Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 10

Many of the plants eaten by the mice had large numbers of crystals in the epidermis. Druses were the most abundant, but raphid crystals also were seen. Every slide contained at least one species of plant which contained druses. Such crystals are composed mostly of calcium oxalate (Esau, 1960:41). In Mesa Verde, families of plants having crystals include: Boraginaceae, Chenopodiaceae, Compositae, Cruciferae, Leguminosae, Liliaceae, Malvaceae, Ornargraceae, Rosaceae, and Saxifragaceae. Calcium oxalate is a highly insoluble compound and is innocuous if it pa.s.ses through the gastro-intestinal tract without being absorbed. In rats of the genus _Neotoma_, some calcium oxalate pa.s.ses through the intestines unchanged, but large amounts of calcium are absorbed through the intestine. The urine of pack rats is creamy in color and contains calcium carbonate. It is not understood how these rats metabolize the highly toxic oxalic acid, when converting calcium oxalate to calcium carbonate (Schmidt-Nielsen, 1964:147-148). Apparently calcium oxalate pa.s.ses through the intestine unchanged in both species of _Peromyscus_, for their urine is clear and yellowish.

Although both species of mice appear to prefer plants having soft leaves, some plants having coa.r.s.e leaves also are eaten. Many of the slides contained isolated sclerids. The stomach contents of one individual of _P. truei_ contained a small fragment of the epidermis of _Yucca_. This fragment may have come from a young shoot. It is unlikely that _Peromyscus_ would eat the larger, coa.r.s.er leaves of _Yucca_.

Pinyon and juniper nuts were found in nests of all mice. Captive mice were especially fond of pinyon nuts, and these probably provide a substantial part of the diet of _Peromyscus_ in the autumn and early winter. The winter staple of _P. truei_ appears to be juniper seeds.

Nesting sites of this mouse often could be located by the mounds of discarded seeds lying nearby.

Both species eat pinyon and juniper seeds; since _P. truei_ lives in the forest, it has better access to these foods than does _P. maniculatus_.

Mice remove the embryos of juniper seeds by chewing a small hole in the larger end of the seed. The seed coats of juniper are extremely hard, and a considerable amount of effort must be expended to remove the embryo. Captives discarded the resinous and pithy, outer layers of juniper berries. Individuals of _P. truei_ are adept climbers. Since many juniper berries remain on branches throughout the winter, the ability of these mice to forage in the trees would be especially advantageous when snow covers the ground.

WATER CONSUMPTION

_Peromyscus maniculatus_ is ubiquitous, occurring in habitats ranging from mesic boreal forests to arid southwestern deserts. Most subspecies of _P. maniculatus_ live in moderately mesic or near-mesic environments, but a few have adapted to arid conditions. It has been a.s.sumed that the success of _P. maniculatus_ in inhabiting such diverse habitats is a.s.sociated with its adaptability to different kinds of food and varying amount of available water (Williams, 1959b:606).

Throughout its range _P. maniculatus_ coexists with one or more other species of _Peromyscus_ that are more restricted in distribution.

_Peromyscus truei_ is one such species.

Both species live under xeric or near-xeric conditions, for the climate of Mesa Verde is semi-arid. Other than a few widely-scattered springs, there are no sources of free water on the top of the Mesa Verde land ma.s.s; thus animals inhabiting the park must rely upon moisture in the plants and other foods they eat, or upon dew.

Several investigators have studied water consumption in mice of the genus _Peromyscus_ (Table 7). Dice (1922) did so for the prairie deer mouse, _P. m. bairdii_, and the forest deer mouse, _P. leucopus noveboracensis_, under varying environmental conditions. He found that both species drank about the same amounts of water per gram of body weight, and that food and water requirements did not differ sufficiently to be the basis for the habitat differences between these species.

Neither of his samples was from an arid environment. Chew (1951) studied water consumption in _P. leucopus_, and recently reviewed the literature on water metabolism of mammals (Chew, 1965). In his studies of five subspecies of two species of _Peromyscus_, Ross (1930) found significant differences in water consumption between species but not between subspecies within a species. One of the subspecies of _P. maniculatus_ tested was from a desert region, whereas the other two were from mesic areas along the coast of California.

Lindeborg (1952) was the first to measure water consumption of both _P.

m. rufinus_ and _P. t. truei_, the species and subspecies with which my experiments are concerned. Lindeborg also tested the ability of five races of _Peromyscus_ to survive reduced water rations. Unfortunately, the subspecies chosen for these experiments did not include _P. t.

truei_ or _P. m. rufinus_. Lindeborg (1952:25) found that the "amounts of water consumed by various species of _Peromyscus_ from different habitats within the same climatic region were not conclusively different." However, he did find significant differences between some subspecies from different geographical areas. For example, he found no significant difference in water consumption between _P. m. bairdii_ from Michigan and either _P. m. blandus_ or _P. m. rufinus_ from New Mexico, but he found a highly significant difference between _P. l.

noveboracensis_ from Michigan and _P. l. tornillo_ from New Mexico.

