Chapter IV: Part 4

The only significant changes in weight occurred when mice were fed low protein food (Table 10). Individuals of _P. truei_ lost 15.72 per cent and individuals of _P. maniculatus_ lost 10.03 per cent of their total body weights on this diet. This indicates that food having a protein content of more than 10 per cent but less than 23 per cent is required for maintenance of weight in these animals.

Although knowledge of the amount of water consumed, _ad libitum_, by adult mice is valuable information, maintenance of the population depends upon reproduction and dispersal of young individuals. My trapping data indicate that only two to three per cent of the adults live long enough to breed in consecutive breeding seasons. In spring, the breeding population is composed largely of mice that were juveniles or subadults during the latter parts of the breeding season. Therefore, the critical time for the population may well be the time when the season's young are being produced. Any unfavorable circumstances, such as a shortage of food or water, that would affect pregnant or lactating females would be of primary importance to the integrity of the population.

TABLE 9--A Comparison of Mean Daily Water Consumption of Mice on
High Protein Diets. Numbers in Parentheses Are Average Values;
All Others Are Ranges of Values.

Column headings:

A: Temperature
B: Relative humidity
C: Investigator

================+===========================+=========+=======+=========
| Mean daily H_{2}O | | |
| consumption | | |
Species +-------------+-------------+ A | B | C
| cc./gm. wt. | Total cc. | | |
----------------+-------------+-------------+---------+-------+---------
_P. m. osgoodi_ | (0.27-0.54) | (4.6-9.3) | 18-22 C | 10-20 |Williams,
| | | | | 1959
----------------+-------------+-------------+---------+-------+---------
| (0.496) | (10.74) | | |
_P. m. rufinus_ | 0.186-0.764 | 4.54-16.57 | 20-23 C | low |Douglas
----------------+-------------+-------------+---------+-------+---------
| (0.653) | (19.57) | | |
_P. t. truei_ | 0.429-1.031 | 13.28-30.28 | 20-23 C | low |Douglas
----------------+-------------+-------------+---------+-------+---------

One would assume that pregnant and lactating females require more water than non-pregnant females. One might also assume that juveniles require different amounts of water and food than adults. Juveniles have less dense pelage than adults, and probably are affected more by their immediate environment because of their relatively poor insulation. Juveniles might also be in an unfavorable situation insofar as water conservation is concerned, because they are actively growing, and in most cases, acquiring new pelage; it is well known that these are times of stress for the individual.

TABLE 10--Weights of Mice at Start and Finish of Experiments,
Showing Changes in Weight and Mean Weights, and Means of Changes
in Weight (mean delta).

========================================================================
_Peromyscus truei truei_
----+---------------------+----------------------+----------------------
| Lab Chow | Hog Chow | Corn
+------+------+-------+-------+------+-------+-------+------+-------
No. |Start | End |[Delta]| Start | End |[Delta]| Start | End |[Delta]
----+------+------+-------+-------+------+-------+-------+------+-------
1 | 31.0 | 31.3 | 0.3 | 31.3 | 32.3 | 1.0 | 32.3 | 29.0 | 3.3
5 | 31.1 | 30.5 | 0.6 | 30.5 | 32.8 | 2.3 | 32.8 | 28.7 | 4.1
6 | 27.6 | 27.1 | 0.5 | 27.1 | 29.5 | 2.4 | 29.5 | 27.3 | 2.2
7 | 28.0 | 26.3 | 1.7 | 26.3 | 27.5 | 1.2 | 27.5 | 22.2 | 5.3
13 | 25.8 | 30.6 | 4.8 | 30.6 | 27.0 | 3.6 | 27.0 | 22.2 | 4.8
14 | 26.9 | 30.7 | 3.8 | 30.7 | 31.4 | 0.7 | 31.4 | 27.3 | 4.1
15 | 25.4 | 29.4 | 4.0 | 29.4 | 29.8 | 0.4 | 29.8 | 24.0 | 5.8
16 | 33.0 | 32.9 | 0.1 | 32.9 | 30.5 | 2.4 | 30.5 | 26.0 | 4.5
19 | 37.6 | 38.1 | 0.5 | 38.1 | 31.8 | 6.3 | 31.8 | 22.0 | 9.8
20 | 23.5 | 25.8 | 2.3 | 25.8 | 26.2 | 0.4 | 26.2 | 22.9 | 3.1
----+------+------+-------+-------+------+-------+-------+------+-------
[=Y]| 28.9 | 30.2 | 1.8 | 30.2 | 29.8 | 2.0 | 29.8 | 25.2 | 4.7
----+------+------+-------+-------+------+-------+-------+------+-------

