=============================+========+======+========+=====_Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park, Colorado

Species of Plant | Leaves | Stem | Flower | Seeds

-----------------------------+--------+------+--------+-------

_Amelanchier utahensis_ | - | - | - | +

_Calochortus gunnisonii_ | + | + | - | +

_Chaenactis douglasii_ | 0 | 0 | - | -

_Chrysothamnus depressus_ | 0 | 0 | 0 | -

_Chrysothamnus nauseosus_ | + | 0 | 0 | -

_Comandra umbellata_ | + | + | - | -

_Erigeron speciosus_ | + | + | - | -

_Eriogonum alatum_ | - | - | - | +

_Juniperus osteosperma_ | - | - | - | +

_Lupinus caudatus_ | 0 | 0 | + | -

_Lithospermum ruderale_ | 0 | 0 | - | 0

_Mellilotus alba_ | + | + | + | +

_Mellilotus officinalis_ | + | + | + | -

_Orthocarpus purpureo-albus_ | + | + | + | +

_Pedicularis centranthera_ | + | + | - | -

_Penstemon linarioides_ | + | + | - | +

_Pinus edulis_ | - | - | - | +

_Polygonum sawatchense_ | + | + | - | 0

_Solidago petradoria_ | 0 | 0 | 0 | -

_Viguiera multiflora_ | 0 | 0 | 0 | 0

-----------------------------+--------+------+--------+-------

Plant material eaten by captives of both species included _Calochortus

gunnisonii_--stem and seeds; _Comandra umbellata_--leaves and stem;

_Eriogonum alatum_--seeds; _Penstemon linarioides_--leaves and stem;

_Pinus edulis_--seeds; and _Juniperus osteosperma_--seeds.

Plant materials refused by both species of mice included the leaves and

stem of _Chaenactis douglasii_, the leaves, stem and seeds of

_Lithospermum ruderale_, and the leaves, stem and flowers of _Solidago

petradoria_.

Cricetine rodents chew plant and animal foods thoroughly; contents of

their stomachs appear as finely-particulate fragments. These fragments

invariably contain pieces of epidermis from ingested plants. Due to the

presence of cutin in the cell walls, epidermis is last to be digested.

Microscopic analysis of plant epidermis is useful in helping to

determine food habits of various animals (Dusi, 1949; Williams, 1955,

1959a; Brusven and Mulkern, 1960; Johnson, 1962). The microscopic

analysis of stomach contents provides a practical method of determining

which plants are eaten by rodents. Contents of stomachs and intestines

were removed from mice caught in snap traps, and from preserved

specimens. The contents were placed on a piece of bolting silk, washed

thoroughly with running water, stained with iron-hematoxylin and mounted

on slides, or stored in 70 per cent ethanol (Williams, 1959a; Douglas,

1965).

TABLE 6--Plants, or Parts of Plants, Eaten by Captive Individuals of

_P. maniculatus_ in Mesa Verde National Park, Colorado. 0 = not

eaten, + = eaten, - = not offered.

=============================+========+======+========+=======

Species of Plant | Leaves | Stem | Flower | Seeds

-----------------------------+--------+------+--------+-------

_Artemisia ludoviciana_ | 0 | 0 | - | -

_Calochortus gunnisonii_ | 0 | + | - | +

_Chaenactis douglasii_ | 0 | 0 | - | -

_Comandra umbellata_ | + | + | - | -

_Erigeron speciosus_ | 0 | 0 | - | -

_Eriogonum alatum_ | - | - | - | +

_Juniperus osteosperma_ | - | - | - | +

_Lappula redowskii_ | 0 | 0 | - | +

_Lithospermum ruderale_ | 0 | 0 | - | 0

_Orthocarpus purpureo-albus_ | 0 | 0 | + | +

_Penstemon linarioides_ | + | + | + | -

_Pinus edulis_ | - | - | - | +

_Purshia tridentata_ | + | + | - | -

_Sitanion hystrix_ | 0 | 0 | - | 0

_Solidago petradoria_ | 0 | 0 | 0 | -

_Sphaeralcea coccinea_ | + | + | - | +

_Stipa comata_ | 0 | 0 | - | +

_Viguiera multiflora_ | + | + | - | -

-----------------------------+--------+------+--------+-------

In order to analyze these epidermal fragments, a collection of plants

was made within the park. Slides of the epidermis of these plants were

prepared and analyzed for diagnostic characters (Douglas, 1965:197-199).

Features such as the stomatal arrangement in relation to subsidiary

cells; the types of trichomes, scales and glands; the cellular

inclusions such as starch grains, mucilage and resins are of taxonomic

value (Metcalfe and Chalk, 1950). The configuration of the anticlinal

cell walls is useful in separating species that are similar in other

respects (Douglas, 1965:199).

The following species of plants, and other food items, were identified

in the stomach or intestinal contents of _Peromyscus maniculatus_:

_Agropyron smithii_

_Artemisia_ sp.

_Eriogonum umbellatum_

_Lupinus ammophilus_

_Penstemon linarioides_

_Phlox hoodii_

_Stipa comata_

Arachnid legs

Stomach and intestinal contents of _P. truei_ contained the following

food items:

_Artemisia nova_

_Artemisia_ sp.

_Penstemon_ cf. _barbatus_

_Penstemon_ cf. _linarioides_

_Poa fendleriana_

Arachnid legs

_Eriogonum_ sp.

_Gutierrezia sarothrae_

_Yucca_ sp.

Chitin

Feathers

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 passes through the gastro-intestinal tract without being

absorbed. In rats of the genus _Neotoma_, some calcium oxalate

passes 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 passes 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 coarse 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, coarser 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 assumed that the

success of _P. maniculatus_ in inhabiting such diverse habitats is

associated 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

mass; 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

assumption 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_

------------+------+---------------+-------+---------------+--------

------------+------+---------------+-------+---------------+--------

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_

------------+------+---------------+-------+---------------+--------

------------+------+---------------+-------+---------------+--------

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, shelled 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 scraps, 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 constituents of the diet other than

protein, would all be expected to affect the body weight (Chew,

1965:145-147).

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 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

----------------+-------------+-------------+---------+-------+---------

_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

----------------+-------------+-------------+---------+-------+---------

Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park, Colorado · 第 6 / 8 段