This subspecies is associated with contrasting types of soil and vegetation. It is seemingly absent from the upper pinyon-juniper sage flats and areas grown to chaparral, but is fairly common on the gravelly benches dotted with junipers, and in the washes issuing from the canyons on the desert slope. It is present in small numbers in the Joshua tree association.
In 1951 the numbers of sonoriensis were noticeably less than in 1948; probably this was correlated with the series of dry winters in this period. In December, 1948, this animal was one of the most common rodents in Mescal Wash, 200 trap-nights yielding thirteen specimens; but in November, 1951, none was taken. In parts of the juniper belt, where an average of about six sonoriensis was taken per 100 trap-nights in 1948, the average had dropped to one per 100 trap-nights in 1951.
Specimens of this species from the desert slope of the mountains have been assigned to the subspecies sonoriensis. Those from Blue Ridge tend toward sonoriensis in color, and may be considered as intergrades between this subspecies and gambeli.
This species was active on nights when the temperature was as low as 10° F., and individuals were trapped in the juniper belt in December, 1948, when four inches of snow lay on the ground.
Gray-pelaged juveniles were taken on the desert slope in December, 1948, and a female taken in Mescal Canyon on December 22 of this year carried four embryos near term.
Specimens examined.—Total, 11, distributed as follows: Los Angeles County: 8 mi. E and 4 mi. S Llano, 4000 ft., 6 (4 PC); Mescal Canyon, 4800 ft., 5.
Peromyscus boylii rowleyi (J. A. Allen)
Brush Mouse
The main range of this mouse in the San Gabriel Mountains lies between 1600 and 6000 feet elevation on the Pacific slope of the Mountains, thus encompassing much of the chaparral and oak woodland associations. It was the most common mammal in the oak woodland association in the lower foothills and often was trapped there on leaf mold beneath the oaks. While trapping for shrews I regularly took this species in riparian growth right down to the edge of the water. In San Antonio Canyon many boylii were trapped beneath logs and dense vegetation, and on wet seepage slopes adjacent to the creek.
This species shows a definite predilection for rocky habitats where these occur in the chaparral. In heavy lilac brush near Camp Baldy Peromyscus boylii was outnumbered by P. californicus, yet where talus slopes or boulder piles occurred boylii was more numerous. At the head of Cow Canyon amid boulders beneath scrub oak, bay, and big cone-spruce, this species was especially abundant and no other Peromyscus was taken.