_Peromyscus maniculatus rufinus_
----+---------------------+----------------------+----------------------
| Lab Chow | Hog Chow | Corn
+------+------+-------+-------+------+-------+-------+------+-------
No. |Start | End |[Delta]| Start | End |[Delta]| Start | End |[Delta]
----+------+------+-------+-------+------+-------+-------+------+-------
2 | 23.0 | 20.7 | 2.3 | 20.7 | 21.1 | 0.4 | 21.1 | 18.6 | 2.5
3 | 22.7 | 23.1 | 0.4 | 23.1 | 23.8 | 0.7 | 23.8 | 20.7 | 3.1
4 | 22.0 | 21.1 | 0.9 | 21.1 | 21.8 | 0.7 | 21.8 | 21.3 | 0.5
8 | 26.3 | 28.1 | 1.8 | 28.1 | 15.8 | 2.3 | 25.8 | 23.8 | 2.0
9 | 21.5 | 24.0 | 2.5 | 24.0 | 25.1 | 1.1 | 25.1 | 21.8 | 3.3
10 | | | | | | | 22.5 | 20.0 | 2.5
11 | 21.0 | 22.1 | 1.1 | 22.1 | 20.8 | 1.3 | 20.8 | 19.0 | 1.8
12 | 22.3 | 23.2 | 0.9 | 23.2 | 21.3 | 1.9 | 21.3 | 20.4 | 0.9
17 | 18.9 | 20.0 | 1.1 | 20.0 | 19.2 | 0.8 | 19.2 | 19.4 | 0.2
18 | 17.0 | 17.5 | 0.5 | 17.5 | 19.5 | 2.0 | 19.5 | 17.3 | 2.2
21 | 18.9 | 18.1 | 0.8 | 18.1 | 20.2 | 2.1 | 20.2 | 17.3 | 2.9
----+------+------+-------+-------+------+-------+-------+------+-------
[=Y]| 21.4 | 21.8 | 1.2 | 21.8 | 21.8 | 1.3 | 21.9 | 19.9 | 2.2
----+------+------+-------+-------+------+-------+-------+------+-------

Lindeborg (1950:76) found that 15 days before parturition, pregnant and non-pregnant females of _P. m. bairdii_ drank about the same amounts of water, that females consumed more water after the young were born and until they were weaned, and that water consumption increased with an increase in weight in young, growing individuals. He found that in the later stages of pregnancy, females of _P. m. bairdii_ required 36 per cent more water than non-breeding females; at 14 days after parturition, nursing females required 111 per cent more water than non-breeding females, and at weaning time, 158 per cent more water. Dice (1922:35) reported a 217 per cent increase in drinking of _P. m. bairdii_ before parturition, and 171 per cent increase while nursing.

Several females of both species were bred prior to the start of the experiments described herein. As a consequence, it was possible to determine water and food consumption for lactating females of each species, and later, for their litters. Pregnant and lactating females, and newly-weaned litters, were fed laboratory chow throughout this experiment. The litters were separated from their mothers as soon as the young were observed to be eating, or no later than 33 days after birth.

Table 11 shows the amounts of water and food consumed by two females of each species while they were either in the later stages of pregnancy, or were nursing. Although the data in Table 11 do not cover the full developmental time of the litters involved, it is obvious that both lactating females of _P. truei_ and one female of _P. maniculatus_ consumed more water than the average for their species (Table 7). Water and food consumption was measured for both females of _P. truei_ while they were nursing. The female that gave birth to litter A was left in the cage with the male for several days after the litter was born, resulting in another litter being born about 27 days after the first. Therefore, the record of this female represents an extreme case of stress (probably a common occurrence in nature) in which a female is nursing one litter while she is pregnant with a second.

The record of the female of _P. truei_ that gave birth to litter B is the most complete, including data from the fifth day after parturition until the young were weaned on the thirty-third day after parturition. The record of the female of _P. maniculatus_ that gave birth to litter C covers the last 10 days of nursing before the young were weaned. After being separated from her litter, this female drank more than the average amounts of water, on both high and low protein diets. Although the food and water were lost several times for the female of _P. maniculatus_ with litter D, the period of time covered by the 14 days when water and food consumption were measured includes times just prior to parturition and to weaning of the young.

TABLE 11--Water and Food Consumed by Nursing Females of _P. truei_
and _P. maniculatus_. Consumption Is Calculated on the Basis of
Amount (Milliliters or Grams) Consumed per Gram of Body Weight
per Day, as well as Total Amounts Used per Day.

Column headings:

A: Water used
B: No. days
C: Average weight
D: ml. H_{2}O/gm./day
E: Total water/day
F: No. in litter
G: Food used
H: gms. food/gm./day
I: Total food/day

