Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park, Colorado

by Charles L. Douglas

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.