Part 5

TABLE 3.--Minimum wet and dry thermal conductances (mL O{2}·g^{-1}·h^{-1}·°C^{-1}) of Procyon lotor_ in summer and winter. Means of values were calculated from equations 3 and 4 (s.d. = standard deviation and n = number of observations).

----------------------+---------------------------------------- | Thermal conductance Season and sex |---------------------------------------- | Wet ±s.d. (n) Dry ±s.d. (n) ----------------------+---------------------------------------- Summer | Captive, both sexes | 0.0256 ±0.0028 (18) 0.0246 ±0.0019 (12) Winter | Captive, female | 0.0172 ±0.0023 (10) 0.0161 ±0.0027 (6) ----------------------+----------------------------------------

C{mw} was calculated for each season from metabolic measurements made at all air temperatures below T{lc} (Table 3). Because evaporative water loss was not measured at temperatures below freezing, C{md} was calculated only from metabolic determinations made at air temperatures between T{lc} and 0°C. There was no difference between males and females in summer for either C{mw} or C{md} (mL O{2}·g^{-1}·h^{-1}·°C^{-1}). Data for each sex were combined to give a summer average of 0.0256 ±0.0028 for C{mw}, and 0.0246 ±0.0019 for C{md} (Table 3). These summer conductances were 49% higher (p<0.005) than those calculated for winter females (0.0172 ±0.0023, and 0.0161 ±0.0027 for C{mw} and C{md}, respectively; Table 3). C{mw} and C{md} were not different from each other in either summer or winter, which indicated that in both seasons evaporative water loss contributed very little to heat dissipation at temperatures below T{n}. Comparisons of thermal conductances calculated on the basis of metabolic body size (Mellen, 1963) gave the same results.

EVAPORATIVE WATER LOSS

Evaporative water loss increased as chamber temperature increased in both summer and winter (Figures 4, 5). In summer, the pattern of increase was different for females and males. Polynomial regressions for trapped and captive males produced equations that describe a concave relationship between T{a} and evaporative water loss, whereas the equation for females describes a sigmoid curve (Table 4; Figure 4). For females, water loss increased rapidly at temperatures above 25°C (Figure 4). The intercepts and coefficients of the X, X², and X³ terms of the polynomial regression equations (Table 4) were compared (t_-tests) to determine if they differed from each other. The coefficients in the equation for trapped males differed from those for captive females in the X² (p<0.05) and X³ (p<0.025) terms. The intercept and coefficients of the equation for captive males, however, were not different from those for either captive females or trapped males. Although this lack of difference is understandable in the case of trapped males, where the shape of the two curves is similar (concave), it is not so clear for the sigmoid curve of captive females (Figure 4). Perhaps the lack of difference in this case is simply due to the small number of observations available for captive males (n = 10; Table 4). Nonetheless, in summer at 35°C, both captive and trapped males relied less on evaporative cooling than did captive females (Figure 4).

In winter, males and females had similar rates of evaporative water loss across the full range of temperatures tested (Figure 5). Therefore, data for both sexes were combined. The intercept and coefficients of this equation (Table 4) did not differ from those for summer females, but they did differ from those in the regression for trapped males in the X² (p<0.05) and X³ (p<0.025) terms. As was the case for females in summer, rates of water loss for winter animals increased most rapidly at temperatures above 25°C (Figure 5).

TABLE 4.--Polynomial regression equations describing evaporative water loss (mg·g^{-1}·h^{-1}) of Procyon lotor in summer and winter (X = chamber temperature (°C), Y = evaporative water loss, n = number of observations, R² = coefficient of determination, and SEE = standard error of estimate).

--------------+-------------------------------------------------------- Season and sex| Equation (n) R² --------------+-------------------------------------------------------- Summer | Trapped male |Y = 0.1899 + 0.0114·X + 0.0011·X² - 0.00002·X³ (32) 0.86 SEE | 0.0885 0.0223 0.0015 0.00003 Captive male |Y = 0.2174 + 0.0192·X + 0.0009·X² - 0.00003·X³ (10) 0.73 SEE | 0.3983 0.0834 0.0048 0.00008 Captive | female |Y = 0.0127 + 0.0943·X - 0.0060·X² + 0.00013·X³ (31) 0.64 SEE | 0.2218 0.0547 0.0036 0.00006 Winter | Captive, | both sexes |Y = 0.1550 + 0.0426·X - 0.0025·X² + 0.00006·X³ (57) 0.80 SEE | 0.0734 0.0192 0.0013 0.00002 --------------+--------------------------------------------------------

THERMOREGULATION AT LOW TEMPERATURES

Body Temperature

Body temperatures in Figure 6 are those recorded during metabolic measurements from animals equipped with surgically implanted, temperature-sensitive radio transmitters. Each point was recorded during the lowest level of oxygen consumption at each T{a}. In both summer and winter, T{b}'s were lowest during metabolic measurements at T{a}'s around T{lc}. At T{a}'s below T{lc}, T{b}'s increased (Figure 6), which is an unusual response. Under similar conditions, other procyonids either maintain a nearly constant T{b} or allow it to fall slightly (Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, 1983; Chevalier, 1985). For our raccoons, confinement in the metabolism chamber at low temperatures must have stimulated a greater than necessary increase in metabolic rate such that heat production exceeded heat loss, which caused T_{b} to become elevated.

TABLE 5.--Regression equations describing oxygen consumption (mL O{2}·g^{-1}·h^{-1}) of Procyon lotor_ at temperatures below their lower critical temperature (I = x-intercept (°C), n = number of observations, R² = coefficient of determination, SEE = standard error of estimate for the y-intercept (a) and slope (b), X = chamber temperature (°C), and Y = oxygen consumption).

