Peromyscus truei
Pinyon Mouse

* Information adapted from Mammalian Species No. 161, pp. l-5,  The American Society of Mammalogists, December 3, 1981
 

GENERAL STATEMENT
Pinyon mice are large-eared, medium-sized mice. Color varies from a pale yellowish-brown (P. t. chlorus) to brownish black (P. t. dyselius and gratus) and the fur is long and silky. They are found in the western United States from central Oregon and southern Wyoming to the north and down into central and southern Mexico to the south.

DESCRIPTION
Pelage:  Color varies from a pale yellowish-brown (P. t. chlorus) to brownish black (P. t. dyselius and gratus) and the fur is long and silky. The dorsal tail stripe is slightly darker than the back. The tail is covered with short hairs except at the tip where longer hairs project. An ochraceous pectoral spot may be present or absent. The dorsum of the hindfeet is white or dusky.

Weight:  Adult weights range from 24-39 g.

Size and Measurements:  Three groups of populations of pinyon mice each have certain characteristics separating them, one from another. In one group (P. t. gilberti, sequoiensis, dyselius, ntontipinoris, chlorus, martireusis, and lagurcae), the tail is longer than the body, the ear relative to the length of the hindfoot is small, and the inflation of the auditory bullae is not so great as in the next group. Selected measurements (mm) of a population from Kern County, California, are: length of tail, 106; length of body, 99; length of hindfoot, 23.9; length of ear, measured dry from the notch, 23.4; greatest length of skull, 29.1; breadth of braincase, 13.3.

In a second group consisting of those populations east of the Sierra Nevada-Cascade axis and north of Mexico (P. t. truei, nevadensis, and preblei), the tail is shorter than the body, the ear relative to the length of the hindfoot is large, and the inflation of the auditory bullae is great. Selected measurements (mm) of a population for this group from Valencia County, New Mexico, are: length of tail, 88; length of body, 100; length of hindfoot, 23.1; length of ear measured dry from the notch, 23.7; greatest length of skull, 28.5; breadth of braincase, 13.1. The subspecies P. t. comanche should fit in group two on a geographical basis, but the tail is longer than the body (102.7 to 95.1, respectively) and the ear relative to the hindfoot is short (measured in the flesh, 21.9 to 23.2, respectively) (Schmidly, 1973).

In the third group, consisting of populations in mainland Mexico (P. t. gentilis, gratus, erasmus, and zapotecae), the tail is longer than the body, the ear is small relative to the hindfoot, and auditory bullae not greatly inflated. Selected measurements (mm) of a population from Tlalpan, Federal District, Mexico, are: length of tail, 106; length of body, 94; length of hindfoot, 22.6; length of ear measured dry from the notch, 19.9; greatest length of skull, 27.3; breadth of braincse, 13.0. The fundamental number of the chromosomes of group 3 is 54; FN is 62 in the other groups (Zimmerman et al., 1975).

Greatest length of skull in adults is usually between 27.0 and 30.0 mm and alveolar length of the maxillary toothrow between 4.2 and 4.5 mm. The braincase is large and vaulted and the zygomatic arches are weak, do not bulge laterally, and converge anteriorly.

FORMAL DIAGNOSIS
Mice of the species P. truei have relatively and actually large ears, with the ear measured from the notch equal to or longer than the hind foot in most populations. Total length averages about 190 to 200 mm. The tail is longer than the head and body in populations west of the Sierra Nevadas, Baja California, and mainland Mexico; the tail is shorter than the body in most United States populations east of the Sierra Nevadas and Cascade Mountains. The hindfoot in most populations is 22 mm or more in length. The skull  is of medium size but the auditory bullae are large and well inflated. The upper cheek-teeth have the characteristic features of the subgenus Peromyscus, with a well-developed mesoloph in the upper first and second molars.

Peromyscus truei is most often confused with Peromyscus boylii, P. difficilis, P. leucopus, P. californicus, and P. bullatus. P. truei differs from P. boylii in having an ear that is as long as or longer than the hindfoot, rather than only about 70 to 80% of the length of the hindfoot, tail that is more heavily haired and with less pronounced annulations, auditory bullae that are more inflated, and chromosomes that differ in the number of kinds.

