Fig. 5: Diagram showing the major associations of understory and overstory vegetation in a trapping grid located south of Far View Ruins, Mesa Verde National Park, Colorado.
As the forest floor begins to slope into the drainage, the ground becomes rocky and shrubs assume more importance in the understory. Most of this shrubby zone is on the slope; on the western side this zone abuts pinyon-juniper woodland, and on the eastern side is bordered byArtemisia tridentatain the sandy bottom of the drainage. Shrubs become more abundant and pinyon and juniper trees become less abundant as one approaches the drainage. In the vegetation maps, this brushy zone is delimited on the east by a heavy line passing vertically through the middle of the grid (Figs.5-8). The codominant shrubs in the understory of this zone areAmelanchier utahensis,Artemisia novaandPurshia tridentata. The three most abundant plants on the ground areArtemisia ludoviciana,Chrysothamnus depressusandPenstemon linarioides.
The drainage occupies most of unit N and parts of Units I, J and M. Unit N is at the head of the drainage; the ground slopes rapidly southward and the bottom of the drainage in unit J is approximately 50 feet lower than in unit N. The canopy cover of the drainage isArtemisia tridentata(Fig. 5). The same three plants that are most abundant in the ground cover of the slope are also most abundant in the drainage.
Fig. 6: Diagram showing the most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.
The eastern slope of the drainage is covered with oak chaparral (Quercus gambelii); this zone occupies parts of units J, L, M, and P.Artemisia ludoviciana,Solidago petradoria, andViguiera multiflora(goldeneye), are the most abundant plants of the ground cover.
Mixed shrubs (Amelanchier,Cercocarpos, andFendlera) form large islands in the oak chaparral, in units K, L and P. The brushy areas of oak and mixed shrub give way at the top of the slope to pinyon-juniper forest with an understory ofArtemisia novaandPurshia tridentata. The three most abundant plants in the ground cover of the shrub zones areSolidago petradoria,Balsamorrhiza sagittata, andComandra umbellata. The eastern part of unit O hasAmelanchier utahensisin the understory, in addition toArtemisia novaandPurshia tridentata(Fig. 5). The northeastern corner of unit O is in pinyon-juniper woodland with an understory ofCercocarpos montanus.
Fig. 7: Diagram showing the second most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.
There are two relatively pure stands of sagebrush in the grid: one is in unit N, and the other in unit F and part of unit G. As figures5to8show, unit N has a relatively pure stand ofArtemisia tridentata(big sagebrush),withArtemisia ludoviciana,Agropyron smithii(western wheatgrass), andKoeleria cristata(Junegrass), being most abundant in the ground cover.Artemisia tridentataandArtemisia novaform the overstory in unit F and part of G. The three most abundant plants in the ground cover there areChrysothamnus depressus,Solidago petradoria, andPenstemon linarioides(Figs.6-8).
Fig. 8: Diagram showing the third most abundant species of plants in the ground cover of the trapping grid south of Far View Ruins.
Microclimates of Different Habitats
Four microclimatic stations were established in units D, F, L and M of 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.
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 4shows 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 ofP. 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. maniculatuslives 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 4shows 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.
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 isArtemisia tridentata.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. trueiandP. maniculatusoccur 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. trueilives 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 14shows 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.
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.
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.SiteMaximum Temps.Maximum R. H.JJASOJJASOFar View Sagebrush89918677746884828871Drainage86918578788794939684Oak Brush86888276815778808066Pinyon-Juniper-Poa75807466645983828858Minimum Temps.Minimum R. H.JJASOJJASOFar View Sagebrush42535042311824252921Drainage36484538262126272930Oak Brush42525042321925303121Pinyon-Juniper-Poa44545042342230293225Mean Temps.Mean R. H.JJASOJJASOFar View Sagebrush66726860524354544846Drainage61706558525460606252Oak Brush64706659563851555644Pinyon-Juniper-Poa60676254494156556042
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.