Lindeborg also found that the subspecies of _Peromyscus_ that consumed the least water, and that were best able to survive a reduced water ration, were those from the more xeric climatic areas.

Some mammals may be able to change their diets in times of water stress, and thereby compensate for a shortage of water. At such times, _Dipodomys_ selects foods with high percentages of carbohydrates and conserves water by reducing the amounts of nitrogenous wastes to be excreted (Schmidt-Nielsen _et al._, 1948).

Williams (1959b) found that _P. m. osgoodi_ from Colorado drank more water on a diet rich in protein than on one rich in carbohydrates. But, her mice on a high carbohydrate diet used less than a normal amount of water for a period of only five weeks; at the end of the five weeks they were drinking about as much as they had been when on the control diet of laboratory chow. Likewise, mice adjusted to the high protein diet by consuming more water; but by the end of the fifth week their daily water consumption approximated the amount drunk when fed on laboratory chow.

Because of these results, Williams questioned the validity of the a.s.sumption that _P. maniculatus_ is able to inhabit a diversity of habitats because of its adaptability with respect to food and water requirements.

I conducted a series of experiments on water and food consumption by individuals of _P. truei_ and _P. maniculatus_. It was thought that if there were differences in water or food consumption, or both, knowledge of them might help to explain the obvious differences in habitat preferences of these two species in Mesa Verde National Park.

In August of 1965, 30 individuals of _P. truei_ and _P. maniculatus_ were trapped in Mesa Verde National Park at elevations of 7000-8400 feet, and transported to Lawrence, Kansas, where the experiments were carried out.

Mice were housed in individual metal cages (10 x 7.5 x 5 inches), having removable tops of wire mesh, and an externally-mounted water bottle that had a drop-type spout extending into the cage. Cages were on one of five shelves of a movable tier of shelving, and were rotated randomly, from one shelf to another, each week. A layer of dry wood shavings covered the bottom of each cage. A control cage was similarly equipped.

The mice were kept in a room in which temperature and photoperiod were controlled. The ambient air temperature of this room was 20 to 23 degrees Centigrade throughout the experiments, and averaged 21 degrees.

Humidity was not controlled, but remained low throughout the experiments. The room was illuminated for eight hours each day, from about 9 A. M. to 5 P. M.

The animals were fed at least once a week, at which time all remaining food was weighed and discarded, and the remaining water was measured.

Tap water was used in all of the experiments. The cages were cleaned each week. Each time the cages containing mice were handled, the control cage was handled in the same way. The amount of evaporation was determined each week by measuring the water remaining in the bottle of the control cage.

Water and food consumption of individuals of _P. maniculatus_ and _P.

truei_ were measured when the mice were fed diets of differing protein content. To my knowledge, the only other study in which water consumption was measured for mice of the genus _Peromyscus_ on diets of different protein contents was by Williams (1959b). Because of the limited number of animals available, it was decided that the best results could be obtained by placing all individuals on the same diet for a predetermined number of weeks, then on a second diet for a certain period, and so on.

Each mouse was weighed at the beginning, at the mid-point, and at the end of each experiment. The mice were weighed on the same days, at times when they were inactive. Because weights of individual mice differ, water and food consumption was calculated on the basis of the amount consumed per gram of body weight per day. All foods were air-dry and contained a negligible amount of water.

First, food and water consumption was measured for nine individuals of each species on a diet of Purina Laboratory Chow. This chow contains not less than 23 per cent protein and 4.5 per cent fat, and about 57 per cent carbohydrate. Since the mice had been maintained on this diet for several months prior to the experiments, food and water consumption was measured for a period of only two weeks. Individuals of _P. truei_ consumed more total water and more water per gram of body weight than individuals of _P. maniculatus_ (Table 7).

Next, 10 mice of each species were placed on a diet of Purina Hog Chow for a period of four weeks. This chow contains not less than 36 per cent protein and one per cent fat, and about 42 per cent carbohydrate. Both species increased their daily water consumption immediately after being placed on this diet (tables 7 and 11). On the high protein diet, _P.

truei_ again consumed much more water than did _P. maniculatus_ (tables 7 and 9).

TABLE 7--Food and Water Consumption of _Peromyscus maniculatus_ and _P. truei_ When Fed Diets of Different Protein Content. Food and Water Consumption Are Determined for the Grams, or Milliliters, Consumed per Gram of Body Weight per Day; Daily Totals Are also Given.

