the trapping grid to record air temperatures and relative humidities at
ground level. These sites were chosen as being representative of larger
topographic or vegetational areas within the grid. Belfort
hygrothermographs were installed on June 10, 1964, and were serviced
once each week through October 31, 1964, at which time the stations were
dismantled. Each station consisted of a shelter 18 by 9 by 11.5 inches,
having a false top to minimize heating (Fig. 9). The shelters were
painted white. Several rows of holes, each one inch in diameter, were
drilled in all four sides of each shelter, to provide circulation of
air. The holes were covered by brass window screening to prevent entry
of insects and rodents. Preliminary tests with several U. S. Weather
Bureau maximum and minimum thermometers, suspended one above the other,
from the top to the bottom of the shelter, revealed that there was no
stratification of air within the shelters. Nevertheless, each shelter
was placed so that the sun did not strike the sensing elements of the
hygrothermograph inside it.
[Illustration: FIG. 9: (above) Photograph of microclimatic shelter
built to house hygrothermograph. False top minimizes heating, and
ventilation holes are covered with screening. (below) Photograph
showing shelter in use.]
Accuracy of the hair elements was checked by means of a Bendix-Friez
battery driven psychrometer, in periods when humidity conditions were
stable (on clear days the relative humidity is at its lowest limits and
is "stable" for several hours during early afternoon).
The four microclimatic stations were in the following places: 1) a stand
of big sagebrush near Far View Ruins; 2) a pinyon-juniper-muttongrass
association; 3) a stand of big sagebrush at the head of a drainage; and
4) a stand of Gambel oak on a southwest-facing slope of the drainage.
Table 4 shows monthly averages of maximum and minimum air temperatures
and relative humidities at each of the four sites. Vegetation and
microclimates of the sites are discussed below.
_Far View Sagebrush Site, 7,650 feet elevation_
The shelter housing the hygrothermograph was next to the stake of
station F4a in the trapping grid (Fig. 10), in a stand of big sagebrush
on the flat, middle part of the mesa top, approximately 100 yards
southwest of Far View Ruins. The sagebrush extends approximately 200
feet in all directions from the station (Fig. 5). Pinyon pine and Utah
juniper trees are encroaching upon this area, and scattered trees are
present throughout the sagebrush. This area is one of the habitats of
_P. maniculatus_.
Sagebrush tends to provide less shade for the ground than pinyon-juniper
woodland, and therefore the surface temperatures of the soil rise
rapidly to their daily maximum. In mid-June, air temperatures rise
rapidly from 6 A. M. until they reach the daily maximum between 2 and 4
P. M. Shortly after 4 P. M. the air temperatures decrease rapidly and
reach the daily low by about 5 A. M.
Relative humidities follow an inverse relationship to air temperatures;
when air temperatures are highest, relative humidities approach their
lowest values. Thus, on clear days, humidities decrease during the day,
reaching a minimum slightly later than air temperatures attain their
maximum. Unless it rains, the highest humidities of the day occur
between midnight and 6 A. M.
_Drainage Site, 7,625 feet elevation_
This site was in the bottom of the drainage that runs through the
eastern side of the trapping grid, and through parts of units M, N, I,
and J. The site was at station M4d on a level bench at the head of the
drainage (Fig. 11). Southward from the station the drainage deepens
rapidly, and the bottom loses approximately 25 feet in elevation for
every 200 feet of linear distance. _P. maniculatus_ lives here.
The microclimate of the drainage differs markedly from that of other
stations. The major difference is attributable to the topography of the
drainage itself. Nocturnal cold air flows from the surrounding mesa top
to lower elevations. A lake of cold air forms in the bottom of the
drainage; the depth of the lake depends in part upon the depth of the
drainage. The same phenomenon occurs in canyons and causes cooler night
time temperatures on the floor of canyons than on adjacent mesa tops
(Erdman, Douglas, and Marr, in press). Drainage of cold air into lower
elevations affects both nocturnal air temperatures and relative
humidities. Table 4 shows that maximum air temperatures in the drainage
did not differ appreciably from those at other stations. Mean minimum
temperatures, however, were considerably lower in the drainage than at
the other sites. This phenomenon is reflected also in the mean air
temperatures at this station.
[Illustration: FIG. 10: (above) Photograph of microclimatic station
at the Far View Sagebrush Site, at trapping station F4a in the
grid south of Far View Ruins. Dominant vegetation is _Artemisia
tridentata_.]
