Part 20

Adaptations to Environment

Each of the two species of Peromyscus illustrates 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 grass 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 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.

LITERATURE CITED

ANDERSON, S.

1961. Mammals of Mesa Verde National Park, Colorado. Univ. Kansas Publ., Mus. Nat. Hist., 14:29-67, 2 pls., 3 figs.

ARRHENIUS, G., and E. BONATTI

1965. The Mesa Verde loess, pp. 92-100, in Contributions of the Wetherill Mesa Archeological Project, Memoirs Soc. Amer. Archeol., 19; American Antiquity, 31, No. 2, Pt. 2.

ASDELL, S. A.

1964. Patterns of mammalian reproduction. Comstock Publ. Co., Ithaca, viii + 1-670 pp.

BAILEY, V.

1931. Mammals of New Mexico. N. Amer. Fauna, 53:1-412, 22 pls., 58 figs.

BEIDLEMAN, R. G.

1954. October breeding of Peromyscus in north central Colorado. Jour. Mamm., 35:118.

BRUSVEN, M. A., and G. B. MULKERN

1960. The use of epidermal characteristics for the identification of plants recovered in fragmentary condition from the crops of grasshoppers. North Dakota Agricultural Exp. Sta., Fargo, Research Rept., 3:3-11.

BURT, W. H.

1940. Territorial behavior and populations of some small mammals in southern Michigan. Misc. Publ. Mus. Zool., Univ. Michigan, 45: 1-58, 2 pls.

CALHOUN, J. B., ed.

1948-1956. Annual reports of the North American census of small mammals. Distributed by the editor, National Institutes of Health, Bethesda 14, Maryland.

1959. Population dynamics of vertebrates release No. 10, Revised sampling procedure for the North American census of small mammals (NACSM). pp. 1-12, Distributed by the editor, Nat. Inst. Health, Bethesda 14, Maryland.

CHEW, R. M.

1951. The water exchanges of some small mammals. Ecological Monographs, 21:215-225.