=====================+=======+====+=======+======+=======+===
Female | A | B | C | D | E | F
---------------------+-------+----+-------+------+-------+---
_P. truei_ (A) | 447 | 17 | 33.00 | .796 | 26.29 | 3
_P. truei_ (B) | 676 | 28 | 32.70 | .738 | 24.14 | 3
_P. maniculatus_ (C) | 191 | 10 | 19.45 | .983 | 19.10 | 5
_P. maniculatus_ (D) | 133 | 14 | 24.35 | .224 | 5.46 | 6
---------------------+-------+----+-------+------+-------+---
Female | G | B | C | H | I | F
---------------------+-------+----+-------+------+-------+---
_P. truei_ (A) | 214.7 | 26 | 33.00 | .250 | 8.26 | 3
_P. truei_ (B) | 120.5 | 24 | 32.70 | .153 | 5.02 | 3
_P. maniculatus_ (C) | 47.8 | 10 | 19.45 | .246 | 4.78 | 5
_P. maniculatus_ (D) | 180.1 | 21 | 27.42 | .312 | 8.58 | 6
---------------------+-------+----+-------+------+-------+---

It is interesting that the female of _P. maniculatus_ with litter C used much more than the average amount of water for the species, and even more per gram of body weight than lactating females of _P. truei_. Conversely, water consumption of the female with litter D was within one standard deviation of the mean for all adults of _P. maniculatus_. I infer that at least some lactating females of _P. maniculatus_ are better adapted to aridity than are some lactating females of _P. truei_.

Table 11 also shows food consumption of the four females discussed above. All females, with the exception of the female with litter D, consumed amounts of food that lie within one standard deviation of the means for their species. The female with litter D had the most young, consumed the most food but drank the least water of the four females. Later, when separated from her litter and placed on the low protein diet, this female drank only .046 milliliters of water per gram of body weight per day. This figure is less than one-third of the average amount (.174) for this species (Table 7).

The records of water and food consumption for litters A, C, and D are given in Table 12; the mice in litter B persisted in placing wood shavings in the opening of the spout on their water bottle, causing loss of the water. The data show that mice in all three litters had an average water and food consumption within one standard deviation of the mean for adults of their respective species (Tables 7 and 12). It is interesting that juveniles of both species require no more food and water per gram of body weight than adults. This indicates that if a young animal survives the rigors of postnatal life until it is weaned, it is then at no disadvantage as far as food and water consumption are concerned. This would be greatly advantageous to the species, as a population, for the young could disperse immediately upon weaning, and go into any areas that would be habitable for adults of the species.

TABLE 12--Food and Water Consumed by Young Mice in Litters, After
Weaning. Consumption Is Calculated on the Basis of the Amount
(Milliliters or Grams) Consumed per Gram of Litter Weight per
Day; Total Amounts Are Shown and Can Be Divided by Litter Size
for Average Individual Consumption. Litter Sizes Are as
Follows: A=3; C=5; D=6.

=====================+=======+=========+=====+=========+========+======
| Total | | | Average | ml. | Total
Litter | water | Total | No. | total |H_{2}O/ | water
| used |corrected|days | weight |gm./day | /day
---------------------+-------+---------+-----+---------+--------+------
_P. truei_ (A) | 1207 | 1120 | 57 | 58.30 | .337 | 19.64
_P. maniculatus_ (C) | 1427 | 1340 | 57 | 76.14 | .308 | 23.50
_P. maniculatus_ (D) | 700 | 670 | 31 | 58.80 | .367 | 21.61
---------------------+-------+---------+-----+---------+--------+------
| Total | | Average | Gms./ | Total
Litter | food | No. | total |gms. wt.| food
| used |days | weight | /day | /day
-----------------------------+---------+-----+---------+--------+------
_P. truei_ (A) | 651.2 | 50 | 58.30 | .223 | 13.02
_P. maniculatus_ (C) | 743.8 | 57 | 76.14 | .171 | 13.04
_P. maniculatus_ (D) | 471.1 | 31 | 58.80 | .258 | 15.19
-----------------------------+---------+-----+---------+--------+------

The young of pregnant and lactating females are the animals in the population most likely to be affected by a deficient supply of water. Drought could reduce the water content of the vegetation to such a level that pregnant or lactating females might find it difficult, if not impossible, to raise litters successfully. If such a drought persisted throughout an entire breeding season, the next year's population would be reduced in numbers, for even under normal climatic conditions it is almost exclusively the juveniles that survive from one breeding season to the next. If such a hypothetical drought occurred, lactating females of _P. truei_ would be in a more critical position than lactating females of _P. maniculatus_.

In order to determine how much water was available to mice in the peak of the breeding season, samples of the three most common plants in the study area were collected each week for analysis of their moisture content. Plants were placed in separate plastic bags that were sealed in the field. About a dozen plants of each species were used in each determination. Only the new tender shoots of the plants were collected, for it was assumed that mice would eat these in preference to the tougher basal portions of the plants. The plants were taken immediately to the laboratory and were weighed in the bag. Then the bag was opened and it and the contents placed in an incubator at 85 degrees Fahrenheit for a period of at least 72 hours. About 48 hours were required to dry the plants to a constant weight. The dried plants were weighed and their percentages of moisture were determined. Plants lose some water upon being placed in a closed bag; small drops of water appear immediately on the inner surface of the bag. Therefore, the bag must be weighed at the same time as the plants and the weight of the dried bag must be subtracted later.

