Growth
Growth of captiveP. maniculatusandP. trueiis discussed in several reports. One of the most complete is that of McCabe and Blanchard (1950) onP. m. gambeliiandP. t. gilbertiin California. A detailed discussion of the dentition inP. trueiand wear of the teeth in different age groups is given by Hoffmeister (1951). Molt in these species has been considered by a number of authors (Collins, 1918; McCabe and Blanchard, 1950; Hoffmeister, 1951; Anderson, 1961). The report by McCabe and Blanchard is valuable because molt is compared between the two species from the first to the twenty-first week of postnatal development.
Fig. 18: Scatter diagram of postnatal growth of captive mice, showing increase in length of bodies from birth to 70 days of age. The records forP. trueirepresent 11 individuals of five litters; those forP. maniculatusrepresent 17 individuals of four litters.
The thoroughness of the above-mentioned studies is readily apparent to those who have worked with mice of the genusPeromyscus. Nevertheless, the ecology of local populations ofP. maniculatusandP. trueias reported for the San Francisco Bay area (McCabe and Blanchard, 1950) has little relationship to the ecology of mice of other subspecies of these species, in southwestern Colorado. Indeed, the preferred habitats, and to some extent the behavior, differ strikingly in Colorado and California.
Fig. 19: Graphs showing postnatal growth of solitary captive individuals ofP. trueiandP. maniculatus, representing the only young in each of two litters.
Figures18and19show that some litters grow appreciably faster thanothers, but the end results are about the same. Since the young were measured at irregular intervals, statistical procedures for calculating confidence limits of the curves were not applicable.
Solitary young reared by one female of each species, attained maximum size more rapidly than animals having litter mates (Fig. 19). Nevertheless, solitary individuals and individuals from litters all reach essentially the same size 50 days after birth.
The gestation time ofP. trueiis several days longer than that ofP. maniculatus, and the young oftrueiare fewer and heavier than those ofmaniculatus. As would be expected,trueiremains in the nest longer and nurses longer thanmaniculatus.
Young of each species grow rapidly for the first month, and attain, in that time, the largest percentage of their adult size; they grow rapidly up to sometime between the thirtieth and fiftieth days. Thereafter the rate of growth diminishes and the animals begin to gain weight rather than continuing to extend the lengths of the body and appendages.
Figure 19reveals that the appendages of youngmaniculatusattain most of their length about a week earlier than those oftruei. Youngtrueiacquire mobility and coordination somewhat later than youngmaniculatus, but both species are seemingly equal in these respects by about the end of the second week.
Length of gestation period, number and size of embryos, amount of time spent in the nest, and time required for bodily growth are all of major importance in determining the relative success oftrueiandmaniculatus. These parameters will be considered further in the discussion.
Parental Behavior
In the laboratory, pregnant females were supplied with either kapok, cotton, or a piece of burlap with which to make a nest. The kapok or cotton was used directly by the mice in constructing a hollow, compact, moundlike nest. When burlap was used for nest building, the female first completely frayed the cloth by chewing it into a fluffy mass of fibers.
When the top of a nest was opened to inspect young, the female would attempt to pull the nesting material back into shape by means of forefeet and teeth. The mother's defensive posture was to cover the young with her body, often lying over them and facing upward, toward the investigator. In this semi-recumbent position, the female would attack the investigator's fingers with her forefeet and teeth. Often the female would stand bipedally and use the forefeet and teeth to mount the attack. If at this time a young chanced to wander away from the mother, she would quickly pick it up and place it in the nest at her feet.
When disturbed, females of both species, but especiallyP. maniculatus, often dove headlong under their nest or into the wood shavings on the floor of the cage. This type of retreat was most often used when young were nursing. Time is required even by the mother to disengage nursing young, and this mode of escape is the most expedient. The mother disengaged nursing young by licking around their faces and pushing with her paws.
