Chapter III: Part 3

Toward the beginning and end of the breeding season the testes and accessory organs of wild mice were small and probably produced few if any sperm. At these times some males apparently were so frightened by being handled that the testes were retracted into the inguinal canals. It would have been easy to consider such males as having abdominal testes when in fact they did not. In such cases the scrotum usually was noticeably enlarged; it was found also that in many cases the testes returned to the scrotal position if the mouse was held gently for a few minutes. Careful handling of animals was found to prevent, or at least retard, retraction of the testes. Retraction of the testes from the scrotum was not a problem at the height of the breeding season when the testes were engorged.

I had originally assumed that all adult males would be fertile throughout the breeding season, and that any males with abdominal testes would be subadults or young of the year. This assumption was an oversimplification; all adult males did not reach breeding condition at the same time of year. My data do not support a firm conclusion, for it is difficult to follow non-captive individuals throughout a breeding season, owing to sporadic appearance of animals in traps. Nevertheless, observations of mice that were trapped a number of times indicated the following:

1) Some adult males that had abdominal testes in the middle of July reached breeding condition as late as late August and even late September.

2) Some juvenal males had scrotal testes at the time their postjuvenal molt was just beginning to be apparent on their sides. Most juvenal males did not have scrotal testes, and many juveniles that appeared repeatedly in traps from mid-July through late October did not attain breeding condition. A mouse that was a juvenile in mid-July must have been born in mid-June.

3) Apparently animals born early in the breeding season may reproduce later in that season, whereas those born later in the breeding season tend not to breed until the following year.

Possibly cooler evening temperatures in July and August, due to the relatively larger amounts of precipitation in those months, inhibit reproductive development of late-born young. Most plants have ceased vegetative growth and have produced seeds by this time; but the interrelationships between growing seasons, climatic conditions, and reproductive physiology are unknown.

Only one adult of each species had scrotal testes after late September; the _P. truei_ had scrotal testes on October 24, 1963, and the _P. maniculatus_ had scrotal testes on October 15 of that year.

GROWTH

Growth of captive _P. maniculatus_ and _P. truei_ is discussed in several reports. One of the most complete is that of McCabe and Blanchard (1950) on _P. m. gambelii_ and _P. t. gilberti_ in California. A detailed discussion of the dentition in _P. truei_ and 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.

The thoroughness of the above-mentioned studies is readily apparent to those who have worked with mice of the genus _Peromyscus_. Nevertheless, the ecology of local populations of _P. maniculatus_ and _P. truei_ as 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.

Figures 18 and 19 show that some litters grow appreciably faster than others, 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 of _P. truei_ is several days longer than that of _P. maniculatus_, and the young of _truei_ are fewer and heavier than those of _maniculatus_. As would be expected, _truei_ remains in the nest longer and nurses longer than _maniculatus_.

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 19 reveals that the appendages of young _maniculatus_ attain most of their length about a week earlier than those of _truei_. Young _truei_ acquire mobility and coordination somewhat later than young _maniculatus_, 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 of _truei_ and _maniculatus_. 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 especially _P. 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 any defense 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 of _P. truei_ gave 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 of _P. truei_ lost or killed all but one young of her litter; at about the same time, a _P. maniculatus_ and all but one of her young inexplicably died. Since the remaining young _maniculatus_, a male, was just weaned and was considered expendable, I placed him in the cage with the female _truei_ and her 33-day-old, male offspring. The reaction to the newcomer was unexpected. The female immediately covered the _P. maniculatus_ and her own young and prepared to defend them against me. Later, when the _P. maniculatus_ was disturbed, he had only to emit a squeak and the female _truei_ would run to cover and protect him. When the young male of _P. truei_ was 69 days old the female kept him out of the nest, but still kept the male _maniculatus_ in 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 male _truei_ was left in the cage with his mother and the _P. maniculatus_ from 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 the _P. maniculatus_ was introduced into the cage, the female _truei_ continued to cover him with her body whenever there was a disturbance. The male _maniculatus_ not only tolerated this attention, but ran under the female _truei_ when 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 of _Peromyscus_ in nature.

