Part 3

Values of oxygen consumption, evaporative water loss, and body temperature were plotted as a function of chamber air temperature. Linear regressions of oxygen consumption at temperatures below the thermoneutral zone (T_{n}), and evaporative water loss at temperatures above freezing, were determined with the SAS (1982) GLM procedure. Lower critical temperature (T_{lc}) was determined graphically from intersection of the line representing [.H]_{b} and the regression line representing oxygen consumption below T_{n}. Slopes and intercepts of regression lines, as well as other mean values, were compared with _t_-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). Unless indicated otherwise, data are expressed as mean +- standard deviation (s.d.).

ESTIMATING INTRINSIC RATE OF NATURAL INCREASE

We employed the method first described by Cole (1954) to calculate r_{max}:

1 = e^{-r_{max}} + b.e^{-r_{max}(a)} - b.e^{r_{max}(n+1)} Eq. 2

where a is potential age of females first producing young, b is potential annual birth rate of female young, and n is potential age of females producing their final young. After life-history data were substituted into Eq. 2, r_{max} was determined by trial and error substitution (Hennemann, 1983).

Because r_{max} represents the genetically fixed, physiologically determined maximum possible rate of increase, data on earliest possible age of female reproduction, highest possible birth rate of female young, and longest possible female reproductive life span were used for a, b, and n, respectively. Calculated values, therefore, represent physiologically possible, not ecologically possible, intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984; McNab, 1984b). Values of n were derived from longevity records for captive animals, and as these were all large values of similar duration (14-16 years), they had very little effect on r_{max}. All species considered have one litter per year, and because their sex ratios at birth are about 50:50, variation in b was due to differences in litter size. Therefore, age of first reproduction and litter size had the greatest effect on r_{max}. Intrinsic rate of increase scales to body mass (Fenchel, 1974), and we removed this effect by comparing each calculated r_{max} with the value expected (r_{maxe}) on the basis of body mass (Hennemann, 1983).

COMPARISON OF ADAPTIVE UNITS

Dimensionless numbers for each of the four variables used in calculating composite scores were derived as follows. Ratios of measured to predicted values were used for basal metabolism (H_{br}) and minimum wet thermal conductance (C_{mwr}). Thermoregulatory ability at low temperatures is closely related to the ratio H_{br}/C_{mwr} (McNab, 1966). This ratio was used, therefore, to gauge each species' cold tolerance. For D_{d} we used the ratio of food categories actually used by a species to the total number of food categories taken by all species tested (D_{dr}). The ratio of calculated to expected intrinsic rates of natural increase was used to derive r_{maxr}. Composite scores were calculated as

Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3 Eq. 3

The correlation between number of climates these species occupy and their composite scores was tested by linear regression.

$Results$

BODY MASS

According to monthly live-trapping records, the body mass of free-ranging female raccoons increased from 3.6 +-0.6 kg during summer to 5.6 +-0.8 kg in early winter, and the mass of free-ranging males increased from 4.0 +-0.5 to 6.7 +-0.9 kg during the same interval. These seasonal changes in body mass were due to fluctuations in the amount of body fat and represent a mechanism for storing energy during fall for use in winter. In summer, captive and trapped male and captive female raccoons had the same body mass (4.73 +-0.61, 4.41 +-0.70, and 4.67 +-0.88 kg, respectively, Table 2). Mass of captive females did not change between seasons, whereas captive males were heavier in winter than summer (p<0.005; Table 2). This seasonal change in mass of our captive males was of a much smaller magnitude (0.6 kg) than that observed for wild males (2.7 kg). During winter, captive males (5.34 +-1.39 kg) were heavier than captive females (4.49 +-0.98 kg; p<0.005; Table 2). Thus, our captive animals maintained a body mass throughout the year that was intermediate to the range of values found for wild raccoons in the same area.

TABLE 2.--Body mass in kg and basal metabolism (mL O_{2}.kg^{-0.75}.h^{-1}) of _Procyon lotor_ in summer and winter (s.d. = standard deviation and n = number of observations).