Nursing females of both species tolerated the male parent in the nest. A male and female often sat side by side in the nest and by means of their bodies participated in covering the young. Males were not observed to attempt anydefense of the nest, or of the young. Females were tolerant of older young in the nest when another litter was born and was being nursed. In one nest, a female ofP. trueigave birth to a litter of three when her older litter was 29 days old. The three older young continued to nurse until they were 37 days old, at which time they were removed from the cage. The female appeared tolerant of this nursing by members of the older litter, but appeared to give preference to the wants of the younger offspring.
One female ofP. trueilost or killed all but one young of her litter; at about the same time, aP. maniculatusand all but one of her young inexplicably died. Since the remaining youngmaniculatus, a male, was just weaned and was considered expendable, I placed him in the cage with the femaletrueiand her 33-day-old, male offspring. The reaction to the newcomer was unexpected. The female immediately covered theP. maniculatusand her own young and prepared to defend them against me. Later, when theP. maniculatuswas disturbed, he had only to emit a squeak and the femaletrueiwould run to cover and protect him. When the young male ofP. trueiwas 69 days old the female kept him out of the nest, but still kept the malemaniculatusin the nest with her. Although the female was somewhat antagonistic to her own young, she did not injure him, but only kept him out of the nest. The maletrueiwas left in the cage with his mother and theP. maniculatusfrom September 23 to December 10. None of the mice had any apparent cuts on the ears or tail to indicate fighting. As much as seven months after theP. maniculatuswas introduced into the cage, the femaletrueicontinued to cover him with her body whenever there was a disturbance. The malemaniculatusnot only tolerated this attention, but ran under the femaletrueiwhen frightened. "Adoption" of young of another species has been reported for a number of animals, but, without further evidence, it is not possible to postulate that such adoptions occur between species ofPeromyscusin nature.
Young males are tolerated by their mothers after weaning. One young malemaniculatuswas left in the cage with his mother from the time of his birth in autumn until late February of the following year. A litter was born on February 24. A young maleP. trueiwas also left in the cage with his mother until he had acquired most of his postjuvenal pelage; the female and male usually sat together in the cage.
Females of both species sometimes eat their young when the young die shortly after birth. One female of each species killed three of her four young, and ate their brains and viscera. In one of these cases, the female, ofP. maniculatus, also died; the female ofP. trueiwas the same one that adopted the survivingP. maniculatus. The femaletrueicontinued to nurse her one remaining young for at least several days after killing three of his litter mates. A reason for this cannibalism might have been that I had fed these mice for several weeks on a mixture of grains low in protein content. Inadequacy of this diet for nursing females may have caused them to become cannibalistic. The feed of all captives was changed to Purina Laboratory Chow after the young were killed.
Transportation of Young
Females of both species transported their young either by dragging them collectively while the young were attached to mammae, or by carrying them one at a time in the mouth. Since mice of the subgenusPeromyscushave threepairs of nipples, they probably transport only six young collectively. Svihla (1932:13) has stated that both pectoral and inguinal teats are used in transporting young, in contrast to Seton's reputed assertion that only inguinal nipples were used. But Svihla neglected to cite Seton's complete statement. Seton (1920:137) recorded a litter of three as using only the inguinal mammae, but on the following page recorded the use of both inguinal and pectoral mammae by another litter of four. My findings agree with those of Svihla. Nursing females of both species were removed periodically from cages by lifting them by the tail. The young would hang onto the mammae and the female would clutch the young to her with all four feet. Young two weeks old or older crawled behind the mother while nursing.