The three kinds of plants chosen were among the most widely distributed species in the study area, and all three grow close to the ground, within reach of mice. Stems and leaves of two of the plants, _Comandra umbellata_ and _Penstemon linarioides_, were readily eaten by captive animals. Mice also were observed to eat leaves of _Comandra_ after being released from metal live traps. The third species, _Solidago petradoria_, differs from the other two in having a short woody stem that branches at ground level. The more succulent shoots arise from this woody stem. The leaves of _Solidago_ are coarse and were not eaten by captive mice. Nevertheless, this species was chosen because it is widely distributed and has the growth form of several other species of plants in the area.

The graph in Figure 20 shows that _Comandra_ contains the highest percentage of water through most of the summer. Water content of both _Penstemon_ and _Comandra_ was greatly reduced in the dry period that occurred in early July. _Solidago_ maintained a relatively constant percentage of moisture; perhaps its woody stem serves for water storage. The rains of July and August increased the percentage of moisture in the plants, but not to the extent expected. Neither _Solidago_ nor _Comandra_ reached the levels of hydration of early June. All plants were collected at or about 11 A. M. At night, when mice are active, these plants would be expected to contain a higher percentage of water than in the daytime.

The data in Figure 20 indicate that mice probably are not endangered by water shortages in most years. The average percentage of moisture in the plants studied was as follows: _Comandra umbellata_ 62.33 per cent; _Solidago petradoria_ 53.0 per cent; _Penstemon linarioides_ 49.28 per cent. If a mouse were to eat ten grams of plant material containing 50 per cent moisture, it would provide him with five grams of food and five grams of water, both of which exceed the minimum daily needs for non-pregnant adults of either species.

The data indicate that there are sufficient differences in water consumption between _P. maniculatus_ and _P. truei_ to account for their habitat preferences in Mesa Verde National Park. In years having average precipitation, water present in the vegetation has the potential for providing enough moisture for the needs of both species. Extended drought would affect individuals of _P. truei_ more adversely than individuals of _P. maniculatus_.

PARASITISM

Ectoparasites were collected by placing specimens of _Peromyscus_ in separate plastic bags soon after death, adding cotton saturated with carbon tetrachloride, closing the bag for about five minutes, then brushing the fur of the specimen above a sheet of white paper. The ectoparasites were sorted and sent to specialists for identification. Endoparasites were saved when stomach and intestinal contents were examined. Larvae of botflies were collected from mice in the autumn of 1962, placed in sand in containers, and kept over winter until they hatched. Eyelids of alcoholic specimens were inspected for mites by an authority on these organisms.

In 1961, the incidence of parasitism by botflies was the highest for the period 1960-1966. _P. maniculatus_ was more heavily infected with warbles than was _P. truei_. In 84 individuals of _P. maniculatus_ taken in September 1961, from Morfield Ridge, 32.1 per cent had warbles. The average number of warbles per animal was 1.24, and it was not uncommon to find two or three warbles per mouse. Sixty-nine per cent of the warbles were in the third instar stage, and the rest were in the second instar stage. Warble infestation was higher in the first half of September (40 per cent of mice infected) than in the second half of the month (30 per cent infected), but a larger percentage of the warbles were found (69 per cent) in the second half of the month.

In October 1961, 12.9 per cent of 62 _P. truei_ were infected with warbles. The average number of warbles per infected mouse was 1.37. Seventy-three per cent of the warbles were in the third instar stage; the rest were in the second instar stage. Warble infestation was higher in the first half of October (16 per cent of the mice infected) than in the second half of the month (5.5 per cent infected). These mice were collected from several localities on Chapin Mesa, in pinyon-juniper woodland.

In Mesa Verde the greatest incidence of infestations is in late September and early October. This agrees with the finding of other investigators (Sealander, 1961:58).

Sealander (1961) investigated hematological values in deer mice infected with botflies, and found that infected mice had significantly lower concentrations of hemoglobin than non-infected mice. Myiasis, associated with infection by _Cuterebra_, is likely to lead to a lowering of the physiological resistance of a segment of the population, and perhaps to a subsequent decline in the population (Sealander, 1961:60).

Mice infected by warbles were less agile than non-infected mice. Other investigators also have reported awkwardness in locomotion in infected mice (Scott and Snead, 1942:95; Sealander, 1961:58). Test and Test (1943:507) noted that parasitized mice did not appear to be emaciated, and this was also true of parasitized mice at Mesa Verde. Healed wounds, where warbles had emerged, were apparent on a number of mice. The warbles, and wounds, usually were found on the flanks and backs of the mice. The large, third instar larvae weighed about one gram apiece; there is little doubt that such large larvae induce trauma in their hosts.

The highest rate of infestation by botflies occurred in 1961, the year in which the population density of _P. maniculatus_ was near its peak. The population of this species was reduced considerably in 1962, and remained low through 1964. In 1965, the density of _P. maniculatus_ appeared to be increasing. Other investigators have reported that increased incidence of _Cuterebra_ infestation in deer mice coincides with lower population densities and with a downward trend in the population (Scott and Snead, 1942:95; Wilson, 1945). My data indicate that this may not be the situation in Mesa Verde.