+----------------------------------------------------- Season and sex | Body mass, +-s.d., (n) Basal metabolism, +-s.d., (n) ----------------+----------------------------------------------------- Summer | Trapped male | 4.41 +-0.70 (52) 780 +-112 (20) Captive male | 4.73 +-0.61 (22) 680 +-102 (8) Captive female| 4.67 +-0.88 (41) 618 +- 92 (13) Winter | Captive male | 5.34 +-1.39 (31) 704 +- 81 (19) Captive female| 4.49 +-0.98 (42) 667 +-139 (25) ----------------+-----------------------------------------------------

BASAL METABOLIC RATE

Within thermoneutrality, [.H]_{b} (mL O_{2}.g^{-1}.h^{-1}) was 0.54 +-0.09 for trapped males in summer, 0.46 +-0.07 for captive males in summer, 0.42 +-0.07 for captive females in summer, 0.47 +-0.06 for captive males in winter, and 0.46 +-0.10 for captive females in winter (Figures 2, 3). Ratios of these measured values to those predicted by the Kleiber (1932, 1961:206) equation are 1.28, 1.12, 1.02, 1.17, and 1.09, respectively. To minimize the effect of body size (Mellen, 1963) and to facilitate comparisons between sexes and seasons and between captive and trapped animals, basal metabolism also was calculated as a function of metabolic body size (mL O_{2}.kg^{-0.75}.h^{-1}; Table 2). Based on this analysis, trapped summer males had a higher basal metabolism than captive males (p<0.025) or females (p<0.005) in either season (Table 2). There was no difference in basal metabolism between captive males and females in either summer or winter, and there was no seasonal difference in their basal metabolic rates (Table 2).

MINIMUM THERMAL CONDUCTANCE

Minimum wet and dry thermal conductances were calculated using Eqs. 4 and 5

C_{mw} = [.H]_{r} / (T_{b} - T_{a}) Eq. 4

C_{md} = ([.H]_{r} - [.E]_{eq}) / (T_{b} - T_{a}) Eq. 5

where C_{mw} is wet and C_{md} is dry conductance (mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}); [.H]_{r} is the lowest resting metabolic rate measured at each temperature (mL O_{2}.g^{-1}.h^{-1}); [.E]_{eq} is oxygen equivalent for heat lost by evaporation [[.E]_{eq} = mL O_{2}.g^{-1}.h^{-1} = [.E].[lambda]/[gamma], where [.E] is evaporative water loss (mg.g^{-1}.h^{-1}), [lambda] is heat of vaporization for water (2.43 J/mg), and [gamma] is heat equivalent for oxygen (20.097 J/mL)]; T_{b} is body temperature ( deg.C); and T_{a} is chamber air temperature ( deg.C). Only data from animals equipped with temperature-sensitive radio transmitters were used for these calculations.

TABLE 3.--Minimum wet and dry thermal conductances (mL O_{2}.g^{-1}.h^{-1}. deg.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 deg.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}. deg.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 deg.C (Figure 4). The intercepts and coefficients of the X, X squared, and X cubed 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 X squared (p<0.05) and X cubed (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 deg.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 X squared (p<0.05) and X cubed (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 deg.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 ( deg.C), Y = evaporative water loss, n = number of observations, R squared = coefficient of determination, and SEE = standard error of estimate).

+-------------------------------------------------------- Season and sex| Equation (n) R squared --------------+-------------------------------------------------------- Summer | Trapped male |Y = 0.1899 + 0.0114.X + 0.0011.X squared - 0.00002.X cubed (32) 0.86 SEE | 0.0885 0.0223 0.0015 0.00003 Captive male |Y = 0.2174 + 0.0192.X + 0.0009.X squared - 0.00003.X cubed (10) 0.73 SEE | 0.3983 0.0834 0.0048 0.00008 Captive | female |Y = 0.0127 + 0.0943.X - 0.0060.X squared + 0.00013.X cubed (31) 0.64 SEE | 0.2218 0.0547 0.0036 0.00006 Winter | Captive, | both sexes |Y = 0.1550 + 0.0426.X - 0.0025.X squared + 0.00006.X cubed (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 (Mueller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Mueller 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 ( deg.C), n = number of observations, R squared = coefficient of determination, SEE = standard error of estimate for the y-intercept (a) and slope (b), X = chamber temperature ( deg.C), and Y = oxygen consumption).

+------------------------------------------------------ Season | SEE and sex | ----------- | Equation (n) R squared 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 deg.C, whereas for females it was 25 deg.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 deg.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 deg.C for trapped males, 37.6 deg.C for captive males, and 41.1 deg.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}. deg.C^{-1}).

_Winter_

During winter T_{lc} for both sexes decreased to 11 deg.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 deg.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 ( deg.C), n = number of observations, R squared = coefficient of determination, X = chamber temperature ( deg.C), and Y = oxygen consumption).

+-------------------------------------- Group| Equation (n) R squared 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 deg.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 deg.C and 40.4 deg.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}. deg.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 (Mueller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Mueller and Rost, 1983; Chevalier, 1985).

_Summer_

During summer our data suggested that the upper critical temperature (T_{uc}) was higher than 35 deg.C. The lowest rates of oxygen consumption at T_{a} = 35 deg.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 deg.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 deg.C (Figure 6). The one exception (a male) maintained its T_{b} at 39.3 deg.C. At T_{a} = 35 deg.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 deg.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_