In order to analyze these epidermal fragments, a collection of plants was made within the park. Slides of the epidermis of these plants were prepared and analyzed for diagnostic characters (Douglas, 1965:197-199). Features such as the stomatal arrangement in relation to subsidiary cells; the types of trichomes, scales and glands; the cellular inclusions such as starch grains, mucilage and resins are of taxonomic value (Metcalfe and Chalk, 1950). The configuration of the anticlinal cell walls is useful in separating species that are similar in other respects (Douglas, 1965:199).
The following species of plants, and other food items, were identified in the stomach or intestinal contents of Peromyscus maniculatus:
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Agropyron smithii Artemisia sp. Eriogonum umbellatum Lupinus ammophilus Penstemon linarioides Phlox hoodii Stipa comata Arachnid legs |
Stomach and intestinal contents of P. truei contained the following food items:
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Artemisia nova Artemisia sp. Penstemon cf. barbatus Penstemon cf. linarioides Poa fendleriana Arachnid legs Eriogonum sp. Gutierrezia sarothrae Yucca sp. Chitin Feathers |
Many of the plants eaten by the mice had large numbers of crystals in the epidermis. Druses were the most abundant, but raphid crystals also were seen. Every slide contained at least one species of plant which contained druses. Such crystals are composed mostly of calcium oxalate (Esau, 1960:41). In Mesa Verde, families of plants having crystals include: Boraginaceae, Chenopodiaceae, Compositae, Cruciferae, Leguminosae, Liliaceae, Malvaceae, Ornargraceae, Rosaceae, and Saxifragaceae. Calcium oxalate is a highly insoluble compound and is innocuous if it passes through the gastro-intestinal tract without being absorbed. In rats of the genus Neotoma, some calcium oxalate passes through the intestines unchanged, but large amounts of calcium are absorbed through the intestine. The urine of pack rats is creamy in color and contains calcium carbonate. It is not understood how these rats metabolize the highly toxic oxalic acid, when converting calcium oxalate to calcium carbonate (Schmidt-Nielsen, 1964:147-148). 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. Captive mice were especially fond of pinyon nuts, and these probably provide a substantial part of the diet of Peromyscus in the autumn and early winter. The winter staple of P. truei appears to be juniper seeds. Nesting sites of this mouse often could be located by the mounds of discarded seeds lying nearby.
Both species eat pinyon and juniper seeds; since P. truei lives in the forest, it has better access to these foods than does P. maniculatus. Mice remove the embryos of juniper seeds by chewing a small hole in the larger end of the seed. The seed coats of juniper are extremely hard, and a considerable amount of effort must be expended to remove the embryo. Captives discarded the resinous and pithy, outer layers of juniper berries. Individuals of P. truei are adept climbers. Since many juniper berries remain on branches throughout the winter, the ability of these mice to forage in the trees would be especially advantageous when snow covers the ground.