Part 15

Wormian bones and other abnormalities in the roofing bones are noted, as follows:

K. U. 76090, P. maniculatus, young: The interparietal is divided; the divided suture is in line with the suture between the parietals. The interparietal is 7.8 millimeters long.

K. U. 76091, P. maniculatus, young: A wormian bone, 0.5 millimeter by 0.2 millimeter, lies between the anterior border of the interparietal and the posterior border of the left parietal, at a point midway between the center line of the skull and the posterolateral border of the parietal bone.

C. L. D. 248, P. maniculatus, adult: An oval wormian bone, 1.1 millimeters long and 0.6 millimeter wide, lies between the parietals at their posterior margin; the long axis of the bone is parallel to the long axis of the skull.

C. L. D. 246, P. maniculatus, juvenal: The interparietal is divided equally by a suture. An oval wormian bone, 0.3 millimeter long and 0.1 millimeter wide, lies between the frontals, midway between the anterior and posterior borders of these bones.

C. L. D. 656, P. maniculatus, young: A small, rounded wormian bone lies between the right parietal and interparietal, lateral to the posterior junction of the suture between the parietals. This bone extends anteriorly into the parietal bone from the suture of the interparietal and parietal. This bone is 0.7 millimeter wide, and extends 0.6 millimeter into the parietal.

C. L. D. 662, P. maniculatus, subadult: An elongated, diamond shaped wormian bone closes the suture between the parietal bones. This bone is 2.3 millimeters long and 0.8 millimeter wide.

K. U. 34735, P. truei, old: The anterior one-quarter of the left parietal bone is slightly depressed; and the posterior one-third of the left frontal and anterior one-quarter of the left parietal are thin and sculptured. This malformation of the roofing bones posterior to the orbit probably is not the result of a break, for the orbital part of the frontal bone is normal. The frontal-parietal sutures are in the normal positions on both sides of the skull.

The above-mentioned anomalies do not appear to be correlated with age or locality at which the specimens were taken. Apparently such anomalies are present throughout the population, but in a small percentage of specimens.

FOOD HABITS

Mice of the genus Peromyscus are known to eat a wide variety of plants and arthropods, and to be highly opportunistic in selection of food (Cogshall, 1928; Hamilton, 1941; Williams, 1955, 1959a; Jameson, 1952; Johnson, 1962). In order to determine possible food preferences, captive mice of both species were fed plants indigenous to Mesa Verde. Entire plants were used whenever possible; available seeds also were offered (Tables 5, 6). All feeding experiments were replicated with at least six different individuals in order to minimize the trends resulting from individual preferences or dislikes. The mice of each species tended to be consistent in their feeding.

The plant species listed in Tables 5 and 6 were those that were eaten or rejected by a majority of the individuals tested.

Plant material eaten by P. maniculatus and refused by P. truei included only the leaves and stem of Viguiera multiflora. Plant material eaten by P. truei and refused by P. maniculatus included the leaves of Calochortus gunnisonii and the leaves and stem of Erigeron speciosus.

TABLE 5--Plants, or Parts of Plants, Eaten by Captive Individuals of P. truei in Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.

=============================+========+======+========+======= Species of Plant | Leaves | Stem | Flower | Seeds -----------------------------+--------+------+--------+------- Amelanchier utahensis | - | - | - | + Calochortus gunnisonii | + | + | - | + Chaenactis douglasii | 0 | 0 | - | - Chrysothamnus depressus | 0 | 0 | 0 | - Chrysothamnus nauseosus | + | 0 | 0 | - Comandra umbellata | + | + | - | - Erigeron speciosus | + | + | - | - Eriogonum alatum | - | - | - | + Juniperus osteosperma | - | - | - | + Lupinus caudatus | 0 | 0 | + | - Lithospermum ruderale | 0 | 0 | - | 0 Mellilotus alba | + | + | + | + Mellilotus officinalis | + | + | + | - Orthocarpus purpureo-albus | + | + | + | + Pedicularis centranthera | + | + | - | - Penstemon linarioides | + | + | - | + Pinus edulis | - | - | - | + Polygonum sawatchense | + | + | - | 0 Solidago petradoria | 0 | 0 | 0 | - Viguiera multiflora | 0 | 0 | 0 | 0 -----------------------------+--------+------+--------+-------

Plant material eaten by captives of both species included Calochortus gunnisonii--stem and seeds; Comandra umbellata--leaves and stem; Eriogonum alatum--seeds; Penstemon linarioides--leaves and stem; Pinus edulis--seeds; and Juniperus osteosperma--seeds.

Plant materials refused by both species of mice included the leaves and stem of Chaenactis douglasii, the leaves, stem and seeds of Lithospermum ruderale, and the leaves, stem and flowers of Solidago petradoria.

Cricetine rodents chew plant and animal foods thoroughly; contents of their stomachs appear as finely-particulate fragments. These fragments invariably contain pieces of epidermis from ingested plants. Due to the presence of cutin in the cell walls, epidermis is last to be digested.

Microscopic analysis of plant epidermis is useful in helping to determine food habits of various animals (Dusi, 1949; Williams, 1955, 1959a; Brusven and Mulkern, 1960; Johnson, 1962). The microscopic analysis of stomach contents provides a practical method of determining which plants are eaten by rodents. Contents of stomachs and intestines were removed from mice caught in snap traps, and from preserved specimens. The contents were placed on a piece of bolting silk, washed thoroughly with running water, stained with iron-hematoxylin and mounted on slides, or stored in 70 per cent ethanol (Williams, 1959a; Douglas, 1965).

TABLE 6--Plants, or Parts of Plants, Eaten by Captive Individuals of P. maniculatus in Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.

=============================+========+======+========+======= Species of Plant | Leaves | Stem | Flower | Seeds -----------------------------+--------+------+--------+------- Artemisia ludoviciana | 0 | 0 | - | - Calochortus gunnisonii | 0 | + | - | + Chaenactis douglasii | 0 | 0 | - | - Comandra umbellata | + | + | - | - Erigeron speciosus | 0 | 0 | - | - Eriogonum alatum | - | - | - | + Juniperus osteosperma | - | - | - | + Lappula redowskii | 0 | 0 | - | + Lithospermum ruderale | 0 | 0 | - | 0 Orthocarpus purpureo-albus | 0 | 0 | + | + Penstemon linarioides | + | + | + | - Pinus edulis | - | - | - | + Purshia tridentata | + | + | - | - Sitanion hystrix | 0 | 0 | - | 0 Solidago petradoria | 0 | 0 | 0 | - Sphaeralcea coccinea | + | + | - | + Stipa comata | 0 | 0 | - | + Viguiera multiflora | + | + | - | - -----------------------------+--------+------+--------+-------

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:

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:

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.