The method of transporting young in the mouth has been mentioned by Seton (1920:136) and described by Lang (1925) and Hall (1928:256). These authors report that the mother picks the young up in her paws, and places it ventral-side up in her mouth, with her incisors around it. The young are not picked up by the skin on the nape of the neck, as are the juveniles of dogs and cats. I have found that females of both species ofPeromyscuscarry their young ventral-side up in their mouth while the young are small, and sometimes when the young are older. Generally, when females ofP. trueimoved young weighing more than 10 grams, the female grasped the young from the dorsal side, across the thorax just posterior to the shoulders, and held them with the incisors more or less around the animal. Perhaps this method was used with older young because of the observed tendency of the larger young to resist being turned over and grasped from the ventral side, and because their increased weight would have made it difficult, if not impossible, for the mother to pick them up with her paws. The young rarely resisted the efforts of the mother to move them by this method; when grasped across the thorax by the mother, the young would remain limp until released. Some females ofP. trueiwould drag almost fully grown young back into the nest in this manner. I have not observed older young of a comparable age to be moved by females ofP. maniculatus. The females ofP. maniculatusappear to be somewhat less concerned than those ofP. trueifor the welfare of their young once they are mobile and close to being weaned.
The following listing describes changes in postnatal development of young, of each species, from birth to nine weeks of age.
Changes Owing to Increase in Age
Increase in length of limb bones, changes in proportion of bones in the skull, eruption and degree of wear of teeth, and changes in pelage can be used to ascertain relative age. Different investigators might choose different limits for the three categories young, subadult, and adult. Museum specimens were assigned to one of five age groups listed below mostly on the basis of tooth wear, essentially as described by Hoffmeister (1951:1).
Juvenile: M3 just breaking through bony covering of jaw or showing no wear whatsoever.Young: M3 worn smooth except for labial cusps, and M1 and M2 showing little or no wear.Subadult: M3 worn smooth; labial cusp may persist, but is well worn; M1 and M2 having lingual cusps worn, but not smooth; labial cusps showing little wear.Adult: Lingual cusps worn smooth and labial cusps showing considerable wear; labial cusp of M3 may persist.Old: Cusps worn smooth; not more than one re-entrant angle per tooth discernible, frequently none.
Juvenile: M3 just breaking through bony covering of jaw or showing no wear whatsoever.
Young: M3 worn smooth except for labial cusps, and M1 and M2 showing little or no wear.
Subadult: M3 worn smooth; labial cusp may persist, but is well worn; M1 and M2 having lingual cusps worn, but not smooth; labial cusps showing little wear.
Adult: Lingual cusps worn smooth and labial cusps showing considerable wear; labial cusp of M3 may persist.
Old: Cusps worn smooth; not more than one re-entrant angle per tooth discernible, frequently none.
For live animals examined in the field, criteria based on pelage and breeding condition were used, as follows:
Juvenile: Only gray, juvenal pelage present.Young: Subadult pelage apparent on lateral line or on sides; body usually smaller than in adults.Subadults: Subadult pelage having mostly replaced juvenal pelage; mice often as large as adults; testes of males often abdominal in breeding season; gray juvenal pelage may persist on head of some individuals.Adult: Adult pelage present; body usually largest of all animals in population; females may have enlarged mammae from nursing previous litters; testes of males usually scrotal in breeding season; gray pelage may be present on head of some individuals.
Juvenile: Only gray, juvenal pelage present.
Young: Subadult pelage apparent on lateral line or on sides; body usually smaller than in adults.
Subadults: Subadult pelage having mostly replaced juvenal pelage; mice often as large as adults; testes of males often abdominal in breeding season; gray juvenal pelage may persist on head of some individuals.
Adult: Adult pelage present; body usually largest of all animals in population; females may have enlarged mammae from nursing previous litters; testes of males usually scrotal in breeding season; gray pelage may be present on head of some individuals.
Old individuals in the field could not be distinguished from adults; hence any animals that appeared older, or more developed, than subadults were classified as adults.
InP. truei, subadult pelage appears first on the lateral line or on the flanks; new pelage is ochraceous and contrasts markedly with the gray juvenal coat. InP. maniculatus, the subadult pelage contrasts less with the juvenal coat; the new pelage progresses from anterior to posterior over the body in the same manner as intruei, but replaces the juvenal coat in a less distinct manner than intruei. As a result, contrast often is lacking between juvenal and subadult pelages inmaniculatusmaking it difficult to assign an individual to one of these two age categories when examined in the field. In museum specimens, the subadult pelage is much more noticeable because it can be compared with the pelages of other specimens. The subadult pelage inP. maniculatusis duller than the adult pelage: InP. trueithe subadult and adult pelages appear to have an equal sheen.