The intestines or stomachs of almost all individuals of _P. maniculatus_ contained parasites. Endoparasites were less abundant in individuals of _P. truei_. This heavier infestation of _P. maniculatus_ by tapeworms, roundworms, and spiny-headed worms probably reflects the larger proportion of insects eaten by _P. maniculatus_ than by _P. truei_.

The most common endoparasite encountered was the nematode, _Mastophorus numidica_ Seurat, 1914; it was found in the stomachs of many individuals of both species of _Peromyscus_. This nematode has been reported from _Felis ocreata_ in Algeria, _Bitis arietans_ in the Congo, and from the following mammals in the United States: _Canis latrans_, _Peromyscus crinitus_, _P. gossypinus_, _P. maniculatus_, _P. truei_, _Onychomys leucogaster_, _Dipodomys ordii_, _Reithrodontomys megalotis_, and _Eutamias minimus_.

Individuals of _P. maniculatus_ obtained on the northern end of Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At this time, the population of _P. maniculatus_ was high, but on a downward trend.

My data and observations lead me to conclude that individuals of _P. maniculatus_ are more heavily parasitized by both botflies and endoparasites than are individuals of _P. truei_. The reasons for this unequal amount of parasitism in two species of mice occurring in the same general area remain obscure.

The kinds of endoparasites and ectoparasites collected from _P. maniculatus_ and from _P. truei_ are listed below (m = present in _P. maniculatus_, t = present in _P. truei_).

ACARINA: Ixodidae: _Dermacentor andersoni_ mt, _Ixodes angustus_ mt, _Ixodes spinipalpis_ m. Laelaptidae: _Androlaelaps glasgowi_ m. Myobiidae: _Blarinobia_ sp. m. Trombiculidae: _Euschoengastia lanei_ mt, _Euschoengastia criceticola_ m, _Euschoengastia dicipiens_ t, _Euschoengastia peromysci_ m, _Leewenhoekia americana_ m, _Trombicula loomisi_ m.

DIPTERA: Cuterebridae: _Cuterebra cyanella_ mt.

SIPHONAPTERA: _Callistopsyllus deuterus_ m, _Catallagia decipiens_ m, _Epetedia stanfordi_ mt, _Malaraeus sinomus_ mt, _Malaraeus telchinum_ mt, _Megarthroglossus procus_ mt, _Monopsyllus wagneri wagneri_ mt, _Orchopeas leucopus_ mt, _Peromyscopsylla hesperomys adelpha_ mt, _Phalacropsylla allos_ t, _Rhadinopsylla sectilis goodi_ t, _Stenistomera macrodactyla_ m, _Stenoponia_ (_ponera_ or _americana_) mt.

CESTODA: _Choanotaenia_ sp. m, _Hymenolepis_ sp. t.

NEMATODA: _Mastophorus numidica_ mt, _Syphacia obvelata_ mt, _Trichuris stansburyi_ t.

ACANTHOCEPHALA: _Moniliformis clarki_ mt.

PREDATION

In order to determine the relative numbers of each species of _Peromyscus_ that were taken on a seasonal basis by predators, scats of coyotes and foxes were collected from trails and roads at least twice each month, from September 1963 through August 1964. Scats were identified, labeled and dried; all bones and samples of hair were later removed from each scat. Scats that were intermediate in size between the droppings of foxes and coyotes, and that could not be identified readily in the field, were not collected. Bones from the scats were identified to species, and hair was identified to genus or species by comparing color patterns or cuticular patterns with samples from known mammals. More than 200 impression slides and whole mounts of guard hair and underfur were prepared.

Seven individuals of _P. truei_ and three individuals of _P. maniculatus_ were represented in 114 coyote scats (Table 13). Both species of _Peromyscus_ comprised only 3.9 per cent of the 253 items of food represented in the 114 scats. Rabbits, _Sylvilagus_ sp. and mule deer, _Odocoileus hemionus_ were the major food items of coyotes. Mice of the genus _Peromyscus_ apparently were preyed upon mostly in autumn (September through November), when mouse populations were near their yearly peaks.

Foxes also prey upon _Peromyscus_ in the park. One _P. truei_ was represented in the 16 scats of foxes that were analyzed. This individual was taken in the winter quarter (December through February).

The bobcat may be an important predator upon _Peromyscus_ in this region, but few scats of this animal were found. Since these could not be assigned to a specific month, they were not saved for analysis. Anderson (1961:58) believed that bobcats and gray foxes were the most abundant predators in the park. My observations over a period of two years led me to conclude that coyotes were more abundant than foxes and that foxes were, in turn, more abundant than bobcats.

TABLE 13--Food Present in 114 Coyote Scats Collected at Mesa Verde
National Park each Month from September 1963 through August 1964.