[Illustration: FIG. 11: (below) Photograph of microclimatic station
at the Drainage Site, in the bottom of a shallow drainage at
trapping station M4d of the grid south of Far View Ruins.]
The drainage site had the highest humidities of all stations each month
in which data were collected (Table 4). Relative humidities of 90 to 100
per cent were common in the drainage, but occurred at other stations
only in rainy periods. For example, in the month of August, 26 of the
daily maximum readings were between 95 and 100 per cent at the drainage
site, but at the other stations relative humidities were above 95 per
cent for an average of only nine nights. Minimum humidities were about
the same for all stations, since they are affected by insolation
received during the day, and not by the drainage of cold air at night.
_Oak Brush Site, 7,640 feet elevation_
The station was in an oak thicket at trapping station L4a, 250 feet
south and 50 feet east of the drainage site on a southwest-facing slope
of about 30 degrees (Fig. 12). The station was on the lower third of the
slope, approximately 15 feet higher than M4d, the station in the bottom
of the drainage. _P. truei_ and _P. maniculatus_ occur together in this
area.
Air temperatures and relative humidities at this station did not differ
appreciably from mean temperatures and humidities at the other stations.
The unusual feature is the lack of evidence of cold air drainage. The
lake of cold air in the bottom of the drainage apparently is too shallow
to reach this station. This site is near the head of the drainage, and
the cold, nocturnal air probably moves rapidly down slope into the
deeper parts of the canyon, rather than piling up at the shallow head of
the drainage.
In spite of the shade afforded the ground by the oak brush, temperatures
reached the same maximum values as at the drainage site, owing to the
orientation of the slope. South-facing slopes receive more direct
insolation throughout the day and throughout the year than north-facing
slopes and mesa tops (Geiger, 1965:374). In Mesa Verde, south-facing
slopes tend to be more arid; snow melts rapidly, and most of this
moisture evaporates. As a consequence, south-facing slopes have less
soil moisture and more widely-distributed vegetation than north-facing
slopes where snows often persist all winter and melt in spring. (For a
detailed discussion of climates on northeast-versus-southwest-facing
slopes in Mesa Verde, see Erdman, Douglas, and Marr, in press.)
_Pinyon-Juniper-Muttongrass Site, 7,600 feet elevation_
The station was in the trapping grid at D5b (Fig. 13). The
pinyon-juniper woodland surrounding this site resembles much of the
woodland on the middle part of the mesa. The forest floor is well shaded
by the coniferous canopy, and muttongrass is the dominant plant in the
ground cover. _P. truei_ lives in this habitat.
The climate at this site is moderate. Shade from the canopy greatly
moderates the maximum air temperatures during the day; minimum air
temperatures, however, are about the same as at the other stations
(Table 4). Mean temperatures are somewhat lower at this site than at the
others because of the lower maximum temperatures. Relative humidities do
not differ markedly from those at other stations.
Figure 14 shows hygrothermograph traces at all stations for a typical
week. An interesting phenomenon is illustrated by several of these
traces. By about midnight, air temperatures have cooled to within a few
degrees of their nightly low. At this time, heat is given up by the
surface of the ground in sufficient quantities to elevate the air
temperature at ground level. This release of reradiated energy lasts
from one to several hours, then air temperatures drop to the nightly low
just before sunrise. A depression in the percentage of relative humidity
accompanies this surge of warmer air. On some nights winds apparently
disturb, or mix, the layers of air at ground level. On such nights the
reradiation of energy is not apparent in the traces of the thermographs.
Reradiation of energy is restricted to ground level, and traces of
hygrothermographs in standard Weather Bureau shelters, approximately
four feet above the ground surface, at other sites on the mesa top did
not record it.
[Illustration: FIG. 12: (left) Photograph of microclimatic station
at the Oak Brush Site, at trapping station L4a of the grid south
of Far View Ruins. (right) General view of the stand of Gambel
oak in unit L of the trapping grid.]
[Illustration: FIG. 13: Photograph of microclimatic station at the
Pinyon-Juniper-Muttongrass Site, at trapping station D5b of the
grid south of Far View Ruins. Grass in the foreground is
muttongrass, _Poa fendleriana_.]
The instruments used in this study were unmodified Belfort
hygrothermographs containing as sensing units a hair element for
relative humidity and a Bourdon tube for air temperatures. The hair
element, especially, does not register changes in humidity at precisely
ground level; rather, it reflects changes in the layer of air from about
ground level to about a foot above. Thus data from these instruments
give only approximations of the conditions under which mice live while
they are on the ground.