In early winter, the postjuvenal pelage acquired by young individuals ofP. trueiwas thick and luxuriant and indistinguishable from the winter pelage of adults. My observations lead me to conclude that individuals born late in the breeding season molt from juvenal summer pelage directly into winter adult pelage. Technically, this new coat is the postjuvenal one, yet it cannot be distinguished as such after the molt is completed.
Anomalies and Injuries
Anatomical anomalies were rare in the individuals ofPeromyscusthat I examined. When anomalies were found they were striking, principally because of their low rate of occurrence.
One female ofP. truei, born in captivity, had a congenital defect of the pinna of the right ear, noted on the fifteenth day after birth. Closer examination then and later revealed that the pinna was normal in all respects except that the tip was missing. The tip showed no evidence of injury. When the mouse was subadult, this defective pinna was approximately half as long as the normal pinna. The topmost part of the defective pinna was somewhat more constricted in circumference than the normal one.
On September 11, 1963, a subadult male ofP. trueiwas captured that had five functional toes on its right front foot, the only one of more than 175 individuals caught and handled in the field that exhibited polydactyly. The front foot was examined closely in the field, but it could not be determined how or where the extra bones of the sixth toe articulated.Peromyscusnormally has four full-sized toes on each front foot, and a small inner toe hardly more than an enlarged tubercle, having no nail.
A few mice of both species had broken toes or claws torn off. Such injuries were more common on toes of the hind foot. In several instances the toes were shortened, as if by marking, although the animals concerned had beenmarked earlier by clipping toes other than the injured toes. The reason for these injuries is not apparent, although they could have been caused by fighting, or from having been caught in doors of Sherman live traps.
Toes of several mice were swollen and inflamed due to small glochids of cacti that were stuck in them. Apparently the mice had stepped on the glochids by chance, for I found no evidence thatPeromyscusof either species eats cacti.
OneP. trueihad a broken tail; three other individuals had tails about one-half normal length. OneP. maniculatushad a shortened tail. Some of these injuries probably were caused by the Sherman live traps; several individuals ofP. trueiwere released after having been caught by the tail by the spring-loaded door of these traps.
On October 17, 1963, an adultP. trueihad a bleeding penis; when this mouse was recaptured on October 25, the injury was healed.
Losses Attributed to Exposure in Traps
Observations of wild mice caught in live traps suggest that metabolic maturity is reached later than physical and reproductive maturity. In such trapping, it became apparent that juvenal and young mice suffered from exposure to cold and to heat much more than did subadult or adult mice. Although traps were carefully shaded and ample nesting material and food provided, some mice died in the traps. An overwhelming majority of these mice were juveniles and young.
Traps were checked in the morning, both in the summer and autumn, yet mice died in traps that were barely warm to the touch, in summer, and cool to the touch in autumn. Older mice frequently were found in traps that were warm, or even hot, to the touch; yet the older mice rarely died in such traps. Apparently the tolerance of adults is much greater to heating and chilling. Greater bulk and perhaps longer pelage in adults might provide sufficiently better insulation to account for this difference.
Occasionally juvenal mice were found in traps in a sluggish and weakened condition, especially in autumn when nights were cool. In such cases the mice were either cupped in the hands and warmed until lively enough to fend for themselves, or, if especially weakened, were taken to the laboratory. None of such animals that were returned to the laboratory lived for more than two weeks. Most of those released in the field did not reappear in the traps.
I conclude that juvenal and young mice placed under stress by overheating or cooling die immediately or live only a few days. Subadult and adult animals tolerate more extreme conditions of overheating or cooling, presumably because they are able to regulate their internal temperature better, by either losing or retaining heat more effectively.
Mice found dead in overheated traps had salivated heavily, and may also have licked the fur on their chests to increase heat dissipation. One such adult, ofP. truei, had a wet chest when he was taken from a warm trap; when released, this mouse ran to a nearby plant ofComandra umbellata, and ate a few of the succulent leaves before running off. This individual was trapped several times later in the summer, and apparently suffered no ill effects from the exposure.
