Stands of mixed shrubs, lacking a pinyon-juniper canopy, do not support P. truei. Its absence was noteworthy on Navajo Hill and on the northern end of Wetherill Mesa where only P. maniculatus lived among the mixed shrubs and grassland. On the Mesa Verde, pinyon and juniper trees must be present in order for P. truei to live in an area; and, these trees must be alive. Dead pinyons and junipers still stand in the burned part of Morfield Ridge, but no P. truei were found there.

Although a few individuals of P. truei were taken in stands of sagebrush adjacent to pinyon-juniper woodlands, this species does not ordinarily venture far from the forest.

P. maniculatus lives almost everywhere in Mesa Verde; the preferred habitats are open and grassy with an overstory of mixed shrubs. Individuals of P. maniculatus venture into ecotonal areas lying between grasslands and pinyon-juniper forest, or between sagebrush and pinyon-juniper forest. P. maniculatus is found also in disturbed areas and in stands of sagebrush that occur in clearings of the pinyon-juniper woodland. In such areas, P. maniculatus and P. truei are sympatric; their home ranges overlap and any inter-specific competition that might occur would be expected in these places.

The ability of P. maniculatus to live in many different habitats is correlated in part with its ability to build nests in a variety of sites. Whereas P. truei usually builds nests only in dead branches or logs, P. maniculatus builds nests in such varied places as spaces under rocks, at the bases of rotten trees, and in abandoned tunnels of pocket gophers. This adaptability is advantageous for the dispersal of young individuals and the movement of adults into new areas.

Nesting sites have important bearing on survival of the young. In Mesa Verde the rainy season occurs in July and August, while both species of Peromyscus are reproducing. It is reasonable to assume that young animals that remain dry survive better than those that become wet and chilled. The nestling young of P. truei are in a more favorable position to remain dry and warm than are nestling young of P. maniculatus.

Captives of each species differed in the amounts of water consumed per gram of body weight. Individuals of P. truei consumed more water per gram of body weight than individuals of P. maniculatus. Animals may drink more water than they require when allowed to drink ad libitum, but Lindeborg (1952) has shown that species which consume less water when it is not restricted also fare better on a reduced ration. P. maniculatus appears to be better adapted to aridity than P. truei. The preferred habitats of each species are in accord with these findings.

Within the trapping grid, the most moderate microenvironment, in terms of temperature and humidity, was in the pinyon-juniper forest, where P. truei lives. The temperature extremes were wider in the microenvironments of a thicket of oak brush and of two different stands of sagebrush, where P. maniculatus lives, than in the forest. P. maniculatus tends to live in the harsher, more arid parts of Mesa Verde. Because of its propensity to build nests under things, or in the ground, and because of its ability to use less water per gram of body weight, P. maniculatus is better adapted to withstand harsh environments than is P. truei.

P. truei may be restricted to the pinyon-juniper woodland because of its need for more mesic conditions. Still, Mesa Verde is semi-arid and there are few permanent sources of water available for animals. The primary source of moisture for rodents must be their food. Analysis of the percentages of moisture contained in the three most common plants in the trapping grid showed that P. truei could obtain the required moisture by eating about ten grams of these plants daily; individuals of P. maniculatus would need to eat less in order to satisfy their water needs.

Individuals of P. truei died more frequently in warm live-traps than did individuals of P. maniculatus. This indicates that P. truei can tolerate less desiccation, or a narrower range of temperatures, than can P. maniculatus.

Both species of mice eat some of the same plants, but these plants occur widely. P. truei seems to rely more upon the nuts of pinyons and the seeds of junipers than does P. maniculatus. Mounds of discarded juniper seeds were associated with all nesting sites of P. truei. Bailey (1931:153) also noticed the fondness of this species for pine nuts and juniper seeds. Apparently, the availability of these foods is one of the major factors affecting the distribution of P. truei. However, this is not the only factor, as is shown by the presence of P. maniculatus but lack of P. truei in a juniper-pinyon association with an understory of bitterbrush. This habitat was seemingly too arid for P. truei.

The production of young, and success in rearing them, is essential to continuity of any population. P. maniculatus is favored in this respect, because the females produce more young and wean them sooner than do females of P. truei. In addition, lactating females of P. maniculatus require significantly less water than do females of P. truei. Since young mice of both species require no more water per gram of body weight than do adults, the young can disperse into any area that is habitable by their species. P. maniculatus probably is affected less by prolonged drought than is P. truei. Since lactating females require the most water of any animal in the population, they are the weakest link in the system. Females of Peromyscus are known to reabsorb embryos when conditions are unfavorable for continued pregnancy. If prolonged drought occurred in the reproductive season, and desiccated the vegetation upon which the mice depend for moisture, the populations should diminish the following year. Lactating females of P. truei would be affected more seriously by a shortage of water than would lactating females of P. maniculatus.

Of two species, the one producing the more young probably would be subjected to more parasitism and predation than the species producing fewer young. A favorable season for botflies, Cuterebra sp., revealed that P. maniculatus has a higher incidence of parasitism by these flies than has P. truei; possibly the adult flies concentrate in the open, grassy areas where P. maniculatus is more abundant, rather than in the woodlands where P. truei lives. Perhaps the lower parasitism of P. truei by warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower in P. difficilis, a species closely related to P. truei, than in P. maniculatus (V. Keen, personal communication).

Although predation by carnivores would be expected to be higher on P. maniculatus, because this species does not climb, my data show that more individuals of P. truei were taken by coyotes. I lack confidence in these findings, suspecting that another sample might indicate the reverse. Birds of prey probably catch more individuals of P. maniculatus, because this species lives in more open habitats. My data do not warrant firm conclusions regarding predation.

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.

Captives of P. truei were tolerant of other individuals of the same species, even when kept in close confinement. However, when there was slight shortage of food or water they killed their litter mates, or females killed their young. Only a short period of time was necessary for one mouse to dispatch all others in the litter. The attacked mice were bitten through the head before being eaten; the brains and viscera were the first parts consumed. The population might be decimated rapidly if drought forced this species to cannibalism. 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.

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 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.

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. Apparently P. maniculatus prefers the grassy areas and the thickets of oak brush. Although such habitats have harsh climatic conditions, they offer innumerable hiding places, and thus have great advantage for a species confined to the ground.

The low requirements of water per gram of body weight, the ability to eat diversified foods, the use of varied habitats, the high fecundity, and the ability to use any nook for retreat or nesting make P. maniculatus a successful inhabitant of most parts of Mesa Verde, and indeed, of most of North America.