Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 3

Traps were carefully shaded and a ball of kapok was placed in each trap to provide protection against the killing temperatures that can develop inside. In spite of these precautions, mice occasionally succ.u.mbed from heat or cold. The traps were baited with coa.r.s.ely-ground scratch feed.

Mammals trapped in the grid were inspected for molt, s.e.xual maturity, larvae of botflies, anomalies, and other pertinent data. Each animal was marked by toe- and ear-clipping and then released. Four toes were used on each front foot, and all five toes were used on each hind foot; two toes were clipped on the right front foot to signify number nine. The tip of the left ear was clipped to signify number 100, and the tip of the right ear was clipped to signify 200. If 300 or more animals had been captured, the tip of the tail would have been clipped to represent number 300. A maximum of 799 animals could have been marked with this system, which was used by Shepherd. I continued with it, starting my listings with number one.

Only two mice were caught that had been marked in the previous season by Shepherd.

Live traps were operated in the trapping grid from July 9 through October 25, 1963, and from June 25 through August 21, 1964. Traps were rotated through all stations five different times (35 days) in 1963, and twice (14 days) in 1964. Approximately three man hours were required each day to service and rotate the traps to the next group of stations.

By the autumn of 1964, a total of 282 mice had been captured, marked and released; these were handled 817 times. In 1963, 235 mice were caught for an average of 20 captures per day; in 1964, 47 mice were caught for an average of 9 captures per day.

Calculations of Home Range

A diagrammatic map of the trapping grid was drawn to scale with one centimeter equal to 100 linear feet. Trapping stations were numbered on the diagram to correspond with stations in the field. An outline of this drawing also was prepared to the same scale, but station numbers were omitted. Mimeographed copies of such a form could be placed over the diagrammatic map and marks made at each station where an animal was caught. A separate form was kept for each animal that was caught four or more times.

In calculating home range, it was a.s.sumed that animals would venture half-way from the peripheral stations, at which they were caught, to the next station outside the range. A circle having a scaled radius of 25 feet (half the distance between stations) was inscribed around each station on the periphery of the home range by means of a drafting compa.s.s. The estimated range for each animal was then outlined on the form by connecting peripheries of the circles. Both the inclusive boundary-strip method and the exclusive boundary-strip method (Stickel, 1954:3) were used to estimate the ranges. The area encompa.s.sed within the home ranges was measured by planimetering the outline of the drawing. At least two such readings were taken for each home range; then these planimeter values were converted into square feet.

The customary practice in delimiting home ranges on a scaled map of a grid is to inscribe squares around the peripheral stations at which the animal was trapped, and then to connect the exterior corners of these squares (Stickel, 1954:3). If the distance between stations was 50 feet, such squares would have sides 50 feet long. An easier method is to inscribe a circle having a scaled radius of 25 feet around the peripheral stations by means of a drafting compa.s.s. To my knowledge this method has not been used previously and consequently has not been tested by experiments with artificial populations.

To test the accuracy of this method, a "grid of traps" was constructed by using 8-1/2 by 11 inch sheets of graph paper with heavy lines each centimeter. The intersects of the heavier lines were considered as trap stations. A "home range" of circular shape, 200 feet (4 cm.) in diameter, with an area of 31,146 square feet (0.71 acre), was cut from a sheet of transparent plastic. Another "home range" was made in an oblong shape with rounded ends. This range measured 2 by 65 centimeters (100 by 325 feet) and had an area of 32,102 square feet (0.74 acre). Each plastic range was tossed at random on sheets of graph paper for fifty trials each. The range was outlined on the graph paper, then circles having a scaled radius of 25 feet were inscribed around each "trap station" within the range. The peripheries of the inscribed circles were then connected and the estimated home range was delimited by the exclusive boundary-strip method. The estimated range was measured by planimetering, and the data were compared with the known home range (Table 2).