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 VerdeP. trueiandP. maniculatusoccur together only at the fringes of the pinyon-juniper woodland, where ecotonal areas provide less than optimum habitats for both species. Almost all individuals ofP. trueioccur only in pinyon-juniper woodland, whereasP. maniculatusoccurs 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 fendlerianausually is the dominant grass in the ground cover.P. trueiwas especially abundant along brushy slopes where mixed shrubs (Amelanchier,CercocarposandFendlera) were codominant with pinyon pines and Utah junipers. The pinyon-juniper-mixed shrub area west of Far View Ruins was almost optimum habitat forP. truei.
P. trueiwas abundant on the rocky ridge of Wetherill Mesa near Mug House; the pinyon-juniper woodland here has aCercocarposunderstory, and appears to provide close to optimum conditions for this species.
Not all associations of the pinyon-juniper woodland support large numbers ofP. truei. Pinyon-juniper woodland having a ground cover ofPoa fendleriana, and no shrubs, supports few mice; the woodland on Wetherill Mesa near Long House is an example. Juniper-pinyon woodland having aPurshia tridentataunderstory also supports only a few mice. Such areas occur on the southernends 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. trueiwas 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, butP. trueiappears not to live there.
P. trueitends to avoid stands of sagebrush, or grasslands, lacking pinyon or juniper trees.P. trueimay 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 forP. 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 forP. maniculatuswere 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 ofP. maniculatusventured 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 notP. truei, was taken in the arid pinyon-juniper-bitterbrush stand on the southern end of Wetherill Mesa.P. maniculatusalso was present, in about equal numbers withP. truei, in a pinyon-juniper-muttongrass stand north of Long House. Both of these localities supported only a few mice.
P. maniculatusis 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. trueiandP. maniculatusoccur 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. maniculatusis 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. maniculatuswill be found in the harsher, more arid habitats. If the habitat is so inhospitable that only a few mice can survive there,P. maniculatuswill be present.P. trueiapparently 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 ofP. trueiand three ofP. maniculatuswere 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 ofP. trueiusually 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 byP. truei. Nests ofP. trueiwere 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.
Fig. 15: Diagrams showing estimated home ranges of six individuals of two species ofPeromyscus, 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).
No. 251,P. truei, juvenile. This female ran into a dead juniper log beside station P4b. Chewed cones of pinyon pine and chewed juniper seeds were on the ground. A small nest of shredded juniper bark, and a few leaves of serviceberry, were found inside the log. Chewed pinyon nuts and juniper seeds also were present in the nest.
Fig. 16: (above) Photograph of juniper log at station G3d, which contained the nest ofP. truei# 241.Fig. 17: (below) Photograph of dissected juniper log at station G3d, showing the nest ofP. truei# 241, at end of mattock handle. The nest of shredded juniper bark contained chewed seeds of juniper trees.
No. 267,P. truei, juvenile. This male ran into a fallen juniper log 40 feet southwest of station P7a and then disappeared into a hole leading under an adjacent rock. Dissection of the log revealed many chewed juniper seeds inside and beneath the log, but no nest. I did not overturn the large rock or excavate under it.
No. 268,P. truei, adult. This pregnant and lactating female ran into a hollow branch of a partly-dead juniper tree 10 feet south of station O7d. The limb and base of the tree were hollow, and there were large numbers of chewed juniper seeds nearby. Because of time limitations, the branch was not dissected.
No. 74,P. maniculatus, juvenile. This female ran into a small circular hole in the ground 13 feet north of station J3a. Excavation revealed that this hole led into the abandoned tunnel of a pocket gopher (Thomomys bottae). The tunnel was followed for about four feet, but no nest was found and the tunnel led under a thicket of oak brush which made further excavation impractical (Fig. 15).
No. 247,P. maniculatus, adult. This male was followed to a large nest situated at the base of a stump and under a juniper log lying beside the stump, five feet from station I2c. This large nest was built on the ground and was constructed of grasses (Poa fendleriana,Stipa comata, andKoeleria cristata), and contained a few leaves of Gambel oak. It was the largest nest found. Chewed pinyon nuts were in the nest. (Fig. 15).
No. 276,P. maniculatus, juvenile. This male ran into a small hole at the base of a dead juniper tree 40 feet north of station O2c. It would have been necessary to cut the tree down to uncover the nest, and this was not deemed to be worthwhile.
