Charles L. Douglas

“ 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 of P. maniculatus use a similar arrangement of nests within their home ranges. The population of P. maniculatus was sparse in the trapping grid, and the habitat these mice occupied was such as to make following them extremely difficult. ”

“ However, an adult may have more than half of its range overlapping with that of another adult of the same sex and of the same, or different, species.
In general, the two species tended to be restricted to certain areas of the trapping grid where the respective habitats were more favorable for their needs. Figure 4 shows the parts of the trapping grid utilized by each species. Of course there is overlap in the areas utilized by each species; a few individuals of P. maniculatus may be found in what appears to be P. truei habitat, and vice versa. ”

“ Peromyscus maniculatus has adapted differently to its environment. Small size of body and appendages permit this species to use a variety of nesting sites and hiding places even though it is [500] restricted, by its anatomy, to life on the ground. The tail and hind feet are shorter than in P. truei, and P. maniculatus is an inefficient climber. I have placed individuals in bushes, and found that many walk off into space from a height of several feet. Perhaps the relative smallness of their eyes accounts for their seeming lack of awareness of how high they are above the ground. ”

“ Although knowledge of the amount of water consumed, ad libitum, by adult mice is valuable information, maintenance of the population depends upon reproduction and dispersal of young individuals. My trapping data indicate that only two to three per cent of the adults live long enough to breed in consecutive breeding seasons. In spring, the breeding population is composed largely of mice that were juveniles or subadults during the latter parts of the breeding season. ”

“ By climbing, this species can nest above-ground in the hollow branches of trees, and can rear its young in a comparatively safe setting. The ability to climb also permits vertical as well as horizontal use of a limited habitat. Because of the three-dimensional nature of the home range of truei, its range is actually larger than that of maniculatus although the standard trapping procedures makes the home range of the two appear to be about the same size. Finally, trees may offer safety from predators, and a source of food that probably is the winter staple of this species. ”

“ Graphs showing the distribution of distances between stations at which mice were captured on successive nights in Mesa Verde National Park. Graphs for each species represent records of both males and females.
Individuals of P. truei and P. maniculatus usually do not have mutually exclusive home ranges. When the home ranges for all females or males of one species are drawn on a single map of the trapping grid, almost every one of their ranges overlaps with the range of at least one other mouse. In some instances, the home range of an individual overlaps ranges of several other individuals. ”

“ When the supply of food or water was restored, the captive mice resumed their tolerant nature.
In captivity, P. maniculatus is amazingly tolerant of close confinement with members of the same species; individuals did not tend to kill their litter mates, or their young, even during shortage of food and water. This tolerance, especially under stressful conditions, probably enables P. maniculatus to persist in relatively unfavorable areas. ”

“ 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. ”

“ 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. truei is not found as resident of such areas. As mentioned earlier, individuals of P. truei may venture into such areas to feed if they are adjacent to pinyon-juniper woodland.
An individual of P. truei may have more than one nest within its home range (for example Nos. 105 and 241 cited above) . Each mouse probably has refuges, each containing a nest, strategically located in its home range. ”

“ The length of time females must care for their young influences the rate at which individuals can be added to the population. Females of P. truei nurse their young longer and keep them in the nest longer than do females of P. maniculatus. Although this may enhance the chances of survival of young of P. truei, it also reduces the number of litters that each female can have in each breeding season. Females of P. maniculatus can produce more young per litter, and each female probably can produce more litters per year than females of P. truei. ”

“ Observations of wild mice caught in live traps suggest that metabolic maturity is reached later than physical and reproductive maturity. In such trapping, it became apparent that juvenal and young mice suffered from exposure to cold and to heat much more than did subadult or adult mice. Although traps were carefully shaded and ample nesting material and food provided, some mice died in the traps. ”

“ Females of both species sometimes eat their young when the young die shortly after birth. One female of each species killed three of her four young, and ate their brains and viscera. In one of these cases, the female, of P. maniculatus, also died; the female of P. truei was the same one that adopted the surviving P. maniculatus. The female truei continued to nurse her one remaining young for at least several days after killing three of his litter mates. A reason for this cannibalism might have been that I had fed these mice for several weeks on a mixture of grains low in protein content. ”