Dental Anomalies
Abnormalities in the formation and occlusion, or decay of teeth, are relatively rare in wild mammals. Of all bodily structures, the teeth apparently are under the most rigid genetic controls; they form early in the embryo and follow rigidly specified patterns in their ontogeny. Apparently any deviation from the normal pattern of tooth formation is quickly selected against. All specimens ofP. m. rufinusandP. t. trueiin the collection of the Museum of Natural History at the University of Kansas, and in my collection, were examined for dental anomalies. A total of 317 specimens ofP. m. rufinusand 54 specimens ofP. t. trueiwere examined. The following specimens were found to have abnormalities:
K. U. 69361,P. maniculatus, adult: Small bundles of plant fibers are lodged between all upper teeth and have penetrated the maxilla anterior to the left M1. The maxillary bone is eroded away from the roots of all teeth. The anteriormost roots of both lower first molars are almost completely exposed, because the dentary has been abraded away.
K. U. 76041,P. maniculatus, young: A piece of plant fiber is wedged between the left M2 and M3. The maxillary bone has eroded away from around the roots of M3, indicating the presence of an abscess in this area.
K. U. 69362,P. maniculatus, adult: All teeth in the lower right tooth-row are greatly worn, especially on the lingual side. The labial half of the right M1 is all that remains; decay is apparent both in the crown and roots on the lingual side of this tooth.
K. U. 69397,P. maniculatus, old: The maxillae have eroded away from around the anterior roots of each first upper molar, leaving these roots unsupported.
C. L. D. 231,P. maniculatus, old: The teeth in this female are greatly worn; re-entrant angles are not visible in any teeth. A circular hole, 0.1 millimeter in diameter, exists in the dentine immediately over (when viewed from the underside of the skull) the posterior root of the right M1. The crowns of the teeth are greatly reduced in height, and the dentine is thin.
Anomalies in the Skull
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 boneis 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 genusPeromyscusare 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 (Tables5,6). 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 Tables5and6were those that were eaten or rejected by a majority of the individuals tested.
Plant material eaten byP. maniculatusand refused byP. trueiincluded only the leaves and stem ofViguiera multiflora. Plant material eaten byP. trueiand refused byP. maniculatusincluded the leaves ofCalochortus gunnisoniiand the leaves and stem ofErigeron speciosus.
Table 5—Plants, or Parts of Plants, Eaten by Captive Individuals ofP. trueiin Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.Species of PlantLeavesStemFlowerSeedsAmelanchier utahensis---+Calochortus gunnisonii++-+Chaenactis douglasii00--Chrysothamnus depressus000-Chrysothamnus nauseosus+00-Comandra umbellata++--Erigeron speciosus++--Eriogonum alatum---+Juniperus osteosperma---+Lupinus caudatus00+-Lithospermum ruderale00-0Mellilotus alba++++Mellilotus officinalis+++-Orthocarpus purpureo-albus++++Pedicularis centranthera++--Penstemon linarioides++-+Pinus edulis---+Polygonum sawatchense++-0Solidago petradoria000-Viguiera multiflora0000
Table 5—Plants, or Parts of Plants, Eaten by Captive Individuals ofP. trueiin Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.
Plant material eaten by captives of both species includedCalochortus gunnisonii—stemand seeds;Comandra umbellata—leaves and stem;Eriogonum alatum—seeds;Penstemon linarioides—leaves and stem;Pinus edulis—seeds; andJuniperus osteosperma—seeds.
