Part 8

The average length of home range for 15 males of P. truei was 363 feet (S. D. 105 ft.); for 22 females of this species 326 feet (S. D. 94 ft.); for 14 males of P. maniculatus 286 feet long (S. D. 94 ft.); and for four females of this species 347 feet (S. D. 83 ft.). The mean lengths of range of males and females differed significantly in P. maniculatus, but not in P. truei. However, no difference was demonstrable in mean sizes of ranges between males, or between females, of the two species.

Distance Between Captures

The distance between captures has been used by several investigators as an index of the extent of home range. More short than long distances tend to be recorded when traps are visited at random, and when inner traps of the range are more strongly favored (Stickel, 1954:10).

TABLE 3--Summary of Data for Estimated Home Ranges of Mice from a Wild Population.

================+==================+=====+======+============+========= | | | | Estimated | Type of | Species | Sex | No. | home range | +- S. D. Estimate | | | | in sq. ft. | ----------------+------------------+-----+------+------------+--------- Inclusive | P. truei | M | 16 | 47,333 | 19,286 boundary-strip | " " | F | 22 | 40,666 | 17,566 | | | | | | P. maniculatus | M | 15 | 34,222 | 16,000 | " " | F | 5 | 51,333 | 15,913 ----------------+------------------+-----+------+------------+--------- Exclusive | P. truei | M | 16 | 34,333 | 13,266 boundary-strip | " " | F | 22 | 27,199 | 8,820 | | | | | | P. maniculatus | M | 15 | 26,666 | 10,180 | " " | F | 5 | 37,199 | 10,140 ----------------+------------------+-----+------+------------+--------- Adjusted Length | P. truei | M | 16 | 363 | 105 | " " | F | 22 | 326 | 94 | | | | | | P. maniculatus | M | 14 | 286 | 94 | " " | F | 4 | 347 | 83 ----------------+------------------+-----+------+------------+---------

It is important to know approximately how far mice travel in one night. The distances traveled between captures on successive nights were calculated for all mice. Even animals caught most frequently usually were caught only once or twice on successive nights. Data from animals caught less than four times, and hence not usable for calculations of home range, could be used in calculating the distance between captures on successive nights. Thus the data were sampled in a more or less random manner for each species.

The mean distance traveled between captures on successive nights was determined for adult and non-adult animals (juvenile, young and subadult) of both sexes. Adult males of P. maniculatus traveled an average of 151.66 feet (n = 24); young males of this species traveled an average of 134.28 feet (n = 7). Adult females of P. maniculatus traveled 170.00 feet (n = 4); no data were available for young females.

Adult males of P. truei traveled an average of 169.47 feet (n = 38); and young males traveled 159.44 feet (n = 18). Adult females of this species traveled 155.71 feet between captures (n = 35), while young females traveled 140.66 feet (n = 15).

The means were tested for differences in the distances traveled between young and adult males and between young and adult females of each species, as well as between males and between females of opposite species. In all cases, there were no demonstrable differences in the distance traveled between captures.

One of the more striking journeys between captures was that of number 59, a juvenal male of P. maniculatus, which traveled 1,070 feet between captures on July 16 and 17, 1963. The route between the two capture sites was over the most rugged part of the trapping grid. This datum was excluded from further calculations. The only other animal that approached this distance was a young female P. truei that traveled 750 feet between captures.

Figure 3 shows the distribution of distances traveled by mice of each species between successive captures. Since there were no demonstrable differences between age groups or sexes in the distances traveled, these data represent a composite of the ages and sexes of each species. They show 101-125 feet to be the most prevalent of the distances traveled by both species, and 51-75 feet to have a higher percentage of occurrence among P. maniculatus. These distances indicate that if an animal was trapped on successive nights, it tended to be trapped within the same unit of the grid. It would have been necessary for an animal to travel 200 feet or more in order to be caught in traps in an adjoining unit of the grid.

The distance between captures also was calculated by the more customary method of averaging the distances between sites of capture, regardless of the time intervening between captures.

Only data from mice caught four or more times were used because these individuals probably had home ranges in the study area, whereas those caught fewer than four times may have been migrants.

The mean distance between captures (n = 95) for 15 males and five females of P. maniculatus was 161 feet. Sixteen males and 22 females of P. truei traveled an average of 143 feet between captures (n = 248). For purposes of comparison, these average distances between captures could be considered as radii of the estimated home ranges. When the range for each species is calculated by considering average distance between captures as the radius of the estimated home range, the average range of P. truei is 64,210 square feet, and that of P. maniculatus is 81,392 square feet. Both of these estimations are larger than those made by the inclusive and exclusive boundary-strip method (Table 3), and smaller than those calculated by using adjusted length of range as the radius.

Since it is known that ranges of some animals tend to be longer than wide (Mohr and Stumpf, 1966), calculations of estimated range based on average distance between captures probably are more accurate than those based on adjusted length of range.