“ When frightened, individuals of P. maniculatus ran rapidly in a zig-zag path and dove into the nearest cover. Mice, released from live traps, often stuck their heads under leaves, leaving their bodies exposed. This species tends to hide as rapidly as possible, and remain motionless. This tactic would not be of much value as an escape from carnivores, but it could be effective against birds of prey.
In Mesa Verde, P. maniculatus inhabits the more arid, open areas. When the population is dense, individuals of this species are found also in pinyon-juniper woodland. ”

“ 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 [490] 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. ”

“ Hawks, owls and eagles live in the park. Red-tailed hawks were seen frequently in the burned area on the northern end of Wetherill Mesa. Both hawks and owls probably prey upon Peromyscus in Mesa Verde, for they are well-known predators upon mice and small rodents in other areas. I tried to find owl and hawk nests that were occupied, but located only nests that were abandoned or impossible to reach.
Captive gopher snakes, Pituophis melanoleucus, ate adults of both species of Peromyscus. Gopher snakes probably are the most abundant snake in the park; they feed mostly on mice and other rodents. ”

“ 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. ”

“ 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. ”

“ A zone of woodland, where Artemisia nova replaces A. tridentata as an [447] understory codominant with Poa fendleriana, borders the pinyon-juniper-muttongrass community to the east. The next most abundant plants in the ground cover are Solidago petradoria, Penstemon linarioides and Comandra umbellata (bastard toadflax) . Koeleria cristata (Junegrass) is as abundant as Comandra, but probably is less important as a source of food for mice. ”

“ One of the native mammals is the ubiquitous deer mouse, Peromyscus maniculatus. The geographic range of this species includes most of the United States, and large parts of Mexico and Canada.
Another species of the same genus, the pinyon mouse, P. truei, also lives on the Mesa Verde. The pinyon mouse lives mostly in southwestern North America, occurring from central Oregon and southern Wyoming to northern Oaxaca. This species generally is associated with pinyon pine trees, or with juniper trees, and where the pinyon-juniper woodland is associated with rocky ground (Hoffmeister, 1951:vii) . ”

“ Theoretically, the more often an animal is captured, the more reliable is the estimate of its home range. Most animals, however, rarely are captured more than a few times. The investigator must decide how many captures are necessary before the data seem to be valid for estimating home ranges.
An animal must be trapped at a minimum of three stations before its home range can be estimated, and even then the area enclosed in the triangle will be much less than the actual home range. ”

“ 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. ”

“ 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. ”

“ Canopy vegetation is pinyon-juniper forest. A dense understory was made up of Amelanchier utahensis (serviceberry) , Cercocarpos montanus (mountain [430] mahogany) , Purshia tridentata (bitterbrush) , and Quercus gambelii (Gambel oak) . The ground cover consisted of small clumps of Poa fendleriana (muttongrass) , and Koeleria cristata (Junegrass) , intermingled with growths of one or more of the following:
Seeds of Cercocarpos montanus covered the ground under the bushes in much of the trapping area, and large numbers of juniper berries were on the ground beneath the trees. ”

“ Individuals of P. truei usually build nests in trees, or in hollow logs, and are therefore more abundant in pinyon-juniper woodland where there are many such nesting sites.
Rocks and stones are not necessary in the habitat of P. truei, although this species was most abundant where there was stony soil. The coincidence of rock or stones and a high density of P. truei is thought to be explainable in terms of vegetation. Stony soils support mixed shrubs as well as pinyon and juniper trees ”

“ 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 by Artemisia tridentata in the sandy bottom of the drainage. Shrubs become more abundant and pinyon and juniper trees become less abundant as one approaches the drainage. ”

“ Apparently calcium oxalate passes through the intestine unchanged in both species of Peromyscus, for their urine is clear and yellowish.
Although both species of mice appear to prefer plants having soft leaves, some plants having coarse leaves also are eaten. Many of the slides contained isolated sclerids. The stomach contents of one individual of P. truei contained a small fragment of the epidermis of Yucca. This fragment may have come from a young shoot. It is unlikely that Peromyscus would eat the larger, coarser leaves of Yucca.
Pinyon and juniper nuts were found in nests of all mice. ”

“ 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. ”

“ Therefore, the critical time for the population may well be the time when the season's young are being produced. Any unfavorable circumstances, such as a shortage of food or water, that would affect pregnant or lactating females would be of primary importance to the integrity of the population. ”

“ 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 of P. leucopus sired 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. ”