Figure 7.—Relationship between body temperature and time of day at various months of the year: captive females, open circles; captive males, closed circles. Vertical cross-hatched areas represent civil twilight.
The regression line for Procyon lotor in winter ([Table 5]) extrapolates to zero metabolism at 35.2°C, which is below normal Tb ([Figures 6],[ 7]). This suggests that not all raccoons measured in winter minimized thermoregulatory metabolism or conductances at Ta's below Tlc (Scholander et al., 1950b; McNab, 1980b). To assess this possibility, data for these animals were divided into three groups: (A) females with radio transmitters, (B) females without radio transmitters, and (C) males ([Table 6]). Regression equations of metabolism below Tlc were derived for each group, and based on extrapolated Tb's at zero metabolism, only the two females with implanted radio transmitters (group A) minimized thermoregulatory metabolism and conductance. Had animals in groups B and C also minimized their thermal conductances, while retaining their measured metabolic rates, their rates of heat production would have been disproportionately higher than their rates of heat loss. Equation 4 predicts that under these conditions their body temperatures would have been elevated to 42.0°C and 40.4°C, respectively. Thus, in order to avoid such a large increase in body temperature, animals in groups B and C increased their thermal conductances in preference to lowering their metabolic rates. The regression equation of thermoregulatory metabolism for all winter animals ([Table 5]), therefore, overestimates minimum metabolic cost of temperature regulation below Tlc, and its slope underestimates Cmw. Consequently, the best estimate of Cmw for Procyon lotor in winter is the value calculated for group A animals with [Eq. 4] (0.0172 mL O2·g-1·h-1·°C-1; [Table 3]), and the minimum cost of thermoregulatory metabolism at any Ta below Tlc is best estimated by substituting this value into [Eq. 4] and solving for Ḣr.
Thermoregulation at High Temperatures
Body Temperature
In both summer and winter, Tb's increased during metabolic measurements at Ta's above Tlc ([Figure 6]). This response also was seen during metabolic measurements conducted on other procyonids (Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, 1983; Chevalier, 1985).