Usually the estimated home ranges were not symmetrical, and did not resemble oblongs or circles in outline. Rather, the ranges tended to follow parts of vegetational zones. Since trapping grids are geometrical in form, there is a tendency among investigators to consider home ranges of animals as conforming to geometrical design. This may or may not be the true situation; telemetric studies on larger animals indicate that home ranges do not conform to geometrical design. At present there is a poverty of knowledge concerning methods for determining the precise home ranges of small mammals. Telemetry appears to offer an unlimited potential for studies of this kind.

Individuals of P. truei and P. maniculatus usually do not have mutually exclusive home ranges. When the home ranges for all females or males of one species are drawn on a single map of the trapping grid, almost every one of their ranges overlaps with the range of at least one other mouse. In some instances, the home range of an individual overlaps ranges of several other individuals. In extreme cases an animal's range lies completely within the estimated boundaries of another individual's range. Such an enclosed range was always that of a juvenile or of a young animal. However, an adult may have more than half of its range overlapping with that of another adult of the same sex and of the same, or different, species.

In general, the two species tended to be restricted to certain areas of the trapping grid where the respective habitats were more favorable for their needs. Figure 4 shows the parts of the trapping grid utilized by each species. Of course there is overlap in the areas utilized by each species; a few individuals of P. maniculatus may be found in what appears to be P. truei habitat, and vice versa. In such cases, an inspection of the vegetation usually reveals an intermediate type of habitat--for example, an open sagebrush area in pinyon-juniper woodland--that is habitable for either or both species.

The ranges of P. truei tend to be clustered in the western half of the trapping grid, whereas the ranges of P. maniculatus are clustered in the eastern half of the grid (Fig. 4). The vegetation of the grid and the preferred habitats of each species are discussed in following chapters.

On the basis of the sizes of estimated home ranges, it is possible to compute the approximate number of individuals of each species that occur in each acre of appropriate habitat.

On the basis of an average home range of 30,206 +- 25,545 square feet (one standard deviation) for both male and female individuals of P. truei, there should be approximately 0.781 to 9.345 individuals of this species per acre of pinyon-juniper woodland. An average home range of 29,400 +- 24,570 square feet for males and females of P. maniculatus indicates that the density of this species is between 0.807 and 9.018 animals per acre in mixed shrub or shrub and sagebrush types of vegetation.

Figure 4 shows that approximately 10 of the 16 units of the trapping grid are suitable habitat for P. truei; the remaining six units are habitat of P. maniculatus. From the preceding calculations of density one could expect to find between seven and 90 individuals of P. truei, and between five and 54 individuals of P. maniculatus as residents within the 22.95 acres of the trapping grid. The higher estimates of density appear to be large enough to compensate for any overlapping of home ranges.

The calculation of density of each species within the trapping grid is dependent upon the precision with which the home ranges of individuals can be estimated. At this time, home ranges of small rodents can not be measured with great precision, therefore any such calculations are, at best, only approximations. This does not imply that estimations of home range are of little value; however, calculations of density, using home ranges as a basis, tend to amplify the variance inherent in the data. This amplification is reflected in the wide range between low and high limits of the densities for each species within the trapping grid.

In order to check on the accuracy of the above calculations, an estimate of density was made for each species on the basis of trapping data. Trapping records kept for each animal were checked for the year 1963. More data on home ranges were obtained in that year due to higher population densities than in 1964. If an animal was caught four or more times in 1963, it was considered to be a resident; animals caught in both 1963 and 1964 were considered to be residents even if caught fewer than four times. Mice caught three times, with at least a month elapsing between the first and third captures, were considered to be probable residents. Other animals caught three or fewer times were considered to be migrants.

In 1963, 15 individuals of P. truei were caught four or more times, or in both years, and considered to be residents; six other mice were classed as probable residents. Of P. maniculatus, 18 individuals were classed as residents, and two as probable residents. Thus the trapping data for 1963 indicate that 21 individuals of P. truei and 20 of P. maniculatus were residents of the trapping grid. These estimates lie well within the estimated limits of density of each species, as calculated from data on home range while taking into account the relative proportions of available habitat for each species within the trapping grid. Analyses of trapping data indicate that the density of each species probably is overestimated by calculations of density based on home range data.

Males and females of both species of Peromyscus appeared to be highly individualistic in the amount of area they utilized. Some adult males of P. truei covered large areas, whereas others were relatively sedentary. The same was true of young males of P. truei, although the younger males tended to have smaller ranges than adult males. Most pregnant or lactating females, of both species, tended to use smaller areas for their daily activities than did non-pregnant or non-lactating females. There were notable exceptions to this generality, for some lactating females had exceptionally large home ranges.

Size of home range apparently was not influenced by the location of an animal's range within the grid. Far more data would be needed to correlate minor differences in vegetational associations with sizes of ranges in different parts of the grid.

It is surprising that adults of P. truei do not have larger home ranges than adults of P. maniculatus. P. truei is the larger, more robust animal, capable of rapid running and occasional saltatorial bounding; individuals of this species can traverse large areas with ease. The semi-arboreal nature of P. truei may explain why individuals of this species do not have larger ranges than individuals of P. maniculatus. P. truei has a three-dimensional home range, whereas P. maniculatus has a range that is two-dimensional only (excluding the relatively minor amount of burrowing done by each species).