============================+=============+============
| Number | Percentage
Food Item | of | of total
| occurrences | items
----------------------------+-------------+------------
_Sylvilagus_ sp. | 32 | 12.65
_Spermophilus variegatus_ | 5 | 1.97
_Eutamias_ sp. | 12 | 4.74
_Reithrodontomys megalotis_ | 4 | 1.58
_Peromyscus boylei_ | 2 | 0.79
_Peromyscus maniculatus_ | 3 | 1.18
_Peromyscus truei_ | 7 | 2.76
_Neotoma cinerea_ | 2 | 0.79
_Neotoma mexicana_ | 9 | 3.56
_Neotoma albigula_ | 5 | 1.97
_Neotoma_ sp. | 3 | 1.18
_Microtus longicaudus_ | 1 | 0.39
_Microtus mexicanus_ | 11 | 4.34
_Microtus montanus_ | 1 | 0.39
_Microtus_ sp. | 1 | 0.39
_Odocoileus hemionus_ | 59 | 23.32
Grass | 34 | 13.44
Juniper berries | 23 | 9.09
Pinyon needles | 14 | 5.53
Pinyon nuts | 1 | 0.39
Arthropods | 7 | 2.76
Juniper needles | 3 | 1.18
Rodent or Lagomorph bones | 5 | 1.97
_Sceloporus_ sp. | 1 | 0.39
Unidentified fruit | 2 | 0.79
Rocks | 3 | 1.18
Paper | 4 | 1.58
Soil | 3 | 1.18
Feathers | 5 | 1.97
+-------------+------------
Total | 253 |
----------------------------+-------------+------------

Hawks, owls and eagles live in the park. Red-tailed hawks were seen frequently in the burned area on the northern end of Wetherill Mesa. Both hawks and owls probably prey upon _Peromyscus_ in Mesa Verde, for they are well-known predators upon mice and small rodents in other areas. I tried to find owl and hawk nests that were occupied, but located only nests that were abandoned or impossible to reach.

Captive gopher snakes, _Pituophis melanoleucus_, ate adults of both species of _Peromyscus_. Gopher snakes probably are the most abundant snake in the park; they feed mostly on mice and other rodents. Fur of _Peromyscus_ was found in the stomach of a striped whipsnake, _Masticophis taeniatus_ (Douglas, 1966:734).

DISCUSSION

Five species of _Peromyscus_ inhabit Mesa Verde National Park (Anderson, 1961). Two of these species, _P. crinitus_ and _P. difficilis_ are rare, and none was taken in more than 14,000 trap nights. Several individuals of _P. boylei_ were taken in live traps, but this species could not be regarded as common. The two remaining species, _P. truei_ and _P. maniculatus_, are the most abundant species in the park. Comparison of the habitats and life-cycles of these two forms and analyses of their interrelationships have been the objectives of this study.

The distribution of _P. truei_ in the park is regulated by the presence of living pinyon-juniper woodland where logs and hollow trees of _Juniperus osteosperma_ provide nesting and hiding places, and where seeds of juniper trees and nuts of pinyon trees provide food. Several other investigators have reported _P. truei_ to be associated with trees, but apparently these findings have not assumed the importance they warrant in understanding the ecology of this species. Bailey (1931:152) observed an individual of _P. truei_ nesting in a tree on Conchas Creek, New Mexico, and thought that this species might be more arboreal than was generally supposed. The type specimen of _P. t. truei_ was taken by Shufeldt from a "nest protruding from an opening in the dead and hollow trunk of a small pinon, at least 2 feet above the ground.... The nest, composed of the fine fibers of the inner bark of the pinon, was soon pulled out, and its owner dislodged...." (Shufeldt, 1885:403). Individuals of _P. truei_ usually build nests in trees, or in hollow logs, and are therefore more abundant in pinyon-juniper woodland where there are many such nesting sites.

Rocks and stones are not necessary in the habitat of _P. truei_, although this species was most abundant where there was stony soil. The coincidence of rock or stones and a high density of _P. truei_ is thought to be explainable in terms of vegetation. Stony soils support mixed shrubs as well as pinyon and juniper trees; the additional cover and source of food probably allow a greater abundance of _P. truei_ than would be possible without the shrubs. Secondarily, the rock provides nesting sites for more mice.

Stands of mixed shrubs, lacking a pinyon-juniper canopy, do not support _P. truei_. Its absence was noteworthy on Navajo Hill and on the northern end of Wetherill Mesa where only _P. maniculatus_ lived among the mixed shrubs and grassland. On the Mesa Verde, pinyon and juniper trees must be present in order for _P. truei_ to live in an area; and, these trees must be alive. Dead pinyons and junipers still stand in the burned part of Morfield Ridge, but no _P. truei_ were found there.

Although a few individuals of _P. truei_ were taken in stands of sagebrush adjacent to pinyon-juniper woodlands, this species does not ordinarily venture far from the forest.