Climatic conditions greatly influence trapping success. Larger numbers
of mice generally were caught on nights when humidities were higher than
average. Rain in part of the evening almost invariably resulted in more
mice of each species being caught. This was probably due to increased
metabolism, by the mice, to keep warm. Apparently the mice began
foraging as soon as the rains subsided; mice were always dry when caught
after a rain. Few mice were caught if rains continued throughout the
night and into the daylight hours.
TABLE 4--Monthly Averages of Daily Means for Maximum, Minimum, and
Mean Air Temperatures and Relative Humidities at Four Sites in
Mesa Verde National Park, Colorado.
===================+========================+========================
Site | Maximum Temps. | Maximum R. H.
| J J A S O | J J A S O
| |
Far View Sagebrush | 89 91 86 77 74 | 68 84 82 88 71
Drainage | 86 91 85 78 78 | 87 94 93 96 84
Oak Brush | 86 88 82 76 81 | 57 78 80 80 66
Pinyon-Juniper-Poa | 75 80 74 66 64 | 59 83 82 88 58
| |
| Minimum Temps. | Minimum R. H.
| J J A S O | J J A S O
| |
Far View Sagebrush | 42 53 50 42 31 | 18 24 25 29 21
Drainage | 36 48 45 38 26 | 21 26 27 29 30
Oak Brush | 42 52 50 42 32 | 19 25 30 31 21
Pinyon-Juniper-Poa | 44 54 50 42 34 | 22 30 29 32 25
| |
| Mean Temps. | Mean R. H.
| J J A S O | J J A S O
| |
Far View Sagebrush | 66 72 68 60 52 | 43 54 54 48 46
Drainage | 61 70 65 58 52 | 54 60 60 62 52
Oak Brush | 64 70 66 59 56 | 38 51 55 56 44
Pinyon-Juniper-Poa | 60 67 62 54 49 | 41 56 55 60 42
-------------------+------------------------+------------------------
[Illustration: FIG. 14: Diagram of hygrothermograph traces showing
daily progressions of air temperatures and relative humidities at
each of four microclimatic stations, from the morning of July 1
through the morning of July 8, 1964. Slanting vertical lines on
each chart designate midnight (2400 Hrs.) of each day.]
Nights of high trapping success usually were associated with days having
solar insolation below the average. Insolation was measured with a
recording pyrheliometer at a regional weather station (M-2) on the
middle of Chapin Mesa, at an elevation of 7,150 feet (Erdman, Douglas,
and Marr, in press). This station was approximately one mile south of
the trapping grid; isolation at this site would have been essentially
the same as that received by the trapping grid. Below-average isolation
for one day indicates cloudy conditions, which are accompanied by
increased humidity, but may or may not be accompanied by precipitation.
Trapping on nights preceded and followed by days of average or above
average isolation with average humidities--indicative of clear days and
clear moonlit nights--did not yield appreciably higher catches of mice
than other nights. Hence there was no evidence that mice tended to
avoid, or to seek out, traps on clear moonlit nights.
On cold, humid nights in autumn numerous mice caught in Sherman live
traps succumbed from exposure, even though nesting material (kapok or
cotton) and food were in the traps. Occasionally mice succumbed to heat
when traps were inadvertently exposed to too much sunlight. Apparently
little heat is required to kill individuals of either species. Traps in
which animals died due to excessive heat usually were not hot to the
touch; in most instances the traps were checked before 9:00 A. M.,
several hours before the sun caused maximum heating. Such individuals
may have licked the fur of their chests in an attempt to lower their
body temperatures. Although mice characteristically salivate before
succumbing from heat, these individuals had moist fur over the entire
chest and upper parts of the front legs, indicating licking. Mice killed
by exposure to heat or cold usually were juveniles or young; subadult
and adult individuals of both species were more tolerant. Older animals
would be expected to have better homeostatic controls than younger
individuals.
HABITAT PREFERENCE
In Mesa Verde _P. truei_ and _P. maniculatus_ occur together only at the
fringes of the pinyon-juniper woodland, where ecotonal areas provide
less than optimum habitats for both species. Almost all individuals of
_P. truei_ occur only in pinyon-juniper woodland, whereas _P.
maniculatus_ occurs only in more open habitats, such as grassy meadows
and stands of sagebrush.