The preceding accounts indicate that, in Mesa Verde, nests ofP. trueiusually are associated with hollow juniper logs or branches. In one instance a nest ofP. trueiwas found on the ground, under a rock. Shredded juniper bark, and, in one case, grasses were the materials most commonly used for nest building.
Individuals ofP. maniculatusdid not build nests in trees. One nest was found under a stump and adjacent log. Another site was in the abandoned tunnel of a pocket gopher, and a third was under a large rock. The only nest that was unquestionably built by aP. maniculatuswas constructed of grasses and a few leaves.
It seems unlikely that competition for nesting sites between the two species ofPeromyscusaffects the local distribution of each species. The analysis of nesting sites suggests thatP. trueiis restricted, in Mesa Verde, by the availability of fallen logs, hollow branches, or hollow trunks of juniper trees. My observations lead me to think that within the pinyon-juniper woodland there is a surplus of nesting sites for individuals ofP. truei. Many juniper trees have dead branches, and hollow juniper logs are abundant throughout the forest. It is inconceivable to me that the population ofP. trueicould reach densities sufficient to saturate every nesting site available to them in the trapping grid.
Sagebrush areas, or brushy zones adjacent to the pinyon-juniper woodland usually do not contain juniper logs; when hollow juniper trees or logs are not available,P. trueiis not found as resident of such areas. As mentioned earlier, individuals ofP. trueimay venture into such areas to feed if they are adjacent to pinyon-juniper woodland.
An individual ofP. trueimay have more than one nest within its home range (for example Nos.105and241cited above). Each mouse probably has refuges, each containing a nest, strategically located in its home range. Thus, if a mouse is chased by a predator, or by another mouse, it need notreturn to its main nest, but can seek refuge in one of its secondary nests. These secondary nests were small and were invariably constructed from shredded juniper bark. Some of these nests were little more than a scant handful of shredded bark that formed a platform to sit upon. Other nests were larger and ball-shaped, with one opening on the side. All of the secondary nests that were found were inside hollow juniper logs. The bark used in construction of the nests had, in each case, been transported from nearby living trees. The logs had previously lost their bark through decay.
The evidence indicates that these secondary refuges are prepared with considerable care. Not only is the bark transported for some distance, but it is shredded into a soft mass of fibers. When a mouse first establishes itself in a new area, perhaps it begins several such nests before settling upon the most favorable site. The less desirable sites, if still within the animal's range, are then available (barring competition by a new inhabitant) for outlying refuges.
My data do not indicate whether individuals ofP. maniculatususe a similar arrangement of nests within their home ranges. The population ofP. maniculatuswas sparse in the trapping grid, and the habitat these mice occupied was such as to make following them extremely difficult.
In captivity, both species constructed nests that were indistinguishable to me, when the mice were given cotton, kapok, or pieces of burlap as building material. The cotton or kapok was used directly, but the burlap was shredded into a fine mass of fluffy fibers. The burlap seemed to me to be the best building material, for it maintained its shape best.
Both species constructed nests that resembled inverted bowls. Solitary mice naturally built smaller nests than those built by females with young.
The entrance to the closed nests varied; often the female would bolt through the side of the nest where there was no opening. Sometimes the mice would exit and enter through the top of the nest. In some cases it appeared that the entire nest was closed; probably the occupant had closed the entrance. Such a closed nest would have the advantage of greatly moderating the microenvironment within the nest, and would allow the animal within to remain comfortable with a minimum expenditure of energy. The larger nests found in the trapping grid resembled those built by captives. Nests built of grasses were always larger than those built of juniper bark. Juniper bark is as easily worked into nests as are grasses, in my judgment. Therefore, difficulty of construction of nests from this material probably does not account for the smaller size of the nests composed of bark. I think the difference in insulating characteristics between the two materials probably accounts for the difference in size of the nests.
Reproduction
In Mesa Verde,Peromyscusreproduces from April through September. Reproduction is greatly reduced in the autumn, and most females complete reproduction before October.