_P. maniculatus_ lives almost everywhere in Mesa Verde; the preferred habitats are open and grassy with an overstory of mixed shrubs. Individuals of _P. maniculatus_ venture into ecotonal areas lying between grasslands and pinyon-juniper forest, or between sagebrush and pinyon-juniper forest. _P. maniculatus_ is found also in disturbed areas and in stands of sagebrush that occur in clearings of the pinyon-juniper woodland. In such areas, _P. maniculatus_ and _P. truei_ are sympatric; their home ranges overlap and any inter-specific competition that might occur would be expected in these places.

The ability of _P. maniculatus_ to live in many different habitats is correlated in part with its ability to build nests in a variety of sites. Whereas _P. truei_ usually builds nests only in dead branches or logs, _P. maniculatus_ builds nests in such varied places as spaces under rocks, at the bases of rotten trees, and in abandoned tunnels of pocket gophers. This adaptability is advantageous for the dispersal of young individuals and the movement of adults into new areas.

Nesting sites have important bearing on survival of the young. In Mesa Verde the rainy season occurs in July and August, while both species of _Peromyscus_ are reproducing. It is reasonable to assume that young animals that remain dry survive better than those that become wet and chilled. The nestling young of _P. truei_ are in a more favorable position to remain dry and warm than are nestling young of _P. maniculatus_.

Captives of each species differed in the amounts of water consumed per gram of body weight. Individuals of _P. truei_ consumed more water per gram of body weight than individuals of _P. maniculatus_. Animals may drink more water than they require when allowed to drink _ad libitum_, but Lindeborg (1952) has shown that species which consume less water when it is not restricted also fare better on a reduced ration. _P. maniculatus_ appears to be better adapted to aridity than _P. truei_. The preferred habitats of each species are in accord with these findings.

Within the trapping grid, the most moderate microenvironment, in terms of temperature and humidity, was in the pinyon-juniper forest, where _P. truei_ lives. The temperature extremes were wider in the microenvironments of a thicket of oak brush and of two different stands of sagebrush, where _P. maniculatus_ lives, than in the forest. _P. maniculatus_ tends to live in the harsher, more arid parts of Mesa Verde. Because of its propensity to build nests under things, or in the ground, and because of its ability to use less water per gram of body weight, _P. maniculatus_ is better adapted to withstand harsh environments than is _P. truei_.

_P. truei_ may be restricted to the pinyon-juniper woodland because of its need for more mesic conditions. Still, Mesa Verde is semi-arid and there are few permanent sources of water available for animals. The primary source of moisture for rodents must be their food. Analysis of the percentages of moisture contained in the three most common plants in the trapping grid showed that _P. truei_ could obtain the required moisture by eating about ten grams of these plants daily; individuals of _P. maniculatus_ would need to eat less in order to satisfy their water needs.

Individuals of _P. truei_ died more frequently in warm live-traps than did individuals of _P. maniculatus_. This indicates that _P. truei_ can tolerate less desiccation, or a narrower range of temperatures, than can _P. maniculatus_.

Both species of mice eat some of the same plants, but these plants occur widely. _P. truei_ seems to rely more upon the nuts of pinyons and the seeds of junipers than does _P. maniculatus_. Mounds of discarded juniper seeds were associated with all nesting sites of _P. truei_. Bailey (1931:153) also noticed the fondness of this species for pine nuts and juniper seeds. Apparently, the availability of these foods is one of the major factors affecting the distribution of _P. truei_. However, this is not the only factor, as is shown by the presence of _P. maniculatus_ but lack of _P. truei_ in a juniper-pinyon association with an understory of bitterbrush. This habitat was seemingly too arid for _P. truei_.

Factors Affecting Population Densities

The production of young, and success in rearing them, is essential to continuity of any population. _P. maniculatus_ is favored in this respect, because the females produce more young and wean them sooner than do females of _P. truei_. In addition, lactating females of _P. maniculatus_ require significantly less water than do females of _P. truei_. Since young mice of both species require no more water per gram of body weight than do adults, the young can disperse into any area that is habitable by their species. _P. maniculatus_ probably is affected less by prolonged drought than is _P. truei_. Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females of _Peromyscus_ are known to reabsorb embryos when conditions are unfavorable for continued pregnancy. If prolonged drought occurred in the reproductive season, and desiccated the vegetation upon which the mice depend for moisture, the populations should diminish the following year. Lactating females of _P. truei_ would be affected more seriously by a shortage of water than would lactating females of _P. maniculatus_.

Of two species, the one producing the more young probably would be subjected to more parasitism and predation than the species producing fewer young. A favorable season for botflies, _Cuterebra_ sp., revealed that _P. maniculatus_ has a higher incidence of parasitism by these flies than has _P. truei_; possibly the adult flies concentrate in the open, grassy areas where _P. maniculatus_ is more abundant, rather than in the woodlands where _P. truei_ lives. Perhaps the lower parasitism of _P. truei_ by warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower in _P. difficilis_, a species closely related to _P. truei_, than in _P. maniculatus_ (V. Keen, personal communication).