Young males are tolerated by their mothers after weaning. One young male _maniculatus_ was 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 male _P. truei_ was 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, of _P. maniculatus_, also died; the female of _P. truei_ was the same one that adopted the surviving _P. maniculatus_. The female _truei_ continued 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 subgenus _Peromyscus_ have three pairs 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 of _Peromyscus_ carry their young ventral-side up in their mouth while the young are small, and sometimes when the young are older. Generally, when females of _P. truei_ moved 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 of _P. truei_ would 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 of _P. maniculatus_. The females of _P. maniculatus_ appear to be somewhat less concerned than those of _P. truei_ for 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.

_P. maniculatus_ _P. truei_
----------------------------------+-------------------------------------
FIRST WEEK: At birth, young are | At birth, young are helpless, red
helpless, red overall, small | overall, smaller than _P. truei_,
with wrinkled skin. Pinna of ear | skin wrinkled. Ear, eyes, and
folded over and closed; eyes | digits as in _P. truei_.
closed; digits not separated |
from rest of foot. |
|
Redness diminished by fourth day. | Redness decreases and disappears by
| fourth day.
|
Hair apparent by fifth day; | Hair apparent by fourth day; body
dorsal one-half or two-thirds of | bicolored by end of week.
body more darkly pigmented than |
venter by fourth day. |
|
Young squeak loudly and suck; | Young squeak loudly; sucking more
sometimes crawl, but drag hind | pronounced than in _P. truei_; may
legs. | crawl, but drag hind legs.
----------------------------------+-------------------------------------
|
SECOND WEEK: Appreciable increase | As in _P. truei_.
in size; head about 60 percent |
larger than at birth, by 14th |
day, and still large in |
proportion to body. |
|
Toes on hind foot separated more | As in _P. truei_, but somewhat more
from foot. | advanced.
|
Body well haired by end of week; | Body well haired by end of week;
dorsum dark gray, venter whitish; | dorsum dark gray with brownish
tail bicolored in most, but not | tint; venter whitish; tail
haired. | bicolored in most, but not haired.
|
Pinna of ear unfolded and open by | As in _P. truei_, but development
end of week. | somewhat more advanced.
|
Through day 10, use hind legs to | Crawl well by end of week;
push, but by end of week use legs | difficult to hold, squirm but do
to crawl; difficult to hold, | not bite; agile.
squirm but do not bite. |
|
Walk behind mother while nursing; |
agile. |
----------------------------------+-------------------------------------
|
THIRD WEEK: Eyes open on 16th to | Eyes open on 16th to 20th day,
21st day. | partly open earlier.
|
Gray pelage of dorsum brownish. | Pelage of dorsum brownish; molt
Apparently there is a molt line | line across shoulders progressing
progressing posteriorly from | posteriorly; browner anterior to
nose; the molt line has moved to | line, grayer posterior to it.
shoulder region by end of week; |
pelage anterior to line browner, |
grayer posterior to it. |
|
Tail haired and weakly bicolored | Tail haired and bicolored in all
in some individuals by end of | individuals.
week. |
|
Young walk and jump well; squirm | Young walk and jump well; fight and
but rarely bite. | bite when handled.
----------------------------------+-------------------------------------
|
FOURTH WEEK: Begin to eat solid | Some young eat grain by 24th day;
foods at 23-29 days, but also | others continue to nurse.
nurse. |
|
Molt line about 3/4 inch | Juvenal pelage complete; no sign of
posterior to head. Juvenal pelage | postjuvenal molt.
completed by end of week. Some |
young have brownish hair on front |
legs. |
|
Young roll over on backs and use | As in _P. truei_; also, all jump
feet to ward off litter mates | well, and fight fiercely when
that are dropped into nest, or | handled.
into container, with them. |
----------------------------------+-------------------------------------
|
FIFTH WEEK: Young weaned on 30th | All young weaned before or by end
to 40th day; some nurse beyond | of week; none observed to nurse
30th day if female is lactating. | beyond 30th day, even if female is
| lactating.
|
Juvenal pelage complete and no | Juvenal pelage complete;
postjuvenal molt apparent on | postjuvenal pelage not apparent on
dorsum. | most, but probably present on all,
| and concealed under juvenal pelage.
----------------------------------+-------------------------------------
|
SIXTH WEEK: Postjuvenal pelage | Postjuvenal molt apparent in most
apparent in most individuals | young; almost complete in some,
under juvenal pelage, especially | except above tail and on flanks.
along lateral line. |
----------------------------------+-------------------------------------
|
SEVENTH WEEK: Postjuvenal pelage | Postjuvenal pelage apparent in all
apparent in most young; in some | young; less distinct molt line than
the molt line has progressed well | in _P. truei_.
up on the sides, but not to |
mid-dorsum. |
----------------------------------+-------------------------------------
|
EIGHTH WEEK: All individuals | Growth completed in some
growing; total lengths of 156-170 | individuals; those in larger
millimeters; weight 17-22 grams. | litters have total lengths of
| 128-144 millimeters; weight
| 14-17 grams.
----------------------------------+-------------------------------------
|
NINTH WEEK: Testes partly scrotal | Scrotum in season usually large,
in one male on 59th day. | vaginae open, evidence of coitus
| common. (McCabe and Blanchard,
| 1950:39).
|
New brown pelage encroaching on | Postjuvenal molt completed in some
saddle and on hind legs; | individuals by end of week. New
postjuvenal molt completed in | pelage tends to be concealed under
some individuals by eleventh | juvenal pelage longer than in _P.
week. | truei_.
----------------------------------+-------------------------------------