Plant materials refused by both species of mice included the leaves and stem ofChaenactis douglasii, the leaves, stem and seeds ofLithospermum ruderale, and the leaves, stem and flowers ofSolidago 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 ofP. maniculatusin Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.Species of PlantLeavesStemFlowerSeedsArtemisia ludoviciana00--Calochortus gunnisonii0+-+Chaenactis douglasii00--Comandra umbellata++--Erigeron speciosus00--Eriogonum alatum---+Juniperus osteosperma---+Lappula redowskii00-+Lithospermum ruderale00-0Orthocarpus purpureo-albus00++Penstemon linarioides+++-Pinus edulis---+Purshia tridentata++--Sitanion hystrix00-0Solidago petradoria000-Sphaeralcea coccinea++-+Stipa comata00-+Viguiera multiflora++--
Table 6—Plants, or Parts of Plants, Eaten by Captive Individuals ofP. maniculatusin Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.
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 ofPeromyscus maniculatus:
Agropyron smithiiArtemisiasp.Eriogonum umbellatumLupinus ammophilusPenstemon linarioidesPhlox hoodiiStipa comataArachnid legs
Stomach and intestinal contents ofP. trueicontained the following food items:
Artemisia novaArtemisiasp.Penstemoncf.barbatusPenstemoncf.linarioidesPoa fendlerianaArachnid legsEriogonumsp.Gutierrezia sarothraeYuccasp.ChitinFeathers
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 genusNeotoma, 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 ofPeromyscus, 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 ofP. trueicontained a small fragment of the epidermis ofYucca. This fragment may have come from a young shoot. It is unlikely thatPeromyscuswould eat the larger, coarser leaves ofYucca.
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 ofPeromyscusin the autumn and early winter. The winter staple ofP. trueiappears 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; sinceP. trueilives in the forest, it has better access to these foods than doesP. 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 ofP. trueiare 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.
Water Consumption
Peromyscus maniculatusis ubiquitous, occurring in habitats ranging from mesic boreal forests to arid southwestern deserts. Most subspecies ofP. maniculatuslive in moderately mesic or near-mesic environments, but a few have adapted to arid conditions. It has been assumed that the success ofP. maniculatusin inhabiting such diverse habitats is associated with its adaptability to different kinds of food and varying amount of available water (Williams, 1959b:606).
Throughout its rangeP. maniculatuscoexists with one or more other species ofPeromyscusthat are more restricted in distribution.Peromyscus trueiis one such species.
Both species live under xeric or near-xeric conditions, for the climate of Mesa Verde is semi-arid. Other than a few widely-scattered springs, there are no sources of free water on the top of the Mesa Verde land mass; thus animals inhabiting the park must rely upon moisture in the plants and other foods they eat, or upon dew.
Several investigators have studied water consumption in mice of the genusPeromyscus(Table 7). Dice (1922) did so for the prairie deer mouse,P. m. bairdii, and the forest deer mouse,P. leucopus noveboracensis, under varying environmental conditions. He found that both species drank about the same amounts of water per gram of body weight, and that food and water requirements did not differ sufficiently to be the basis for the habitat differences between these species. Neither of his samples was from an arid environment. Chew (1951) studied water consumption inP. leucopus, and recently reviewed the literature on water metabolism of mammals (Chew, 1965). In his studies of five subspecies of two species ofPeromyscus, Ross (1930) found significant differences in water consumption between species but not between subspecies within a species. One of the subspecies ofP. maniculatustested was from a desert region, whereas the other two were from mesic areas along the coast of California.
Lindeborg (1952) was the first to measure water consumption of bothP. m. rufinusandP. t. truei, the species and subspecies with which my experiments are concerned. Lindeborg also tested the ability of five races ofPeromyscusto survive reduced water rations. Unfortunately, the subspecies chosen for these experiments did not includeP. t. trueiorP. m. rufinus. Lindeborg (1952:25) found that the "amounts of water consumed by various species ofPeromyscusfrom different habitats within the same climatic region were not conclusively different." However, he did find significant differences between some subspecies from different geographical areas. For example, he found no significant difference in water consumption betweenP. m. bairdiifrom Michigan and eitherP. m. blandusorP. m. rufinusfrom New Mexico, but he found a highly significant difference betweenP. l. noveboracensisfrom Michigan andP. l. tornillofrom New Mexico. Lindeborg also found that the subspecies ofPeromyscusthat consumed the least water, and that were best able to survive a reduced water ration, were those from the more xeric climatic areas.
