Summer
During summer our data suggested that the upper critical temperature (Tuc) was higher than 35°C. The lowest rates of oxygen consumption at Ta = 35°C occurred after 1.5 to 2.5 hours of exposure to that temperature. Prolonged exposure to this temperature in summer did not make animals restless, and their rate of oxygen consumption was very stable throughout each measurement. Body temperature responses at Ta = 35°C were recorded from two males and two females that had implanted radio transmitters. With the exception of one male, Tb's were maintained near 38°C ([Figure 6]). The one exception (a male) maintained its Tb at 39.3°C. At Ta = 35°C, summer males had rates of evaporative water loss that were lower than those of summer females ([Figure 4]). At this temperature, males dissipated 35% ± 6% and females 56% ± 18% of their metabolic heat via evaporative water loss. Thus, at Ta = 35°C, males must have utilized modes of heat transfer other than evaporative cooling (convective and conductive heat transfer) to a greater extent than females.
Winter
Body temperature, evaporative water loss, and metabolic data indicated that, in winter, Tuc was very close to 35°C. In winter, the lowest level of oxygen consumption was recorded during the first hour after the chamber had reached Ta = 35°C. Unlike summer, animals became restless after the first hour at 35°C, at which point their oxygen consumption increased and showed a high degree of variability. Body temperature responses at 35°C were recorded from both females that had implanted radio transmitters. In one case, Tb rose from 37.9°C at the end of the first hour to 40.5°C by the end of the second hour, and as it did not show signs of leveling off, we terminated the experiment. We exposed that same animal to Ta = 35°C one other time during winter. In that instance, its Tb rose to 40.0°C during the first 30 minutes and was maintained at that level for three hours with no apparent distress. The other female elevated its Tb from 37.3°C to 39.0°C during the second hour at Ta = 35°C and maintained its Tb at that level for two hours. Thus, during winter, prolonged exposure to Ta = 35°C stimulated more of an increase in Tb than it did in summer. During winter, both males and females increased evaporative water loss at Ta = 35°C ([Figure 5]) but only to the extent that they dissipated 35% ± 10% of their metabolic heat production. Thus, even in winter, convective and conductive heat transfers were still the most important modes of heat loss at this temperature.
Daily Cycle of Body Temperature
The daily cycle of raccoon Tb's during summer and winter are presented in [Figure 7]. In general, Tb's showed a marked circadian cycle in phase with photoperiod. Tb's rose above 38°C for several hours each night but remained below 38°C during daytime. During summer, with the exception of one female whose record was not typical ([Figure 7]), Tb's rose above 38°C shortly after sunset, whereas in winter Tb's did not rise above 38°C until several hours after sunset. Once Tb was elevated it usually remained so until just before or after sunrise ([Figure 7]). During summer, Tb was above 38°C for 85% or more of the time between sunset and sunrise (87% for the female with the typical body temperature pattern, and 85% and 98% for males), whereas in winter it was elevated for only 47%-78% of the time between sunset and sunrise (47% and 61% for females, and 67% and 78% for males). During night, Tb would oscillate between 38°C and about 39°C, such that two peak values occurred. These peak values presumably corresponded to two periods of heightened nighttime activity. During summer, one of these peaks occurred before and the other after 24:00 hours, whereas in winter both peaks occurred after 24:00 hours. With the exception of one female in winter ([Figure 7]), the lowest Tb of the day for both sexes was near 37°C, and this typically occurred during daytime ([Figure 7]).