On the desert slope of the mountains this species is found in the part
of the pinyon-juniper association that is between elevations of 4000 and
5200 feet. The mouse is absent from the higher chaparral and
pinyon-covered slopes, but is present on south slopes in the pinyon belt
where more open growths of pinyons and scrub oaks are interspersed with
yucca. I recorded this pocket mouse from the vicinity of Cajon Pass west
to Valyermo.
The local distribution of _pallidus_ is striking because of its close
positive correlation with the distribution of yucca. On benches around
5000 feet, where yuccas are scattered in their occurrence, _pallidus_ is
nearly always taken near (often right at the base of) this plant. Lower
in the juniper belt the dry rocky south slopes supporting yucca plants
are well populated by _pallidus_, while adjacent flats, and north slopes
grown to antelope brush and scrub oak, are completely uninhabited. Near
the mouth of Grandview Canyon, on steep rocky southern exposures grown
sparsely to burro weed and yucca, one hundred traps produced in one
night eight _pallidus_ and no other rodents. Here many of these pocket
mice were trapped on large fractured rock outcroppings, where most or
all of the mice probably lived in the daytime in the deep cracks; in any
event no burrows were noted near these rocks.
This species prefers barren slopes supporting yucca plants. These plants
produce large seeds which are staple food items for _P. f. pallidus_ and
other rodents during the lean part of the year, that is to say, late
summer and autumn. Many of the dry capsules of the yucca plants were
examined in October, 1951, and these generally still contained a few
seeds. Pocket mice taken in October usually carried in their cheek
pouches seeds of yucca together with some other material, and often they
carried only the seeds of yucca. Probably the wind shakes only a few
seeds out of the capsules at a time, thus tending to drop the seeds over
a fairly long period.
Trapping in winter in the juniper belt revealed that these pocket mice
were not active above ground on nights colder than about 40 deg. F. On
nights when the temperature was about 36 deg. F. none was taken, but on the
one night in late December, 1948, when the minimum was 44 deg. F., several
specimens were taken. In this same area in May 1949, pocket mice were
the most numerous rodents. Because of their evident sensitivity to cold
weather, these mice must remain below ground for weeks at a time during
the cold weather of December and January.
Specimens of _pallidus_ from the desert slope of the San Gabriels are
grayer (less brown) than specimens taken farther southeast in the Mojave
and Colorado deserts. Further sampling of populations of _Perognathus
fallax_ from areas adjacent to the San Gabriels might demonstrate
differences of sufficient magnitude to warrant subspecific distinction
of the San Gabriel population. Possibly, however, the San Gabriel series
manifests only local variation in the race _pallidus_. Grinnell
(1933:54) characterizes the ecological niche of the race _pallidus_ as
being "open, sandy ground, often ... surrounded by rocky slopes,"
whereas these pocket mice in the San Gabriels inhabited gravelly or
rocky juniper-dotted benches.
_Specimens examined._--Total, 11, distributed as follows: Los
Angeles County: 5 mi. E and 4 mi. S Llano, 4500 ft., 7; 2 mi. E
Valyermo, 4500 ft., 3; 4 mi. E Valyermo, 5000 ft., 1.
=Perognathus californicus dispar= Osgood