Pinyon mice were abundant in a variety of associations within the
pinyon-juniper woodland. The highest population densities were in
pinyon-juniper woodland having an understory of mixed shrubs. In such an
association, _Poa fendleriana_ usually is the dominant grass in the
ground cover. _P. truei_ was especially abundant along brushy slopes
where mixed shrubs (_Amelanchier_, _Cercocarpos_ and _Fendlera_) were
codominant with pinyon pines and Utah junipers. The pinyon-juniper-mixed
shrub area west of Far View Ruins was almost optimum habitat for _P.
truei_.
_P. truei_ was abundant on the rocky ridge of Wetherill Mesa near Mug
House; the pinyon-juniper woodland here has a _Cercocarpos_ understory,
and appears to provide close to optimum conditions for this species.
Not all associations of the pinyon-juniper woodland support large
numbers of _P. truei_. Pinyon-juniper woodland having a ground cover of
_Poa fendleriana_, and no shrubs, supports few mice; the woodland on
Wetherill Mesa near Long House is an example. Juniper-pinyon woodland
having a _Purshia tridentata_ understory also supports only a few mice.
Such areas occur on the southern ends of the mesas and are
characterized by widely-spaced trees and little ground cover--a
reflection of the relatively low amounts of precipitation received by
the southern end of the park.
_P. truei_ was not found in grasslands on Navajo Hill, or in meadows at
the southern end of Moccasin Mesa. The old burned areas on the northern
end of Wetherill Mesa and on Morfield Ridge now support numerous grasses
and shrubs, but _P. truei_ appears not to live there.
_P. truei_ tends to avoid stands of sagebrush, or grasslands, lacking
pinyon or juniper trees. _P. truei_ may venture into such areas while
feeding. This species is found in thickets of Gambel oak and in areas
with an overstory of mixed shrubs only when a living pinyon-juniper
canopy is present, or when a woodland adjoins these areas.
Rocky terrain apparently is not a requirement for _P. truei_, since much
of the pinyon-juniper woodland that is free of rocks supports large
numbers. Optimum habitat, however, had a rocky floor. In such places,
rocks probably are of secondary importance, whereas the shrubs and other
plants growing on rocky soils are important for food and cover. Rocks
likely provide additional nesting sites, and allow a larger population
to live in an area than might otherwise be possible.
In Mesa Verde the deer mouse, _P. maniculatus_, prefers open areas
having dense stands of grasses, or brushy areas adjoining open terrain.
This species lives in stands of big sagebrush; in grassy areas having an
oak-chaparral or mixed-shrub-overstory; and in grasslands without
shrubs, such as on the southern end of Moccasin Mesa. Pure stands of
sagebrush did not support large numbers of mice unless there was
additional cover nearby in the form of shrubs or oak brush.
Optimum habitats for _P. maniculatus_ were on Navajo Hill, in the burned
areas on Morfield Ridge, on the northern end of Wetherill Mesa, and in
the grassy areas near the entrance of the park. The trapping areas in
the first three mentioned had heavy growths of grass and an overstory of
shrubs.
Some individuals of _P. maniculatus_ ventured into pinyon-juniper
woodland and entered traps. Such animals usually were found in places
having a heavy understory of sagebrush, or in disturbed places within
the woodland.
_P. maniculatus_, but not _P. truei_, was taken in the arid
pinyon-juniper-bitterbrush stand on the southern end of Wetherill Mesa.
_P. maniculatus_ also was present, in about equal numbers with _P.
truei_, in a pinyon-juniper-muttongrass stand north of Long House. Both
of these localities supported only a few mice.
_P. maniculatus_ is found more frequently in pinyon-juniper woodland
when the population density is high, and when such woodlands adjoin
grasslands or sagebrush areas. As mentioned earlier, _P. truei_ and _P.
maniculatus_ occur together in ecotonal areas between the forest and
grassy or brushy areas. In Mesa Verde the deer mouse inhabits exposed
grassy areas that have mostly shrubs in the open canopy.
_P. maniculatus_ is the first to colonize areas that have been burned;
this species invades such areas as soon as primary successional
vegetation becomes established. It can be stated that in general, _P.
maniculatus_ will be found in the harsher, more arid habitats. If the
habitat is so inhospitable that only a few mice can survive there, _P.
maniculatus_ will be present. _P. truei_ apparently requires the more
moderate conditions found in the pinyon-juniper forest, and this species
does not venture far from the edge of the forest.