Ten of the 20 females ofP. maniculatus, taken in May, contained embryos; five others were lactating. Lactating and pregnant females were collected on May 5, 1962, indicating that reproduction in some females began in early April. In September, 15 of 34 females were pregnant or lactating, whereas in October only two out of 15 females ofP. maniculatuswere reproducing. Onlyone female ofP. maniculatuswas found to contain embryos in October. This large adult was taken on October 3, 1963, and had six embryos, each five millimeters long. She probably would have produced a litter later in October, and would have been nursing into November. A report of October breeding in north-central Colorado described nine of 23 females ofP. maniculatusas being in a reproductive state; seven were lactating and one was pregnant between October 26 and 31, 1952 (Beidleman, 1954:118).
In the Museum of Natural History, the University of Kansas, there are 35 females ofP. maniculatusmore than 144 millimeters in total length taken from Mesa Verde in November, 1957 (Anderson, 1961:53). None of these contained embryos, and no pregnant females have been taken from the park in November.
P. trueiandP. maniculatusreproduce at about the same time. A female ofP. trueiprepared as a specimen on May 10, 1964, contained four embryos, each 20 millimeters long, indicating a breeding time in mid-April. Svihla (1932:19) reported the gestation period for non-lactatingP. trueito be 25 to 27 days and for lactating individuals, 40 days. Lactation tends to increase the gestation period of otherPeromyscusby about five days (Asdell, 1964:266). The gestation period of nine non-lactating females ofP. m. rufinuswas reported by Svihla to be 23 to 24 days. Lactation increased the length of the period of gestation in this subspecies to between 23 and 32 days (mean for seven females 26.57 ± 0.73, Svihla, 1932:19).
Females ofP. trueiwere observed in various stages of reproduction from June through September. Ten of the 20 females ofP. trueitaken in September were reproducing; four contained embryos and the other six were lactating. In October, only one of 17 females caught in snap traps was lactating. Lactating females were caught in live-traps as late as October 23, although most females had ceased reproduction by then. No pregnant or lactating females were observed in November.
InP. maniculatus, puberty has been placed at 32 to 35 days for females weighing 13 grams, and in males at from 40 to 45 days, at weights of 15 to 16 grams (Jameson, 1953:45). InP. truei, the weight of the testes is reported to rise in March and diminish through September, with accessory organs following the same cycle (Asdell, 1964:267). Young ofP. trueinurse for about one month, although some litters may not be weaned until 40 days of age. Young ofP. maniculatusare weaned between 22 and 37 days of age (Svihla, 1932:30).
Twenty-six pregnant females ofP. maniculatus, taken in the breeding seasons of 1961-1964, contained from one to eight embryos each; the mean was 4.65 ± 1.67. Other investigators have found similar mean values in this species (Asdell, 1964:266).
Thirteen females ofP. trueitaken in the breeding seasons of 1961-1964, contained from three to six embryos each; the mean was 4.0 ± .912. Svihla (1932:25) reported litter sizes, at birth, of two to five and a mean of 2.84, in 19 litters. Other investigators have reported litter sizes of one to five with a mean of 3.4, and one to six with a mean of 3.6 (Asdell, 1964:268). ApparentlyP. trueidoes not have more than six young per litter.
In captivity, females of both species began reproduction in early February. These captives had been kept for several months at a temperature of 21 degrees Centigrade, and on a daily photoperiod of 15 hours. Some captive males had enlarged, scrotal testes in January; the extended photoperiod and warm temperatureprobably influenced the breeding condition. In both species testes of wild males caught in autumn after late September and on through the winter were abdominal, except for one male ofP. maniculatuswhich had enlarged, scrotal testes on October 15.
Dates at which different animals arrived at breeding condition varied, in part owing to subadults (young of the year) appearing in the catch from early summer to late autumn. Some adult females appeared to be pregnant or lactating throughout much of the summer and early autumn, whereas other females, that were caught a number of times, apparently reproduced only once in the summer.
Some females may fail to breed even though they are mature enough to do so. One female ofP. trueicaptured eight times (August 30 to September 20) was a juvenile when first caught, and was classed as young (in postjuvenal molt) on September 10. She did not reproduce in her first breeding season, unless she did so after September 20, which is unlikely. Another female ofP. trueiwas an adult when first caught, and was caught 12 times (August 21 to October 25). At no time were her mammae enlarged and she was not lactating or pregnant. It is improbable that she reproduced earlier in the season, for teats of mice that have reproduced earlier usually are enlarged to such a degree that previous parturition is clearly indicated. It was surprising to catch a female, of any age, 12 times in two months without sign of reproductive activity.