Peromyscus truei can be differentiated from P. difficilis by a combination of color features. In P. difficilis, the coloration of the dorsum in adults often is grayish-black, somewhat similar to the juvenile pelage, the color along the lateral line has very little ochraceous color, and the top of the head in adults is often grayish. In addition, in P. truei, the auditory bullae are relatively more inflated, the toothrow is shorter, and chromosomes differ in the number of kinds. In Mexico, the skull of P. truei is noticeably smaller.

Peromyscus truei occurs with P. californicus in parts of its range and differs from it in overall smaller size, shorter hindfoot, smaller skull and shorter toothrow, presence rather than absence of mesolophs of the first and second upper molars, and in differences in the number of kinds of chromosomes.

Peromyscus truei usually does not occupy the same areas as P. leucopus, but where it does P. truei is larger, especially the ear and the tail, the tail is more heavily haired at the tip, the skull is larger, the auditory bullae are much more inflated, and the zygomatic arches are less strongly bowed outward.

Peromyscus truei differs from P. bullatus of Vera Cruz in smaller ears (less rather than more than 24 mm), auditory bullae less inflated, and hindfoot shorter (24 mm or less rather than usually more).

HABITAT
In the United States east of the Sierra Nevada-Cascade axis, pinyon mice are most commonly found "in the pinon belt in close association with pinons or junipers, particularly where these trees grow among rocks or on rocky slopes. No other species of Peromyscus, or any other small rodent, is as exclusively confined to the pinon-juniper belt, or occurs as abundantly in it as does P. t. truei" (Hoffmeister, 1951). In central Oregon, where pinyons are not present, these mice live in rocky outcrops among junipers and sage. In Texas, these mice "occupy a restricted range along the rocky, cedar [juniper]-covered slopes at the northwestern corner of the `Break of the Plains' in western Texas. Locally this species is restricted to rocky situations in cedar forests on the canyon slopes and floors" (Schmidly, 1973). There are exceptions to the occurrence of P. truei in pinyon-juniper. They have been taken in yellow pine forests, in the bristle-cone pine belt, and in a cottonwood-sagebrush riparian situation.

West of the Sierra Nevada-Cascade axis, pinyon mice are found in a variety of habitats. In California, Hoffmeister (1964) pointed out the concordance of these mice and the Upper Sonoran Life-zone and the coastal redwood belt. In much of California, P. truei is closely associated with chaparral on canyon sides, even if the chaparral is among stands of live oaks, eucalyptus, or pine (McCabe and Blanchard, 1950). In the coastal redwood belt, these mice are associated with redwoods, Douglas fir, California laurel, blackberry, madrone, and poison oak (Hoffmeister, 1951). In the desert localities in southern California, pinyon mice are found among pinyons, junipers, Joshua trees, sometimes in chaparral, and even in desert scrub, including sagebrush. In Baia California, these mice are found in pinyon, sometimes in association with junipers and scrub oak.

In mainland Mexico, pinyon mice occupy a variety of habitats. In Oaxaca, they live on the broken faces of limestone cliffs and among boulders at the base, with a sparse vegetative cover of cactus, mesquite, and grasses (Hooper, 1957). In the Distrito Federal, they occupy rocky lava beds that are fissured and broken and partly grown up with sacaton and groundsel. In San Luis Potosi, pinyon mice are found at the bases of rocky slopes, often in dense growths of prickly pear and yucca (Dalquest, 1953); in Durango they live in rocky areas from the Douglas-fir-aspen belt to the rocky arroyos bordering grassy plains (Baker and Greer, 1962). In Coahuila, these mice live in rocky situations where juniper, pinyon, and yucca provide the principal vegetative cover but they are also found in juniper-cactus, pinyon-juniper and yellow pine, and yucca-mesquite associations (Baker, 1956).

NATURAL HISTORY
Feeding:  Pinyon mice are good climbers and frequently go into the trees to forage for it's prefered diet, pinyon nuts and juniper berries. They also are known to dine on other nuts, berries, and seeds, along with insects, plants, and even some mammal matter.

The stomach contents of 16 mice taken in midsummer in Monterey County, California, consisted primarily of insects and some spiders; one contained 60% mammal remains (Bradford, 1974). These mice were not feeding on seeds and leaves of chamise although they lived in chamise chaparral. By late summer, the diet was predominantly acorn mast. Douglas (1969) listed plants eaten or not eaten by captive pinyon mice from southwestern Colorado. He also noted a number of plants that could be identified in the stomach of wild-taken pinyon mice. Douglas felt that the winter staple must be juniper seeds in this part of Colorado, since juniper berries remain on the branches throughout winter.