Although predation by carnivores would be expected to be higher on _P. maniculatus_, because this species does not climb, my data show that more individuals of _P. truei_ were taken by coyotes. I lack confidence in these findings, suspecting that another sample might indicate the reverse. Birds of prey probably catch more individuals of _P. maniculatus_, because this species lives in more open habitats. My data do not warrant firm conclusions regarding predation.

The length of time females must care for their young influences the rate at which individuals can be added to the population. Females of _P. truei_ nurse their young longer and keep them in the nest longer than do females of _P. maniculatus_. Although this may enhance the chances of survival of young of _P. truei_, it also reduces the number of litters that each female can have in each breeding season. Females of _P. maniculatus_ can produce more young per litter, and each female probably can produce more litters per year than females of _P. truei_.

Captives of _P. truei_ were tolerant of other individuals of the same species, even when kept in close confinement. However, when there was slight shortage of food or water they killed their litter mates, or females killed their young. Only a short period of time was necessary for one mouse to dispatch all others in the litter. The attacked mice were bitten through the head before being eaten; the brains and viscera were the first parts consumed. The population might be decimated rapidly if drought forced this species to cannibalism. When the supply of food or water was restored, the captive mice resumed their tolerant nature.

In captivity, _P. maniculatus_ is amazingly tolerant of close confinement with members of the same species; individuals did not tend to kill their litter mates, or their young, even during shortage of food and water. This tolerance, especially under stressful conditions, probably enables _P. maniculatus_ to persist in relatively unfavorable areas.

Adaptations to Environment

Each of the two species of _Peromyscus_ illustrates one or more adaptations to its environment. _P. truei_ is adapted to climbing by possession of long toes, a long tail, and large hind feet. The tail is used as a counterbalance when climbing (Horner, 1954). When frightened, individuals of _P. truei_ often ran across the ground in a semi-saltatorial fashion, bounding over clumps of grass that were as much as 18 inches high. Such individuals usually ran to the nearest tree and climbed to branches 10 to 20 feet above the ground.

Large eyes are characteristic of the _truei_ group of mice, and may be an adaptation to a semi-arboreal mode of life. A similar adaptation is shared by some other arboreal mammals, and of arboreal snakes. The large eyes of _P. truei_ in comparison to those of _P. maniculatus_, probably increase the field of vision, and permit the animal to look downward as well as in other directions.

The above-mentioned adaptations of _P. truei_ permit these graceful mice to use their environment effectively. By climbing, this species can nest above-ground in the hollow branches of trees, and can rear its young in a comparatively safe setting. The ability to climb also permits vertical as well as horizontal use of a limited habitat. Because of the three-dimensional nature of the home range of _truei_, its range is actually larger than that of _maniculatus_ although the standard trapping procedures makes the home range of the two appear to be about the same size. Finally, trees may offer safety from predators, and a source of food that probably is the winter staple of this species.

_Peromyscus maniculatus_ has adapted differently to its environment. Small size of body and appendages permit this species to use a variety of nesting sites and hiding places even though it is restricted, by its anatomy, to life on the ground. The tail and hind feet are shorter than in _P. truei_, and _P. maniculatus_ is an inefficient climber. I have placed individuals in bushes, and found that many walk off into space from a height of several feet. Perhaps the relative smallness of their eyes accounts for their seeming lack of awareness of how high they are above the ground.

When frightened, individuals of _P. maniculatus_ ran rapidly in a zig-zag path and dove into the nearest cover. Mice, released from live traps, often stuck their heads under leaves, leaving their bodies exposed. This species tends to hide as rapidly as possible, and remain motionless. This tactic would not be of much value as an escape from carnivores, but it could be effective against birds of prey.

In Mesa Verde, _P. maniculatus_ inhabits the more arid, open areas. When the population is dense, individuals of this species are found also in pinyon-juniper woodland. Apparently _P. maniculatus_ prefers the grassy areas and the thickets of oak brush. Although such habitats have harsh climatic conditions, they offer innumerable hiding places, and thus have great advantage for a species confined to the ground.

The low requirements of water per gram of body weight, the ability to eat diversified foods, the use of varied habitats, the high fecundity, and the ability to use any nook for retreat or nesting make _P. maniculatus_ a successful inhabitant of most parts of Mesa Verde, and indeed, of most of North America.

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* * * * *

Transcriber's Notes

All obvious typographic errors corrected. The notation [=Y] in Table 10 represents the Mean Value for that column. The notation H_{2}O represents the water molecule where the _{2} represents the subscripted 2. The notation 8-1/2 represents 8 and one half.

Page Correction
==== ==============
429 nuaseosus => nauseosus
430 Orthocarpos => Orthocarpus
450 ludovociana => ludoviciana
456 phrheliometer => pyrheliometer
480 rudale => ruderale
481 rates => rats
482 bases => basis
499 clumbs => clumps