It was found that when calculated by the exclusive boundary-strip method, the circular home range was overestimated by 2.22 per cent. The oblong home range was overestimated by only 1.50 per cent. Stickel (1954:4) has shown that the exclusive boundary-strip method is the most accurate of several methods of estimating home ranges, and in her experiments this method gave an overestimate of two per cent of the known range. Thus, my method of encircling the peripheral stations yields results that are, on the average, as accurate as the more involved method of inscribing squares about the trap stations, and saves a great deal of time as well. My method probably yields better accuracy; a perfect circle is easily drawn by means of a compa.s.s, whereas a perfect square is more difficult to construct without a template.

It is generally understood that the estimated home range of an animal tends to increase in size with each additional capture; this increase is rapid at first, then slows. Theoretically, the more often an animal is captured, the more reliable is the estimate of its home range. Most animals, however, rarely are captured more than a few times. The investigator must decide how many captures are necessary before the data seem to be valid for estimating home ranges.

An animal must be trapped at a minimum of three stations before its home range can be estimated, and even then the area enclosed in the triangle will be much less than the actual home range. Some investigators have plotted home ranges from only three captures (Redman and Selander, 1958:391), whereas others consider that far more captures are needed to make a valid estimate of range (Stickel, 1954:5).

TABLE 2--Summary of Data from Experiments in Calculating Home Ranges for an Artificial Population.

=======+========+=========+==========+========+================+======== Actual Calculated No. Trap Shape area area of range Series of s.p.a.cing of of by exclusive S. D.

trials in ft. range range boundary-strip in ft. method -------+--------+---------+----------+--------+----------------+-------- A 50 50 Circular 31,146 31,782 9,600 B 50 50 Oblong 32,102 32,583 9,466 -------+--------+---------+----------+--------+----------------+--------

In my study, 161 individuals of _P. truei_ were caught from one to 13 times each. The estimated home ranges of 10 individuals of _P. truei_, each caught from eight to 13 times, were plotted and measured after each capture from the fourth to the last. The percentage of the total estimated range represented by the fourth through tenth captures was, respectively, 52, 65, 73, 85, 88, 93, and 96 per cent.

Ninety-seven individuals of _P. maniculatus_ were caught from one to 10 times each. For five individuals that were each caught from seven to 10 times, the percentage of total estimated range represented by the fourth through seventh captures was, respectively, 59, 69, 85, and 93 per cent.

The above percentages do not imply that the true home range of individuals of these species can be reliably estimated after seven or 10 captures; the average percentages do, however, indicate a fairly rapid increase in known size of home ranges between the fourth and tenth captures. The estimated home ranges of _P. maniculatus_ tended to reach maximum known size at about seven captures, whereas the estimated ranges of _P. truei_ tended to attain maximum known size after nine or more captures. The controversy over the number of captures of an individual animal required for a reliable estimate of its home range was not settled by my data.

I initially decided to estimate home ranges for animals caught five or more times and at three or more stations. Of the 282 animals caught and marked, only 48 were caught five or more times. Because of the small numbers of _P. maniculatus_ that were caught five or more times, I wanted to determine whether mice caught four times had an estimated range that was significantly smaller than that of mice caught five times. Eight individuals of _P. maniculatus_ were caught four times each, and it seemed desirable to use the data from these mice if such use was justified. Data from the 48 mice caught five or more times were used for this testing.

By means of a T-test, I compared the estimated ranges of those 48 mice following their fourth capture with ranges estimated after the fifth capture. The results did not demonstrate significant differences between the two sets of estimates; therefore, I decided to use data resulting from four or more captures, and at three or more stations.

Table 3 shows estimations of the home ranges of males and females of each species of _Peromyscus_. When the inclusive boundary-strip method is used, the area encompa.s.sed by the range tends to be larger than the area of the same range when estimated by the exclusive boundary-strip method. Stickel (1954:4) has shown that the inclusive boundary-strip method overestimates the home range by about 17 percent.

a.n.a.lysis of Home Range by Inclusive Boundary-Strip Method

When all age groups were considered, the ranges of 16 males of _P.

truei_ averaged 20,000 to 80,000 square feet (ave. 47,333; S. D.

