Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 11

The data in Table 7 show that both species vary their food intake with changes in diet. Table 10 shows weight changes that took place in individual mice when fed each of the three diets. A change in weight of one gram cannot be considered as important, for the weight of an individual mouse fluctuates depending upon when he last drank, ate, defecated or urinated.

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 circ.u.mstances, 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 a.s.sume that pregnant and lactating females require more water than non-pregnant females. One might also a.s.sume 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 a.n.a.lysis 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 a.s.sumed 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 coa.r.s.e 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_.

[Ill.u.s.tration: FIG. 20: Graph showing percentages of moisture contained during the summer of 1964, by three abundant and widely-distributed species of plants in Mesa Verde National Park, Colorado.]

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 brus.h.i.+ng 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, a.s.sociated 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_.