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.

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.

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.

In _P. 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. In _P. 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 in _truei_, but replaces the juvenal coat in a less distinct manner than in _truei_. As a result, contrast often is lacking between juvenal and subadult pelages in _maniculatus_ making 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 in _P. maniculatus_ is duller than the adult pelage: In _P. truei_ the subadult and adult pelages appear to have an equal sheen.

In early winter, the postjuvenal pelage acquired by young individuals of _P. truei_ was 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 of _Peromyscus_ that I examined. When anomalies were found they were striking, principally because of their low rate of occurrence.

One female of _P. 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 of _P. truei_ was 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. _Peromyscus_ normally 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 been marked 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 that _Peromyscus_ of either species eats cacti.

One _P. truei_ had a broken tail; three other individuals had tails about one-half normal length. One _P. maniculatus_ had a shortened tail. Some of these injuries probably were caused by the Sherman live traps; several individuals of _P. truei_ were released after having been caught by the tail by the spring-loaded door of these traps.

On October 17, 1963, an adult _P. truei_ had 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, of _P. truei_, had a wet chest when he was taken from a warm trap; when released, this mouse ran to a nearby plant of _Comandra 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 of _P. m. rufinus_ and _P. t. truei_ in 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 of _P. m. rufinus_ and 54 specimens of _P. t. truei_ were 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 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_.

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.

