_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.
[Illustration: FIGURE 6.--Relationship between body temperature and
chamber air temperature in summer (panel A), and winter (panel B):
captive females, open circles and solid lines; captive males, solid
circles and dashed lines. Solid vertical lines represent lower
critical temperatures.]
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
-----+--------------------------------------
[Illustration: 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 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}.