Some mammals may be able to change their diets in times of water stress, and thereby compensate for a shortage of water. At such times,Dipodomysselects foods with high percentages of carbohydrates and conserves water by reducing the amounts of nitrogenous wastes to be excreted (Schmidt-Nielsenet al., 1948).
Williams (1959b) found thatP. m. osgoodifrom Colorado drank more water on a diet rich in protein than on one rich in carbohydrates. But, her mice on a high carbohydrate diet used less than a normal amount of water for a period of only five weeks; at the end of the five weeks they were drinking about as much as they had been when on the control diet of laboratory chow. Likewise, mice adjusted to the high protein diet by consuming more water; but by the end of the fifth week their daily water consumption approximated the amount drunk when fed on laboratory chow. Because of these results, Williams questioned the validity of the assumption thatP. maniculatusis able to inhabit a diversity of habitats because of its adaptability with respect to food and water requirements.
I conducted a series of experiments on water and food consumption by individuals ofP. trueiandP. maniculatus. It was thought that if there were differences in water or food consumption, or both, knowledge of them might help to explain the obvious differences in habitat preferences of these two species in Mesa Verde National Park.
In August of 1965, 30 individuals ofP. trueiandP. maniculatuswere trapped in Mesa Verde National Park at elevations of 7000-8400 feet, and transported to Lawrence, Kansas, where the experiments were carried out.
Mice were housed in individual metal cages (10 x 7.5 x 5 inches), having removable tops of wire mesh, and an externally-mounted water bottle that had a drop-type spout extending into the cage. Cages were on one of five shelves of a movable tier of shelving, and were rotated randomly, from one shelf to another, each week. A layer of dry wood shavings covered the bottom of each cage. A control cage was similarly equipped.
The mice were kept in a room in which temperature and photoperiod were controlled. The ambient air temperature of this room was 20 to 23 degrees Centigrade throughout the experiments, and averaged 21 degrees. Humidity was not controlled, but remained low throughout the experiments. The room was illuminated for eight hours each day, from about 9 A. M. to 5 P. M.
The animals were fed at least once a week, at which time all remaining food was weighed and discarded, and the remaining water was measured. Tap water was used in all of the experiments. The cages were cleaned each week. Each time the cages containing mice were handled, the control cage was handled in the same way. The amount of evaporation was determined each week by measuring the water remaining in the bottle of the control cage.
Water and food consumption of individuals ofP. maniculatusandP. trueiwere measured when the mice were fed diets of differing protein content. To my knowledge, the only other study in which water consumption was measured for mice of the genusPeromyscuson diets of different protein contents was by Williams (1959b). Because of the limited number of animals available, it was decided that the best results could be obtained by placing all individuals on the same diet for a predetermined number of weeks, then on a second diet for a certain period, and so on.
Each mouse was weighed at the beginning, at the mid-point, and at the end of each experiment. The mice were weighed on the same days, at times when they were inactive. Because weights of individual mice differ, water and food consumption was calculated on the basis of the amount consumed per gram of body weight per day. All foods were air-dry and contained a negligible amount of water.
First, food and water consumption was measured for nine individuals of each species on a diet of Purina Laboratory Chow. This chow contains not less than 23 per cent protein and 4.5 per cent fat, and about 57 per cent carbohydrate. Since the mice had been maintained on this diet for several months prior to the experiments, food and water consumption was measured for a period of only two weeks. Individuals ofP. trueiconsumed more total water and more water per gram of body weight than individuals ofP. maniculatus(Table 7).
Next, 10 mice of each species were placed on a diet of Purina Hog Chow for a period of four weeks. This chow contains not less than 36 per cent protein and one per cent fat, and about 42 per cent carbohydrate. Both species increased their daily water consumption immediately after being placed on this diet (tables7and11). On the high protein diet,P. trueiagain consumed much more water than didP. maniculatus(tables7and9).