Nesting:  Pinyon mouse usually lives in hollow tunks of juniper trees or under rocks. The nest is constructed of bits of juniper bark, leaves, and grasses. There are frequently some secondary nests of juniper bark scattered around the home territory.

Activity: The home range for these mice in southwestern Colorado (Douglas, 1969) had a mean of 14,427 m²(SD ± 5,878 mz) for males and 12,395 (±5,354) for females, using the inclusive boundarystrip method of calculations. If the exclusive boundary-strip method is used, the mean home range for males is 10,465 m²(±4,043) and for females 8,290 (±2,688). On the average, these mice travelled 52 m between captures for adult males and 49 m for adult females. It is difficult to convert the home range figures into density per hectare because parts of any hectare may be ecologically unsuitable for these mice.

Circadian Rhythm: P. truei is nocturnal, as are all Peromyscus except one subspecies of P. keeni.

Breeding: In southwestern Colorado, breeding occurs from April through September (Douglas, 1969), and in Arizona, from mid-February to mid-November. In California, the average number of young per litter in the wild was 3.5, and in captivity 3.3 (McCabe and Blanchard, 1950). In southwestern Colorado, the number of embryos was 4.0 (SD ± 0.91), with a range of 3 to 6 (Douglas, 1969). Douglas' study of male reproductive capabilities indicated that all males did not reach breeding condition at the same time of the year. Juveniles had scrotal testes at the time the postjuvenal molt was beginning; they are not able to impregnate females unless the testes are scrotal.

McCabe and Blanchard (1950) and Hoffmeister (1951) gave detailed descriptions of newborn and developing young of Peromyscus truei gilberti and Douglas (1969) described young of P. t. truei. At birth, young are hairless and eyes and ears closed. During week 1, young of P. t. truei squeak loudly. In P. t. gilberti they do not squeak until one week old. Between the 16th and 21st day eyes open and ears unfold and open. The body is haired by the 14th day in P. t. truei. Douglas (1969) said that his animals nursed for the first month but McCabe and Blanchard (1950) said that females were without milk in the 4th week after parturition, although the young still attempted to nurse. Postjuvenal molt is discernible by week 7. This molt is complete, or nearly so, by week 10 or 11. Hoffmeister (1951), for P. t. gilberti, and Douglas (1969), for P. t. truei, showed changes in external size with increasing age. Hoffmeister (1951) also discussed changes in the skull with increasing age. Probably at least 88% of growth occurs during the first 35 days; by about day 100, 99%. Cranial elements housing the brain increase least in early postembryonic development; the greatest increase is in the rostral part of the skull.

Longevity is short. Douglas (1969) found that only 2 to 3% of the adults live long enough to breed in consecutive breeding seasons. If I interpret McCabe and Blanchard (1950) correctly, they suggested that only 20% of the young that are out of the nest and feeding for themselves may complete a year of life

KARYOLOGY
The diploid number of chromosomes is 48 and the karyotype consists of 5 pairs of medium to large biarmed autosomes, 3 pairs of small biarmed autosomes, 15 pairs of small to large acrocentric autosomes, and subtelocentric X and small acrocentric Y chromosomes in most populations. Three specimens from Coahuila were reported by Lee et al. (1972) to be like an illustration of Hsu and Arrighi (1968:Fig. 4a), and thus would have 2 pairs of medium to large biarmed autosomes, 2 pairs of small biarmed autosomes, and 19 pairs of acrocentric autosomes.

GENETICS
Fifteen loci were examined for the electrophoretic patterns of 13 proteins by Zimmerman et al. (1975) for four samples of P. truei: Arizona-Utah, New Mexico, Chihuahua, and Durango. Biochemical characters of mice from Grant County, New Mexico, and the sample consisting of mice from 3 mi. SE Santa Barbara and 9 mi. SSE Celulosa, Chihuahua (localities 14 and 15 of Zimmerman et al., 1975) were distinct from the northern populations but no mention was made of the sample to the south in Durango (Zimmerman et al., 1975).

CLASSIFICATION
Order Rodentia, Suborder Myomorpha, Family Muridae, Subfamily Sigmodontinae, Tribe Peromyscini, Genus Peromyscus, Subgenus Peromyscus, Species-group truei.

SUBSPECIES
Currently, 15 subspecies maybe recognized.

