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_.
[Illustration: 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
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.