Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 13

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.

Adaptations to Environment

Each of the two species of _Peromyscus_ ill.u.s.trates one or more adaptations to its environment. _P. truei_ is adapted to climbing by possession of long toes, a long tail, and large hind feet. The tail is used as a counterbalance when climbing (Horner, 1954). When frightened, individuals of _P. truei_ often ran across the ground in a semi-saltatorial fashion, bounding over clumps of gra.s.s that were as much as 18 inches high. Such individuals usually ran to the nearest tree and climbed to branches 10 to 20 feet above the ground.

Large eyes are characteristic of the _truei_ group of mice, and may be an adaptation to a semi-arboreal mode of life. A similar adaptation is shared by some other arboreal mammals, and of arboreal snakes. The large eyes of _P. truei_ in comparison to those of _P. maniculatus_, probably increase the field of vision, and permit the animal to look downward as well as in other directions.

The above-mentioned adaptations of _P. truei_ permit these graceful mice to use their environment effectively. 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.

_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 s.p.a.ce 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 gra.s.sy 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.