WATER CONSUMPTION

_Peromyscus maniculatus_ is ubiquitous, occurring in habitats ranging from mesic boreal forests to arid southwestern deserts. Most subspecies of _P. maniculatus_ live in moderately mesic or near-mesic environments, but a few have adapted to arid conditions. It has been assumed that the success of _P. maniculatus_ in 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 range _P. maniculatus_ coexists with one or more other species of _Peromyscus_ that are more restricted in distribution. _Peromyscus truei_ is 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 genus _Peromyscus_ (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 in _P. leucopus_, and recently reviewed the literature on water metabolism of mammals (Chew, 1965). In his studies of five subspecies of two species of _Peromyscus_, Ross (1930) found significant differences in water consumption between species but not between subspecies within a species. One of the subspecies of _P. maniculatus_ tested 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 both _P. m. rufinus_ and _P. t. truei_, the species and subspecies with which my experiments are concerned. Lindeborg also tested the ability of five races of _Peromyscus_ to survive reduced water rations. Unfortunately, the subspecies chosen for these experiments did not include _P. t. truei_ or _P. m. rufinus_. Lindeborg (1952:25) found that the "amounts of water consumed by various species of _Peromyscus_ from 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 between _P. m. bairdii_ from Michigan and either _P. m. blandus_ or _P. m. rufinus_ from New Mexico, but he found a highly significant difference between _P. l. noveboracensis_ from Michigan and _P. l. tornillo_ from New Mexico. Lindeborg also found that the subspecies of _Peromyscus_ that 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, _Dipodomys_ selects foods with high percentages of carbohydrates and conserves water by reducing the amounts of nitrogenous wastes to be excreted (Schmidt-Nielsen _et al._, 1948).

Williams (1959b) found that _P. m. osgoodi_ from 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 that _P. maniculatus_ is 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 of _P. truei_ and _P. 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 of _P. truei_ and _P. maniculatus_ were 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 of _P. maniculatus_ and _P. truei_ were 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 genus _Peromyscus_ on 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 of _P. truei_ consumed more total water and more water per gram of body weight than individuals of _P. 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 (tables 7 and 11). On the high protein diet, _P. truei_ again consumed much more water than did _P. maniculatus_ (tables 7 and 9).

TABLE 7--Food and Water Consumption of _Peromyscus maniculatus_ and
_P. truei_ When Fed Diets of Different Protein Content. Food and
Water Consumption Are Determined for the Grams, or Milliliters,
Consumed per Gram of Body Weight per Day; Daily Totals Are also
Given.

====================================================================
_Peromyscus maniculatus rufinus_
------------+------+---------------+-------+---------------+--------
Diet | | Food | Total | Water | Total
per cent | No. | /gram | grams | /gram | water
protein | mice | /day +- S. D. | /day | /day +- S. D. | /day
------------+------+---------------+-------+---------------+--------
Lab Chow 23 | 9 | .201 .074 | 4.455 | .262 .183 | 5.751
------------+------+---------------+-------+---------------+--------
Hog Chow 36 | 10 | .238 .060 | 5.232 | .496 .186 | 10.749
------------+------+---------------+-------+---------------+--------
Corn 11 | 11 | .149 .044 | 3.144 | .174 .012 | 3.696
------------+------+---------------+-------+---------------+--------

_Peromyscus truei truei_
------------+------+---------------+-------+---------------+--------
Diet | | Food | Total | Water | Total
per cent | No. | /gram | grams | /gram | water
protein | mice | /day +- S. D. | /day | /day +- S. D. | /day
------------+------+---------------+-------+---------------+--------
Lab Chow 23 | 10 | .216 .070 | 6.353 | .373 .119 | 10.880
------------+------+---------------+-------+---------------+--------
Hog Chow 36 | 10 | .230 .079 | 6.966 | .653 .189 | 19.571
------------+------+---------------+-------+---------------+--------
Corn 11 | 10 | .158 .010 | 4.318 | .332 .016 | 9.034
------------+------+---------------+-------+---------------+--------

The tendency of both species to eat more of the hog chow than they ate when fed standard laboratory chow may reflect a higher palatability of the hog chow. Both species consumed similar amounts of food per gram of body weight, on each of the diets (Table 7). The larger _P. truei_ requires more grams of food per day than the smaller _P. maniculatus_, but this slight difference in food consumption probably has no effect on the distribution of these species within Mesa Verde.

The results obtained with the low protein diet were strikingly different from those of the first two experiments. In this experiment the same groups of mice were placed on a diet of whole, shelled corn for a period of six weeks. The corn contained less than 11 per cent protein, about three per cent fat, and about 80 per cent carbohydrate.