NESTING AND NEST CONSTRUCTION
Ten individuals of _P. truei_ and three of _P. maniculatus_ were
followed to their nesting places. Photographs were taken of the nesting
sites before and after uncovering. Plants or other materials used in
their construction and any commensal arthropods present were saved and
later identified.
Nests of _P. truei_ usually were associated with juniper trees. Dead
branches and trunks of juniper trees decay from the inside, and the
resulting hollows are favored sites for the nests. Pinyon pine trees
tend to decay from the outside and were not used as nesting sites by _P.
truei_. Nests of _P. truei_ were found in hollow trunks and branches of
otherwise healthy juniper trees, and in hollow logs lying on the ground.
The heartwood apparently rots rapidly in juniper trees, but the sapwood
remains intact for many years--even after the tree is lying on the
ground. For example, a part of the pinyon-juniper woodland on the
southern end of Chapin Mesa was burned in 1858, and the hollow trunks of
junipers were still standing in 1966. Almost all of the pinyon pine
trees that were killed by that fire have since decayed; their former
presence is verified only by the crumbling remnants of their trunks that
lie on the ground throughout the burned area.
The following accounts illustrate the preferences of the two species of
mice in selection of nesting sites:
No. 105, _P. truei_, adult. On July 22, 1964, after being released from
a trap, this female ran to a serviceberry bush 10 feet south of station
I4d, preened herself, ate a berry from the bush, and disappeared under a
large rock at the base of the bush. Subsequent excavation revealed a
large nest composed of grasses (_Poa fendleriana_, _Sitanion hystrix_,
_Agropyron smithii_, _Koeleria cristata_), and a few leaves of
serviceberry. There were three entrances to the nest, one on each side
of the rock.
This mouse was captured again on August 12, 1964, released and followed
to a hollow juniper log 15 feet south of station C7b, and 245 feet from
the above nest. This log was dismantled, but no nest was found. A large
number of chewed juniper seeds around the log indicated that this mouse,
or others, had frequented the area.
On August 20, 1964, this female was followed to a large juniper log 20
feet northeast of station I4b. A small nest of shredded juniper bark was
found inside the log, and there were numerous nuts of pinyon pine and
seeds of Utah juniper that had been gnawed open. This site was about 320
feet from that at C7b, and about 240 feet from station I4d (Fig. 15).
No. 118, _P. truei_, young. On August 29, 1963, this male ran into a
hollow branch of a partly dead juniper tree 15 feet south of station
C5d. Part of this branch had been sawed off at some earlier time, and a
hole about one-and-a-half inches in diameter was present in the center
of the remaining part. The branch was not dissected, but probing
revealed that the hole extended far into the branch and enlarged as it
approached the trunk.
No. 177, _P. truei_, adult. This lactating female ran into the hollow
trunk of a juniper 10 feet north of station G7a. Both lateral branches
of the main trunk were rotten and hollow, but the tree appeared to be
healthy. Chewed juniper seeds were present in the trunks and around the
base of the tree.
This female later ran to a juniper log 30 feet north of station N4d.
Apparently there was no permanent nest at this site (Fig. 15).
No. 178, _P. truei_, adult. This female ran into a hollow juniper tree
10 feet south of station H3c. Hundreds of old juniper seeds, with their
embryos chewed out, were present at the base of the tree. The tree was
not cut down.
No. 238, _P. truei_, adult. This male ran into a dead juniper log 10
feet south of station O4b. Chewed juniper seeds were present on the
ground, but no nest was found in the log.
[Illustration: FIG. 15: Diagrams showing estimated home ranges of
six individuals of two species of _Peromyscus_, and location of
these ranges in the trapping grid. Nesting or hiding places are
described in the text, and are indicated on each diagram by an
X. Shaded areas represent home ranges estimated from trapping
records for 1963; outlined, unshaded areas represent estimated
home ranges for 1964.]
No. 241, _P. truei_, adult. This male ran into a small hole at the base
of a juniper tree 25 feet south of station G7c. The hole was at the fork
of the tree, four inches above the ground, and led to a large
subterranean chamber in the basal part of the trunk.
This male later ran into a dead juniper log lying on the ground 20 feet
southwest of station N3b. No nest was found in the log.
After another capture, this mouse ran to a small juniper log 40 feet
southeast of station G3d. There was a nest of shredded juniper bark and
many juniper seeds inside the log (Figs. 15-17).
No. 245, _P. truei_, adult. This female ran into a large, hollow juniper
log 20 feet northwest of station D4d. No nest was seen, but chewed
juniper seeds were noted in and around the log (Fig. 15).