Tamiasciurus douglasii 79T. d. mollipilosus—more inland than the other subspecies in
Washington and Oregon (USA), as well as along the coast
in British Columbia (Canada) and northern California
(USA), and in the Sierra Nevada of northern and western
California (USA). It has a whitish fringe on its tail.
conservation: IUCN status—not listed. Population trend—
no information.
habitat: T. douglasii occurs in single and mixed stands of
conifers: pine (Pinus), fi r (Pseudotsuga and Abies), spruce (Pi-
cea), and hemlock (Tsuga). Douglas’s squirrels are considered
an old growth–dependent species, although a few studies
suggest that the abundance of T. douglasii may not vary con-
siderably with forest age. Research on the eff ects of forest
fragmentation shows that fragment size has little eff ect on
T. douglasii, but that the percentage of primary or old sec-
ondary growth in the surrounding matrix, and its proxim-
ity to primary forests and riparian corridors, has a strong
infl uence.
natural history: Douglas’s squirrel is diurnal and active
throughout the year. Their activity is bimodal from spring
to autumn, with peaks occurring in the morning and late
afternoon. In winter, their activity is restricted to a single
midday peak. When active, this species is generally quite
visible and audible, as a result of the frequent interspecifi c
interactions necessary to maintain territory boundaries.
“Rattle” calls, detectable at well over 100 m, are often fol-
lowed by v igorous cha ses of territoria l i nt r uders. The breed-
ing season of T. douglasii begins in March and continues for
several months. This extended breeding season results from
asynchrony in estrus, which may reduce competition
among dispersing young. A second breeding season is com-
mon when food is abundant and the climate is mild, al-
though reproductive failure can occur when cone crops fail.
Female Douglas’s squirrels are in estrus for a single day dur-
ing the breeding season, within which time several males
converge on the female’s territory and the female mates
with one or more of these males. T. douglasii is a spontane-
ous ovulator, resulting in immediate conception. Litter sizes
range between four and eight (average = 5.36), although lim-
ited data are available. Gestation and development are
poorly documented for this species, but are probably similar
to T. hudsonicus. Individuals reach sexual maturity at 10–12
months; they can reproduce in the fi rst year, but they usu-
ally do not.
T. douglasii defends exclusive territories established
around food supplies, nest sites, and the midden (a single
larder in the center of the territory). Territory size and
shape are both determined by the type, availability, and
distribution of food; territory size is inversely related to
cone crops. Throughout the year, T. douglasii aggressively
defends territory boundaries from conspecifi cs via vocal-
izations and chases. Territories are often bequeathed to
their young upon weaning. The evolution of territoriality
in the genus appears to be primarily related to the avail-
ability of food for larder-hoarding (i.e., cones), the storabil-
ity of cones in the damp middens, and the effi ciency with
which the midden can be defended. T. douglasii feeds heav-
ily on tree seeds, mushrooms, and other fungi as available
throughout the year, but it is also opportunistic when its
primary diet items are scarce. Foods reported in the diet
include the seeds of conifers, the seeds and fruits of angio-
sperms, tree buds, cambium, fruiting bodies of epigeous
(aboveground) and hypogeous (underground) fungi, and
numerous secondary diet items. These studies also give
the energetic value of some of these items. Signifi cant
shifts in diet occur through the year. The mean stomach
content consisted primarily of conifer cones in autumn,
but it dropped through the winter to only 3 percent by
spring, during which time the animals fed extensively on
the cambium of terminal buds and shoots of conifers (50%
by volume). Fungi, especially hypogeous (underground)
truffl es, accounted for 90 percent of their diet in summer.
Cambium and fungi are low in calories but are important
sources of protein, limiting minerals, and water. Popula-
tion density can vary with the size of the cone crops, al-
though the density of T. douglasii is to some extent limited
by its territorial behavior. Population densities across for-
est types ranged from 0.03 to 0.9 animals/ha both between
and within the same forest types. Annual variation in
abundance is due primarily to variation in juvenile recruit-
ment, which appears to be linked directly to annual cone
production. The frequency and mass of T. douglasii in theTamiasciurus douglasii. Photo courtesy A. Coke Smith, www
.cokesmithphototravel.com.