Thermoregulation and Use of Stored Fat at Low Temperatures
Background
Thermoregulation.—At temperatures below a mammal's Tn, heat loss exceeds Ḣb. To maintain Tb 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 (Müller and Kulzer, 1977; Müller and Rost, 1983) all are able to elevate their metabolic rates by 130% above basal when they are exposed to Ta = 0°C. Procyon cancrivorus responds to 0°C with an increase in metabolic rate of 257% above basal (Scholander et al., 1950b). All animals listed have about the same Tlc and Tb, 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 Ta = 0°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 Ta = 0°C ([Table 5]; [Figure 3]). Each year at the completion of its molt, the raccoon's highly insulative pelt is renewed. This lowers their Tlc by 9°C to 15°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 (Tb > 35°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