Minimum thermal conductance C_{mw} for raccoons in winter was
0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1} (Table 3). Based on Thorkelson
and Maxwell's (1974) model we let 1/C_{mw} = 0.65(1/C_{t}) =
1/0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, and 1/C_{a} + 1/C_{d} =
0.35(1/C_{t}). Substituting these values into Eq. 7 and solving
for C_{t} yields 0.0112 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, a value
that is 35% lower than that of the animal alone. Substituting
this value and the value for basal metabolism of winter raccoons
(0.47 mL O_{2}.g^{-1}.h^{-1}; Table 7) into Eq. 4 and solving for
(T_{b} - T_{a}) yields a new temperature differential of 42 deg.C.
Therefore, by using tree dens, raccoons in north central Virginia,
with T_{b} = 37 deg.C (Figure 7), could effectively reduce their T_{lc}
from 11 deg.C to -5 deg.C and markedly reduce their metabolic cost of
thermoregulation.
_Metabolic Advantage of the Den_
Given prevailing winter temperatures in north central Virginia (see
"Materials and Methods"), adult raccoons in that area should be able
to sustain endothermy most of the time they are in their dens by simply
maintaining [.H]_{b}. Depending on the mass of their stored fat, they
could remain in their dens for several weeks without eating (Mugaas and
Seidensticker, ms). The thermal advantage of a den could be further
enhanced during colder temperatures if two or more raccoons occupied it
at the same time and huddled together, and/or if these animals could
reduce C_{mw} even more by lowering T_{b} and cooling their extremities.
Although we do not have any data to verify the second mechanism, there
are many accounts in natural history literature that document raccoons
occupying dens together (Lotze and Anderson, 1979). This habit could be
particularly important for the young of the year and may be one reason
why they often continue to den with their mothers during winter (Lotze
and Anderson, 1979; Seidensticker et al., 1988). Raccoons that live in
colder climates, such as Minnesota, undoubtedly obtain the same
advantage from a den as Virginia animals, but because of their greater
body mass, longer fur, and potentially lower C_{mw}, T_{lc} of a
Minnesota raccoon in a den could be even lower than what we calculated
for Virginia raccoons. Therefore, when they are in their dens, raccoons
living in very cold climates also may be able to maintain homeothermy
with a basal level of metabolism.