_Background_
THERMOREGULATION.--At temperatures below a mammal's T_{n}, heat loss
exceeds [.H]_{b}. To maintain T_{b} under these conditions, metabolic
rate must be increased (Eq. 4). _Procyon lotor_ in summer during its
annual molt (Table 5; Figure 2), _Bassariscus astutus_ (Chevalier,
1985), _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al.,
in prep.), _Nasua narica_ (Scholander et al., 1950b; Mugaas et al.,
in prep.), and _Potos flavus_ (Mueller and Kulzer, 1977; Mueller and Rost,
1983) all are able to elevate their metabolic rates by 130% above basal
when they are exposed to T_{a} = 0 deg.C. _Procyon cancrivorus_ responds to
0 deg.C with an increase in metabolic rate of 257% above basal (Scholander
et al., 1950b). All animals listed have about the same T_{lc} and T_{b},
so the temperature differential producing this response is about the
same for each species. Metabolic ability to defend body temperature
against low ambient temperatures, therefore, is well developed in these
procyonids. Such large increases in metabolic rate are energetically
expensive, and if these animals were routinely exposed to T_{a} = 0 deg.C,
it would be difficult for them to acquire enough food each day to
maintain endothermy. Raccoons in winter pelage, however, need only
elevate their metabolic rate by 47% above basal to maintain endothermy
at T_{a} = 0 deg.C (Table 5; Figure 3). Each year at the completion of its
molt, the raccoon's highly insulative pelt is renewed. This lowers
their T_{lc} by 9 deg.C to 15 deg.C below that measured for them in summer
(Figure 3) and decreases their cost of thermoregulation at low
temperatures. The increased insulative capacity of their pelt is one
of the primary adaptations that has allowed _Procyon lotor_ to extend
its distribution into cold climates.
STORED FAT.--Cyclic fattening is an integral and important part of a
raccoon's annual cycle (Mugaas and Seidensticker, ms); however, it has
not been reported for other procyonids. During winter in parts of the
United States and Canada, raccoons are confined to their dens for
variable periods of time (days to months) depending on the severity of
the weather (Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116;
Sharp and Sharp, 1956; Mech et al., 1968; Schneider et al., 1971).
During this confinement, they do not hibernate but rather enter a state
of "dormancy" and become inactive. While dormant they remain endothermic
(T_{b} > 35 deg.C; Thorkelson, 1972:87-90) and derive most of their energy
requirement from fat reserves accumulated during fall. The rate at which
fat stores are consumed during winter dormancy depends on the
thermoregulatory requirement imposed on them by local weather
conditions, the insulative quality of their pelt, and any advantage
they may gain by seeking shelter in a den.
_Thermal Model of the Raccoon and Its Den_
Heat transfer between an animal and its environment is a function of
the interaction of its body temperature and thermal conductance with
various environmental variables (air temperature, wind speed, vapor
pressure, and thermal radiation). When a raccoon is outside its den,
its thermal conductance (C_{mw}) is the only barrier to heat transfer
with the external environment. However, when it enters a tree den, a
raccoon imposes two other thermal barriers between itself and the
external environment: (1) conductance of the air space between its fur
and the den's walls (C_{a}) and (2) conductance of the den's walls
(C_{d}; Thorkelson, 1972:59-63; Thorkelson and Maxwell, 1974).
Thorkelson and Maxwell (1974) modeled heat transfer of a simulated
raccoon (a water-filled aluminum cylinder equipped with a heater and
covered with a raccoon pelt) in a closed tree den. In their system,
65% of resistance to heat flux was attributable to the pelt, whereas
the remainder (35%) was due to C_{a} and C_{d}. Because resistance is
the inverse of conductance, and resistances for the raccoon and its
den are arranged in series, we can estimate total conductance (C_{t})
of this system with Eq. 7.