19,286). The sizes of home ranges were not significantly different (P > 0.05) between adult and subadult (including juveniles and young) males.

All females of _P. truei_ (22) had ranges encompa.s.sing 16,666 to 83,333 square feet (ave. 40,666; S. D. 17,566). Sizes of home ranges between adult and non-adult females did not differ significantly. The mean range of adult males of _P. truei_ did not differ from that of adult females (P > 0.05).

Fifteen males of _P. maniculatus_ had ranges of 16,666 to 66,666 square feet (ave. 34,222; S. D. 16,000); six adult males had ranges of 33,333 to 53,333 square feet (ave. 38,666). Sizes of home ranges of adult and non-adult males of this species did not differ significantly.

Five females of _P. maniculatus_ had ranges of 33,333 to 76,666 square feet (ave. 51,333; S. D. 15,913); of these, four adults had ranges of 33,333 to 53,333 square feet (ave. 45,000). Sizes of home ranges of adult males of this species did not differ (P > 0.05) from those of adult females.

The ranges of adult males of _P. truei_ were compared with ranges of adult male of _P. maniculatus_; likewise the ranges of adult females of each species were compared. In each case no difference was demonstrable in sizes of ranges between the species.

The largest home range of any _P. truei_ was that of animal number 18, a young male with an estimated home range of 133,333 square feet. This animal was caught only five times, and his home range appeared unusually large in relation to that of other young males of this species; hence some of the widely-s.p.a.ced sites of capture probably represent excursions from the animal's center of activity, rather than the true periphery of his range. These data were, therefore, not used in further computations. Stickel (1954:13) pointed out the advisability of removing such records from data to be used in calculations of home range.

Number eight had the largest home range of any female of _P. truei_; she was captured ten times, and had a range of 83,333 square feet. The vegetation within her range was pinyon-juniper woodland with understories of _Amelanchier_, _Artemisia nova_ and _Purshia_. Most of her home range was in the western half of unit H, but extended into parts of units D, I, G and N.

The largest home range for adult males of either species was number three of _P. truei_; he had a range of 80,000 square feet. The largest range for an adult of _P. maniculatus_ was 66,666 square feet (Table 3).

a.n.a.lysis of Home Range by Exclusive Boundary-Strip Method

Stickel (1954:4) has shown that under theoretical conditions the exclusive boundary-strip method is the most accurate of several methods of estimating home range. This method overestimates the known range by only two percent.

Table 3 shows a comparison of home range calculations obtained for each species, when calculated by inclusive and exclusive boundary-strip methods.

The data for males and for females of each species were compared in the same manner as in the inclusive boundary-strip method. The ranges of 16 male individuals of _P. truei_ encompa.s.sed 14,000 to 56,666 square feet (ave. 34,333; S. D. 13,266); of these, the ranges of 10 adult males were from 23,333 to 53,333 square feet (ave. 39,733). Twenty-two females of this species had ranges of 13,333 to 50,000 square feet (ave. 27,199; S.

D. 8,820). Eighteen adult females had the same extremes, but the average size of range, 28,000 square feet, was larger. Sizes of home ranges of males and females did not differ significantly.

The ranges of fifteen males of _P. maniculatus_ encompa.s.sed 13,333 to 46,666 square feet (ave. 26,666; S. D. 10,180). Of these, six adults had the same extremes in range, but an average size of 31,440 square feet.

The ranges of five females of _P. maniculatus_ varied from 28,000 to 53,333 square feet (ave. 37,199; S. D. 10,140). All but one of these females were adults. The sizes of home ranges of males and females did not differ significantly. No differences were found when ranges of adult males, or adult females, of both species were compared.

Adjusted Length of Home Range

The adjusted length of the range also can be used as an expression of home range. In this method, one-half the distance to the next trapping station is added to each end of the line drawn between stations at either end of the long axis of the range (Stickel, 1954:2).

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 s.e.x 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 s.e.xes. 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.