82 Tamiasciurus hudsonicusalso important, especially in summer and during other peri-
ods of food shortages. Additional secondary food items (in
approximate order of relative importance) include tree buds
and fl owers, fl eshy fruits, tree sap, bark, insects, and other
animal material. During winter, spring, and early summer,
T. hudsonicus can cause serious damage to trees by stripping
bark and girdling the trees to consume phloem and cam-
bium. This species hoards food, primarily conifer cones, in
a larder (midden) that is usually established in the center of
the range near large trees, dense understory, and fallen logs.
Secondary caches also may be common throughout the ter-
ritory. The midden, critical for overwinter survival, pro-
vides a moist environment for maintaining conifer seeds,
which prevents the cones from opening and decreases their
pilferage by competitors. High rates of mutual pilferage of
stored cones have been reported in a Vermont population of
T. hudsonicus. C. C. Smith argued that the storability of coni-
fer cones and the defensibility of the midden specifi cally led
to the evolution of territoriality in the genus, and this social
structure, in turn, probably limits population densities and
the carrying capacity in conifer forests.
T. hudsonicus is highly systematic and selective when
harvesting cones and provisioning the midden. Individual
squirrels retrieve cones throughout their territory and
may stockpile as many as 16,000 cones in a single year. T.
hudsonicus tends to harvest cones fi rst from the species
with the highest energy profi tability (i.e., seed density per
cone) and then move to other species in order of declining
profi tability. In mixed stands in the Pacifi c Northwest, T.
hudsonicus harvests cones in the following order: Pacifi c
silver fi r (Abies amabilis), Douglas fi r (Pseudotsuga menziesii),
Engelmann spruce (Picea engelmannii), and then western
hemlock (Tsuga heterophylla). Squirrels appear to select in-
dividual trees only after cones have been harvested from
all but the species with the least available energy per cone.
T. hudsonicus selects individual cones based on seed num-
ber, the ratio of seed mass to cone mass, cone hardness, the
orientation of the cone to the branch, and the distance
between the point of harvest and the midden. Although
some authors argue that Tamia s c iur u s is unlikely to exert a
strong selective force on cone characteristics, several re-
cent studies suggest otherwise.
This species shows two annual molts: one in spring
through summer, and another during the autumn. The tail
molt occurs only once annually. The structure of the male
and female reproductive tracts of Tamia s c iur u s diff ers sig-
nifi cantly from those of other tree squirrels. The penis is
long, slender, and symmetrical. The Cowper’s gland is re-
duced, the bulbar gland is absent, and the baculum is vesti-
gial. The coiled vagina is unique to Tamia s c iur u s. Seasonal
changes in the structure of male and female reproductive
tracts are detailed by J. N. Layne. Among North American
squirrels, the smaller body size of red squirrels may result
from selection for increased foraging ability and agility in
the smaller branches of conifer trees. Body temperature in
this species is unusually high for mammals. Vascular bun-
dles involved in countercurrent heat exchange may be pres-
ent in the base of the tail. When extended over the body, the
tail increases the insulation of the pelt by 13.7 percent. The
tail also reduces the rate of heat gain by more than 40 per-
cent, through shading and the dissipation of heat.
Red squirrels are spontaneous ovulators and are in es-
trus for only one day during the breeding season. Gestation
averages 33 days and litter sizes (summarized from seven
studies) range from 3.2 to 5.4. T. hudsonicus typically repro-
duces once annually, but females can produce two litters per
year when food is abundant, especially in the eastern por-
tion of the range. Seasonal variation in reproductive anat-
omy was reported for more than 550 animals in New York
state (USA). The young are born pink and hairless; their
eyes open between 26 and 35 days; the pelage is fully devel-
oped by 40 days; and the remainder of their development,
including dentition, is complete by 125 days. The young are
active outside the nest by 7 weeks of age, weaned by 7 weeks,
and fully independent a few weeks after weaning. Maxi-
mum longevity is 10 years. Male-biased adult sex ratios may
result from the diff erential mortality of the sexes. T. hudsoni-
cus shows a Type III survivorship curve, with more than 60
percent mortality occurring in the fi rst year of life, more
than 80 percent by the second, and more than 90 percent by
the third. Squirrels born in a year of abundant seed crops
may show higher survivorship, but food supplementation
often fails to improve survival. The annual popu lation cycle
is characterized by a peak in density during the summer
following recruitment, and a gradual decline through au-
tumn and winter. Red squirrels are probably prey for snakes,
owls, hawks, and a number of carnivorous mammals. Nu-
merous comprehensive surveys of the parasites of red squir-
rels are available.
general references: Benkman and Siepielski 2004;
Bertolino 2009; G. A. Kemp and Keith 1970; Larsen and Bou-
tin 1994; Layne 1954; Rusch and Reeder 1978; C. C. Smith
1965, 1968, 1970, 1978, 1981; Steele 1998.Tamiasciurus mearnsi (Townsend, 1897)
Mearns’s Squirreldescription: Mearns’s squirrels are gray on the dorsum
and head, with a faint brownish tinge extending from the
pelage behind the ears down the middle of the back. A gray