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 (Cmw) 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 (Ca) and (2) conductance of the den's walls (Cd; 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 Ca and Cd. Because resistance is the inverse of conductance, and resistances for the raccoon and its den are arranged in series, we can estimate total conductance (Ct) of this system with [Eq. 7].
| 1/Ct = 1/Cmw + 1/Ca + 1/Cd | Eq. 7 |
Minimum thermal conductance Cmw for raccoons in winter was 0.0172 mL O2·g-1·h-1·°C-1 ([Table 3]). Based on Thorkelson and Maxwell's (1974) model we let 1/Cmw = 0.65(1/Ct) = 1/0.0172 mL O2·g-1·h-1·°C-1, and 1/Ca + 1/Cd = 0.35(1/Ct). Substituting these values into [Eq. 7] and solving for Ct yields 0.0112 mL O2·g-1·h-1·°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 O2·g-1·h-1; [Table 7]) into [Eq. 4] and solving for (Tb - Ta) yields a new temperature differential of 42°C. Therefore, by using tree dens, raccoons in north central Virginia, with Tb = 37°C ([Figure 7]), could effectively reduce their Tlc from 11°C to -5°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 Ḣb. 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 Cmw even more by lowering Tb 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 Cmw, Tlc 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.