P. t. chlorus  Hoffmeistcr, 1941:131. Type locality Lost Horse Mine, S end Little San Bernardino Mts., Riverside Co., California.
P. t. comanche  Blair, 1943:7, see above. See Schmidly (1973:269) for the basis of this combination.
P. t. dyselius  Elliot, 1898:207, see above.
P. t. erasmus  Finley, 1952:265. Type locality 8 mi. NE Durango, 6,200 ft., Durango. Regarded as synonym of geatilis by Baker (1960:321).
P. t. gentilis  Osgood, 1904:61. Type locality Lagos, Jalisco.
P. t. gilberti  (Allen, 1893:188), see above.
P. t. gratus  Merriam, 1898:123, see above (sagax Elliot, pavidus Elliot, and zelotes Osgood are synonyms).
P. t. lagunae  Osgood (1909:172). Type locality La Laguna, Sierra Laguna, Baja California.
P. t. martirensis  (Allen, 1893:187), see above (hemion.otis Elliot a synonym).
P. t. montipinoris  Elliot, 1904:264, see above.
P. t. nevadensis  Hall and Hoffmeister, 1940:401. Type locality 1/2 mi. W Debbs Creek, 6,000 ft., Pilot Peak, Elko Co., Nevada.
P. t. preblei  Bailey, 1936:188. Type locality Crooked River, 20 mi. SE [=12 mi. S, 6 mi. E] Prineville, Crook Co., Oregon.
P. t. sequoiensis  Hoffmeister, 1941:129. Type locality 1 mi. W Guerneville, Sonoma Co., California.
P. t. truei  (Shufeldt, 1885:407), see above (lasius Elliot and megalotis Merriam are synonyms).
P. t. zapotecae  Hooper, 1957:6. Type locality 1 mi. E Tlacolula, 5,700 ft., Oaxaca.

TAXONOMIC HISTORY
Hesperomys truei Shufeldt, 1885:407. Type locality Fort Wingate, McKinley Co., New Mexico.
Hesperomys megalotis Merriam, 1890:63. Type locality Black Tank, Little Colorado Desert, Coconino Co., Arizona.
Sitomys martirensis Allen, 1893:187. Type locality Sierra San Pedro Martir, 7,000 ft., Baja California.
Sitomys gilberti Allen, 1893:188. Type locality Bear Valley, San Benito Co., California.
Peromyscus truei Thomas, 1894:365. First use of name combination.
Peromyscus dyselius Elliot, 1898:207. Type locality Portola, San Mateo Co., California.
Peromyscus gratus Merriam, 1898:123. Type locality Tlalpan, Distrito Federal, Mexico.
Peromyscus pavidus Elliot, 1903a:142. Type locality Patzcuaro, Michoacan.
Peromyscus sagax Elliot, 1903a:142. Type locality La Palma, Michoacan.
Peromyscus hemionotis Elliot, 1903b:157. Type locality Rosarito Divide, Sierra San Pedro Martir, Baja California.
Peromyscus montipinoris Elliot, 1904:264. Type locality Lockwood Valley, near Mt. Pinos, Ventura Co., California.
Peromyscus lasius Elliot, 1904:265. Type locality Hannopee Canyon, 7,500 ft., Panamint Mts., Inyo Co., California.
Peromyscus zelotes Osgood, 1904:67. Type locality Querendaro, Michoacan.
Peromyscus comanche Blair, 1943:7. Type locality Tule Canyon, Briscoe Co., Texas.

REMARKS
Peromyscus truei is the nominal species of the truei group of large-eared mice, which group includes also P. difficilis and P. bullatus. Since the species was revised (Hoffmeister, 1951), three subspecies have been added (comanche, erasmus, zapotecae). The form comanche has had a varied history, being regarded as a distinct species, as a subspecies of P. boylii, and as a subspecies of P. truei. Pinyon mice in areas of open country, as in the pinyon-juniper-rock association, have short tails, large ears, and large bullae. The large ears and bullae may be adaptations for detecting predators at the greatest possible distance. In areas of dense cover, pinyon mice have long tails, short ears, small bullae, and long hindfeet. In such places, detection of potential predators at great distances may not be as important as rapid movements through brush which movement is facilitated by long tails and long, broad hindfeet. Hoffmeister (1951) pointed out that there are clinal changes from the interior coastward (east to west) and southward in decreasing size of the body, increasing length of the tail, decreasing size of the ears, decreasing length of the hindfeet, and decreasing size of the skull.

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