included one period of unusually high rainfall and one year of unusually low rainfall, the normal pattern of seasonal variation of population density was obscured. An examination of the data suggested, however, that the greatest densities were reached in October and November with a second high point in the April-May-June period. These high points generally followed the periods of high levels of breeding activity (Fig. 8). The autumn rise in population may have been due, in part, to the addition of spring and early summer litters to the breeding population, but the rise occurred too late in the year to be explained by that alone. Another factor may have been the spurt in growth of grasses occurring in Kansas in early autumn, in September and October. There was a seeming correlation between high rainfall with rapid growth of grasses and reproductive activity, and, secondarily with high population densities of voles. These relationships are discussed in connection with reproduction. Lowest annual densities were found to occur in January when there is but little breeding activity and when rainfall is low and plant growth has ceased.

Marked deviation from the usual seasonal trends accompanied flood and drought. In the flood of July, 1951, although the study areas were not inundated, the ground was saturated to the extent that every footprint at once became a puddle. Immediately after the floods, on all three areas studied, populations were found to have been drastically reduced. The effect was most severe on the population of House Field, the lowest area studied, and the recovery of the population there was much slower than that of those on the other study areas (Fig. 5). Newborn voles were killed by the saturated condition of the ground in which they lay. The more precocious young of Sigmodon hispidus survived wetting better. They thus acquired an advantage in the competitive relationship between cotton rats and voles. These relationships are discussed more fully in the section on mammalian associates of Microtus.

Adverse effects of heavy rainfall on populations of small mammals have been reported by Blair (1939) and others. Goodpastor and Hoffmeister (1952:370) reported that inundation sharply reduced populations of M. ochrogaster for a year after flooding but that the area was then reoccupied by a large population of voles. Such a reoccupation may have begun on the areas of this study in the spring of 1952 when the upward trend of the population was abruptly reversed by drought. While cotton rats were abundant their competition may have been an important factor in depressing population levels of voles. The population of voles began to rise only after the population of cotton rats had decreased (Fig. 19).

In the unusually dry summer of 1952, there was a marked decline of population levels beginning in June and continuing to August when my field work was terminated. Dr. Fitch (1953, in litt.) informed me that the decline continued through the winter of 1952-53 and into the summer of 1953, until daily catches of Microtus on the Reservation were reduced to 2-10 per cent of the number caught on the same trap lines in the summer of 1951. The drought seemed to affect population levels by inhibiting reproduction, as described elsewhere in this report. A similar sensitivity to drought was reported by Wooster (1935:352) who found M. o. haydeni decreased more than any other species of small mammal after the great drought of the thirties.

No evidence of cycles in M. ochrogaster was observed in this investigation. All of the fluctuations noted were adequately explained as resulting from the direct effects of weather or from its indirect effect in determining the kinds and amounts of vegetation available as food and shelter.

The differences in densities supported by the various habitats were discussed earlier in connection with the analysis of habitats.

HOME RANGE

Home ranges were calculated for individual voles according to the method described by Blair (1940:149-150). The term, home range, is used as defined by Burt (1943:350-351). Only those voles captured at least four times were used for the home range studies. Individuals which included the edge of the trap grid in their range were excluded unless a barrier existed (see description of habitat) confining the seeming range to the study area.

The validity of home range calculations has been challenged (Hayne, 1950:39) and special methods of determining home range have been advocated by a number of authors. The ranges calculated in this study are assumed to approximate the actual areas used by individuals and are considered useful for comparison with other ranges calculated by similar methods, but no claim to exactness is intended. It is obvious, for instance, that many plotted ranges contain so-called blank areas which, at times, are not actually used by any vole (Elton, 1949:8; Mohr, 1943:553). Studies of the movements of mammals on a more detailed scale, perhaps by live-traps set at shorter intervals and moved frequently, are needed to increase our understanding of home range.

In order to test the reliability of the range calculated, an examination of the relationship between the size of the seeming range and the number of captures was made. For the first three months, trapping on House Field was done with a 20 foot grid and throughout the remainder of the study a 30 foot grid was used. The effect of these different spacings on the size of the seeming home range was also investigated. Hayne (1950:38) found that an increase in the distance between traps caused an increase in the size of the seeming home range, but in my study the increased interval between traps was not accompanied by any change in the sizes of the calculated ranges.

The number of captures, above the minimum of four, did not seem to be a factor in determining the size of the calculated monthly range. A seeming relationship was observed between the number of times an individual was trapped and the total area used during the entire time the vole was trapped. Closer examination revealed that the most important factor was the length of time over which the vole's captures extended. Table 2 shows the progressive increase in sizes of the mean range of animals taken over periods of time from one month to ten months.

TABLE 2. RELATIONSHIP BETWEEN HOME RANGE SIZE AND LENGTH OF TIME ON THE STUDY AREA

====================================================================== No. months on area 1 2 3 4 5 6 7 8 9 10 Mean range in acres .09 .09 .10 .14 .13 .17 .22 .22 .26 .24 ----------------------------------------------------------------------

Nothing concerning the home range of Microtus ochrogaster was found in the literature. Several workers, including Blair (1940) and Hamilton (1937c), have studied the home range of M. pennsylvanicus. Blair (1940:153) reported a larger range for males than for females in all habitats and in all seasons represented in his sample. In M. ochrogaster, however, I found that the mean monthly range for both sexes was 0.09 of an acre. Blair (loc. cit.) reported no individuals with a range so small as that mean, but Hamilton (op. cit.:261) mentioned two voles with ranges of less than 1200 square feet. The mean total range used by an individual during the entire time it was being trapped showed a slight difference between the sexes. Males used an average of 0.14 of an acre whereas females used an average of but 0.12 of an acre. This suggested that, as in M. pennsylvanicus (Hamilton, loc. cit.), males tended to wander more than females and to shift their home range more often.

The largest monthly range recorded was 0.28 of an acre used by a female in March, 1951, and calculated on the basis of four captures. The largest monthly range of a male was 0.25 of an acre for a vole caught eight times in November, 1950. The smallest monthly range was 0.02 of an acre; several individuals of both sexes were restricted to areas of this size. Juveniles, not included in the home range study, were usually restricted to 0.01 or, at most, 0.02 of an acre. Seasonal differences in the sizes of home ranges were not significant. However, the voles caught in the winter often enough to be used for home range studies were too few for a thorough study of seasonal variation in the size of home ranges.