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 deg.
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)