Of special interest is the occurrence of this mouse on the desert slope of the mountains; there it was taken beneath scrub oaks in the pinyon-juniper association at the mouth of Mescal Canyon, and amid boulder and debris piles in Mescal Wash at 4000 feet elevation. While manzanita and scrub oak grew in the wash at the points of capture, the animals were actually surrounded by the desert conditions of the Joshua woodland, and associated with such desert forms as Onychomys torridus pulcher and Peromyscus eremicus eremicus.
Immature individuals were taken in October, November, February, and March, and a female with two large embryos was taken near Icehouse Canyon on November 8, 1951.
Specimens examined.—Total, 8, distributed as follows: Los Angeles County: Mescal Wash, 4000 ft., 1; Mescal Canyon, 4800 ft., 2; San Antonio Canyon, 5200 ft., 2; San Antonio Canyon, 4500 ft., 1; San Antonio Canyon, 2800 ft., 1; Thompson Canyon, 1800 ft., 1 (PC).
Peromyscus truei montipinoris Elliot
Piñon Mouse
Only once was this mouse found outside the pinyon-juniper association of the desert slope; in November, 1949, several were collected near Cajon in mixed manzanita, scrub oak, and greasewood chaparral. This was the only Peromyscus of regular occurrence in the pinyon-juniper area, and was recorded from the upper limit of this association, near Jackson Lake, at 6000 feet, to the lower limit of the association at the mouth of Graham Canyon at roughly 4000 feet elevation.
Although in the juniper belt truei often occurs on common ground with Peromyscus maniculatus sonoriensis, the habitat preferences of these animals are generally complementary. Where the mice occur together, traps set in a variety of locations caught Peromyscus maniculatus, but typically traps set amid the brush or on the open ground away from the junipers were productive. On the contrary truei was invariably trapped quite near the junipers and often in association with the large nests of Neotoma fuscipes simplex. In fact traps set right on the beds of litter beneath the junipers were most likely to catch truei. Records kept of trapping localities show that truei was without exception trapped within twenty feet of some treelike shelter such as junipers, pinyons, Joshua tree or scrub oaks. Thus Peromyscus maniculatus occupies the open stretches between the trees, while truei inhabits the ground beneath and immediately adjacent to the trees. In Nevada the piñon mouse prefers rocky areas (Hall, 1946:520). In the San Gabriel Mountains this mouse does not seem to have this predilection.
In the juniper belt truei was second to Dipodomys panamintinus in point of numbers. In the course of 500 trap-nights in the juniper belt twenty-two truei were taken with thirty-six Dipodomys.
I consider my series of Peromyscus truei from the desert slope of the San Gabriels to represent the subspecies montipinoris. The series is closely comparable to specimens of the subspecies montipinoris in the California Museum of Vertebrate Zoology from the Mount Pinos area, but differs from specimens of the race chlorus from the San Bernardino Mountains in certain diagnostic characteristics. In his recent paper on Peromyscus truei, Hoffmeister (1951) considered the populations of this species in the San Gabriels to be of the race chlorus. Hoffmeister had only one specimen available from the San Gabriel Mountains (Lytle Creek, on the Pacific slope) which was intermediate between montipinoris and chlorus, but on the basis of cranial measurements it was referred to the race chlorus. Specimens of Peromyscus truei from the eastern end of the desert slope of the San Gabriel Mountains and the Cajon Pass area would probably demonstrate that the race montipinoris, which occupies the desert slope of the San Gabriels, intergrades with the race chlorus, which occurs in the San Bernardino Range immediately to the east, in the Cajon Pass area. Although montipinoris occurs on the desert slope of the San Gabriels, chlorus may occur on the Pacific slope. I took no specimens of the piñon mouse on the Pacific slope of the San Gabriel Mountains.
In December, 1948, many small juveniles were taken in the juniper belt, and on October 15, 1951, two females trapped at the head of Grandview Canyon had embryos: one three and the other four. On November 13, 1951, a partially gray-pelaged subadult female was trapped which had recently suckled young.