By the end of the first week, on the low protein diet, all mice had reduced their water intake by about half the amount used per day on the high protein diet (Table 7). There was not a statistically significant difference, for either species, between the average amounts of water drunk in the first and in the sixth weeks of the experiment.

The data in Table 7 show that on all three diets, individuals of _P. maniculatus_ drank less water per gram of body weight than individuals of _P. truei_. Variation in water consumption was high; some individuals of _P. maniculatus_ that drank more than the average amount for the species, consumed as much water as some individuals of _P. truei_ that drank less than the average amount. In general, individuals of _P. maniculatus_ drank about half as much water each day as individuals of _P. truei_. Individuals of both species were consistent in their day-to-day consumption.

TABLE 8--Amounts of Mean Daily Water Consumption as Reported in the
Literature for Species of _Peromyscus_. Figures in Parentheses are
Means; Those Not in Parentheses Are Extremes.

Column headings:

A: Mean daily ml./gm. wt./day
B: Water consumption total ml. per day
C: Temperature
D: Humidity
E: Per cent dietary protein
F: Investigator

================+===========+=============+=======+=======+====+=====
| A | B | C | D | E | F
----------------+-----------+-------------+-------+-------+----+-----
| (.262) | (5.70) | | | |
_P. m. rufinus_ | .124-.699 | 2.71-15.07 | 20-23 | low | 23 | [A]
| | | | | |
_P. m. rufinus_ | (.101) | (2.39) | 20-25 | 24-47 | | [B]
| | | | | |
_P. m. osgoodi_ | .16-.25 | 3.2-4.3 | 18-22 | 10-20 | 23 | [C]
| | | | | |
| (.126) | (1.74) | | | |
_P. m. bairdii_ | .082-.177 | 1.12-2.72 | 21 | 25-68 | | [D]
| | | | | |
_P. m. bairdii_ | .124-.182 | (2.37-3.17) | 20-25 | 24-47 | | [B]
| | | | | |
| (.372) | (10.80) | | | |
_P. t. truei_ | .224-.561 | 7.0-16.92 | 20-23 | low | 23 | [A]
| | | | | |
_P. t. truei_ | (.085) | (2.77) | 20-25 | 24-47 | | [B]
| | | | | |
_P. l. nov._ | .057-.117 | 1.36-2.29 | 21 | 25-68 | | [D]
| | | | | |
_P. l. nov._ | | (5.36) | 18 | 62.5 | | [E]
----------------+-----------+-------------+-------+-------+----+-----

[A] Douglas
[B] Lindeborg, 1952
[C] Williams, 1959
[D] Dice, 1922
[E] Chew, 1951

Table 8 shows average water consumption for several species of _Peromyscus_ as reported in the literature, and as determined in my study. It is difficult to compare my results with most of the data in the literature, because of a lack of information as to protein, fat, carbohydrate, and mineral contents of foods used in other studies. Lindeborg (1952) and Dice (1922) fed mice on a mixture of rolled oats, meat scraps, dry skimmed milk, wheat germ, etc. described by Dice (1934). Their data on water consumption in _P. maniculatus_ indicate that this mixture probably is lower in protein content than Purina Laboratory Chow, that was used in my experiments and those of Williams' (tables 8 and 9).

The amount of dietary protein consumed under natural conditions is not known for most wild animals. One index of the minimum amount of protein necessary is the amount required for an animal to maintain its weight. At best, this can be only an approximation of the required amount, for other factors, such as stress, disease, change in tissues during oestrus or gonadal descent, and changes in constituents of the diet other than protein, would all be expected to affect the body weight (Chew, 1965:145-147).

The data in Table 7 show that both species vary their food intake with changes in diet. Table 10 shows weight changes that took place in individual mice when fed each of the three diets. A change in weight of one gram cannot be considered as important, for the weight of an individual mouse fluctuates depending upon when he last drank, ate, defecated or urinated.