==================================================================== _Peromyscus maniculatus rufinus_ ------------+------+---------------+-------+---------------+-------- Diet Food Total Water Total per cent No. /gram grams /gram water protein mice /day S. D. /day /day S. D. /day ------------+------+---------------+-------+---------------+-------- Lab Chow 23 9 .201 .074 4.455 .262 .183 5.751 ------------+------+---------------+-------+---------------+-------- Hog Chow 36 10 .238 .060 5.232 .496 .186 10.749 ------------+------+---------------+-------+---------------+-------- Corn 11 11 .149 .044 3.144 .174 .012 3.696 ------------+------+---------------+-------+---------------+--------

_Peromyscus truei truei_ ------------+------+---------------+-------+---------------+-------- Diet Food Total Water Total per cent No. /gram grams /gram water protein mice /day S. D. /day /day S. D. /day ------------+------+---------------+-------+---------------+-------- Lab Chow 23 10 .216 .070 6.353 .373 .119 10.880 ------------+------+---------------+-------+---------------+-------- Hog Chow 36 10 .230 .079 6.966 .653 .189 19.571 ------------+------+---------------+-------+---------------+-------- Corn 11 10 .158 .010 4.318 .332 .016 9.034 ------------+------+---------------+-------+---------------+--------

The tendency of both species to eat more of the hog chow than they ate when fed standard laboratory chow may reflect a higher palatability of the hog chow. Both species consumed similar amounts of food per gram of body weight, on each of the diets (Table 7). The larger _P. truei_ requires more grams of food per day than the smaller _P. maniculatus_, but this slight difference in food consumption probably has no effect on the distribution of these species within Mesa Verde.

The results obtained with the low protein diet were strikingly different from those of the first two experiments. In this experiment the same groups of mice were placed on a diet of whole, sh.e.l.led corn for a period of six weeks. The corn contained less than 11 per cent protein, about three per cent fat, and about 80 per cent carbohydrate.

By the end of the first week, on the low protein diet, all mice had reduced their water intake by about half the amount used per day on the high protein diet (Table 7). There was not a statistically significant difference, for either species, between the average amounts of water drunk in the first and in the sixth weeks of the experiment.

The data in Table 7 show that on all three diets, individuals of _P.

maniculatus_ drank less water per gram of body weight than individuals of _P. truei_. Variation in water consumption was high; some individuals of _P. maniculatus_ that drank more than the average amount for the species, consumed as much water as some individuals of _P. truei_ that drank less than the average amount. In general, individuals of _P.

maniculatus_ drank about half as much water each day as individuals of _P. truei_. Individuals of both species were consistent in their day-to-day consumption.

TABLE 8--Amounts of Mean Daily Water Consumption as Reported in the Literature for Species of _Peromyscus_. Figures in Parentheses are Means; Those Not in Parentheses Are Extremes.

Column headings:

A: Mean daily ml./gm. wt./day B: Water consumption total ml. per day C: Temperature D: Humidity E: Per cent dietary protein F: Investigator

================+===========+=============+=======+=======+====+===== A B C D E F ----------------+-----------+-------------+-------+-------+----+----- (.262) (5.70) _P. m. rufinus_ .124-.699 2.71-15.07 20-23 low 23 [A]

_P. m. rufinus_ (.101) (2.39) 20-25 24-47 [B]

_P. m. osgoodi_ .16-.25 3.2-4.3 18-22 10-20 23 [C]

(.126) (1.74) _P. m. bairdii_ .082-.177 1.12-2.72 21 25-68 [D]

_P. m. bairdii_ .124-.182 (2.37-3.17) 20-25 24-47 [B]

(.372) (10.80) _P. t. truei_ .224-.561 7.0-16.92 20-23 low 23 [A]

_P. t. truei_ (.085) (2.77) 20-25 24-47 [B]

_P. l. nov._ .057-.117 1.36-2.29 21 25-68 [D]

_P. l. nov._ (5.36) 18 62.5 [E]

[A] Douglas [B] Lindeborg, 1952 [C] Williams, 1959 [D] Dice, 1922 [E] Chew, 1951

Table 8 shows average water consumption for several species of _Peromyscus_ as reported in the literature, and as determined in my study. It is difficult to compare my results with most of the data in the literature, because of a lack of information as to protein, fat, carbohydrate, and mineral contents of foods used in other studies.

Lindeborg (1952) and Dice (1922) fed mice on a mixture of rolled oats, meat sc.r.a.ps, dry skimmed milk, wheat germ, etc. described by Dice (1934). Their data on water consumption in _P. maniculatus_ indicate that this mixture probably is lower in protein content than Purina Laboratory Chow, that was used in my experiments and those of Williams'

(tables 8 and 9).

The amount of dietary protein consumed under natural conditions is not known for most wild animals. One index of the minimum amount of protein necessary is the amount required for an animal to maintain its weight.

At best, this can be only an approximation of the required amount, for other factors, such as stress, disease, change in tissues during oestrus or gonadal descent, and changes in const.i.tuents of the diet other than protein, would all be expected to affect the body weight (Chew, 1965:145-147).