----------------+------------------------------------------------------ Season | SEE and sex | ----------- | Equation (n) R² a b I ----------------+------------------------------------------------------ Summer | Trapped male | Y = 1.09 - 0.0281·X (30) 0.64 0.0353 0.0040 38.8 Captive male | Y = 0.97 - 0.0258·X (12) 0.91 0.0235 0.0025 37.6 Captive female| Y = 1.04 - 0.0251·X (29) 0.78 0.0288 0.0026 41.1 Winter | Captive, | both sexes | Y = 0.68 - 0.0193·X (36) 0.68 0.0157 0.0023 35.2 ----------------+------------------------------------------------------

Summer

During summer, T{lc} for male raccoons was 20°C, whereas for females it was 25°C (Figure 2). Regression equations calculated to describe oxygen consumption at T{a}'s below T{lc} are presented in Table 5. For three groups of summer animals, slopes of regressions are identical. This indicates that minimum conductances of these three groups were equivalent. Intercepts of these equations are different, which suggests a difference in metabolic cost of thermoregulation between these groups (Figure 2); captive males had a lower intercept than either trapped males (p<0.005) or captive females (p<0.05), but there was no difference in intercepts of captive females and trapped males. These regression equations, therefore, also were derived using values of oxygen consumption expressed in terms of metabolic body mass (Mellen, 1963). Relationships between intercepts of these equations are different than those for regressions in Table 5. Intercept for females was intermediate to, and not different from, those of the two groups of males. However, captive males still had a lower intercept than trapped males (p<0.025). Thus, in summer, thermoregulatory metabolism was less expensive for captive than for trapped males, and in spite of a 5°C difference in their T{lc}'s (Figure 2), captive males and females had similar thermoregulatory costs.

Regression lines for three groups of animals in summer extrapolate to zero metabolism at values equivalent to, or greater than, normal T{b}; 38.8°C for trapped males, 37.6°C for captive males, and 41.1°C for captive females (Table 5). Thus, all three groups had minimized thermal conductance at T{a}'s below T{lc} (Scholander et al., 1950b; McNab, 1980b). Minimum wet thermal conductance calculated for raccoons in summer with Eq. 4 (Table 3) is numerically similar to these "slope" values (Table 5), and it was, therefore, considered to be the best estimate of C{mw} for Procyon lotor during that season (0.0256 mL O_{2}·g^{-1}·h^{-1}·°C^{-1}).

Winter

During winter T_{lc} for both sexes decreased to 11°C (Figure 3). Regression equations of thermoregulatory metabolism for males and females in winter are not different from each other in either slope or intercept. These data, therefore, were combined into a single equation (Table 5). Slope and intercept of this equation are both lower (p<0.005 and p<0.05, respectively) than those for summer animals (Table 5). Identical results were obtained from comparisons using regressions derived from oxygen consumption expressed in terms of metabolic body mass (Mellen, 1963). Thermoregulatory costs at any temperature below 20°C were lower for winter than summer animals (Figures 2, 3).

TABLE 6.--Regression equations describing oxygen consumption (mL O{2}·g^{-1}·h^{-1}) of Procyon lotor_ at temperatures below their lower critical temperature in winter (A = females with radio transmitters, B = females without radio transmitters, C = males, I = x-intercept (°C), n = number of observations, R² = coefficient of determination, X = chamber temperature (°C), and Y = oxygen consumption).

-----+-------------------------------------- Group| Equation (n) R² I -----+-------------------------------------- A | Y = 0.63 - 0.0158·X (10) 0.66 40.1 B | Y = 0.72 - 0.0226·X (11) 0.71 32.1 C | Y = 0.69 - 0.0200·X (15) 0.79 34.7 -----+--------------------------------------

The regression line for Procyon lotor in winter (Table 5) extrapolates to zero metabolism at 35.2°C, which is below normal T{b} (Figures 6, 7). This suggests that not all raccoons measured in winter minimized thermoregulatory metabolism or conductances at T{a}'s below T{lc} (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 T{lc} were derived for each group, and based on extrapolated T{b}'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 T{lc}, and its slope underestimates C{mw}. Consequently, the best estimate of C{mw} for Procyon lotor in winter is the value calculated for group A animals with Eq. 4 (0.0172 mL O{2}·g^{-1}·h^{-1}·°C^{-1}; Table 3), and the minimum cost of thermoregulatory metabolism at any T{a} below T{lc} is best estimated by substituting this value into Eq. 4 and solving for [.H]{r}.

THERMOREGULATION AT HIGH TEMPERATURES

Body Temperature

In both summer and winter, T{b}'s increased during metabolic measurements at T{a}'s above T_{lc} (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).

Summer

During summer our data suggested that the upper critical temperature (T{uc}) was higher than 35°C. The lowest rates of oxygen consumption at T{a} = 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 T{a} = 35°C were recorded from two males and two females that had implanted radio transmitters. With the exception of one male, T{b}'s were maintained near 38°C (Figure 6). The one exception (a male) maintained its T{b} at 39.3°C. At T{a} = 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 T_{a} = 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, T{uc} 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 T{a} = 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, T{b} 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 T{a} = 35°C one other time during winter. In that instance, its T{b} 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 T{b} from 37.3°C to 39.0°C during the second hour at T{a} = 35°C and maintained its T{b} at that level for two hours. Thus, during winter, prolonged exposure to T{a} = 35°C stimulated more of an increase in T{b} than it did in summer. During winter, both males and females increased evaporative water loss at T_{a} = 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.