Only one female ofP. maniculatusdid not show reproductive activity. She was a juvenile on July 19 when first caught; a subadult on August 28 when caught the third time, and an adult on October 23 when caught the fifth time.
Burt reported a rest period of a month or more in the summer, in Michigan, during which many females ofP. leucopusdid not reproduce. They began to breed again in late summer at about the time when young of the year began reproducing (Burt, 1940:17, 19). Abundant mast was correlated with reproductivity in autumn, according to Jameson (1953:54), who thought that "food is a basic determinant of the autumn reproduction" ofP. leucopus.
Little has been written about the length of time males remain in breeding condition. Difficulties in determining breeding condition are many. Fertility customarily is determined by sectioning testes and noting the presence or absence, and relative abundance, of sperm. This procedure necessarily sacrifices the individual and indicates the breeding condition at only one moment and for only the individuals sacrificed. My observations of males caught a number of times in live traps shed some light on the breeding condition of males, but the investigator is likely to err in extrapolating physiological data from morphology when he notes whether the testes are abdominal or scrotal and whether they are enlarged or small. It was assumed that testes that have not descended, and that lie within the abdominal cavity, are not capable of producing viable sperm. This is the condition in most juveniles, and in all males during winter. As the breeding condition is attained, testes descend into the scrotum. Soon the testes and their accessory organs enlarge and are readily apparent.
Howard (1950:320) reported that numerous males ofP. leucopussired litters when their testes appeared to be abdominal, and therefore questioned whether the criterion of descended testes is valid as an indicator of breeding condition. My captive males ofP. maniculatusandP. trueidid not sire litters when their testes were abdominal, even though such males were left withadult females for as long as four to five months (August through December). Captive pairs of both species yielded no evidence of reproductive activity until January when, as mentioned earlier, some of the males had scrotal testes. Young were born first in early February, although their parents had been confined together since the preceding August. Jameson reported the testes of fecund males ofP. maniculatusas almost always 8.0 millimeters or larger (Jameson, 1953:50). Testes that are at least partly scrotal must be considered as being capable of producing motile sperm, even though this may not be the case for all individuals.
Toward the beginning and end of the breeding season the testes and accessory organs of wild mice were small and probably produced few if any sperm. At these times some males apparently were so frightened by being handled that the testes were retracted into the inguinal canals. It would have been easy to consider such males as having abdominal testes when in fact they did not. In such cases the scrotum usually was noticeably enlarged; it was found also that in many cases the testes returned to the scrotal position if the mouse was held gently for a few minutes. Careful handling of animals was found to prevent, or at least retard, retraction of the testes. Retraction of the testes from the scrotum was not a problem at the height of the breeding season when the testes were engorged.
I had originally assumed that all adult males would be fertile throughout the breeding season, and that any males with abdominal testes would be subadults or young of the year. This assumption was an oversimplification; all adult males did not reach breeding condition at the same time of year. My data do not support a firm conclusion, for it is difficult to follow non-captive individuals throughout a breeding season, owing to sporadic appearance of animals in traps. Nevertheless, observations of mice that were trapped a number of times indicated the following:
1) Some adult males that had abdominal testes in the middle of July reached breeding condition as late as late August and even late September.
2) Some juvenal males had scrotal testes at the time their postjuvenal molt was just beginning to be apparent on their sides. Most juvenal males did not have scrotal testes, and many juveniles that appeared repeatedly in traps from mid-July through late October did not attain breeding condition. A mouse that was a juvenile in mid-July must have been born in mid-June.
3) Apparently animals born early in the breeding season may reproduce later in that season, whereas those born later in the breeding season tend not to breed until the following year.
Possibly cooler evening temperatures in July and August, due to the relatively larger amounts of precipitation in those months, inhibit reproductive development of late-born young. Most plants have ceased vegetative growth and have produced seeds by this time; but the interrelationships between growing seasons, climatic conditions, and reproductive physiology are unknown.
Only one adult of each species had scrotal testes after late September; theP. trueihad scrotal testes on October 24, 1963, and theP. maniculatushad scrotal testes on October 15 of that year.