_Background_
Thermal conductance is a measure of the ease with which heat is
passively transferred to or from a body through its tissues and pelt.
Within T_{n}, a mammal is able to vary its thermal conductance over a
wide range of values by changing heat transfer characteristics of both
of these layers. Minimum thermal conductance occurs when total heat
transfer through these layers is reduced to its lowest possible rate.
This minimum value, which is the reciprocal of maximum resistance,
occurs, theoretically, but not always practically (see McNab, 1988b),
at the animal's T_{lc} and is best estimated under standard conditions
in a metabolism chamber (McNab, 1980b; Aschoff, 1981). Minimum thermal
conductance scales to body mass (McNab and Morrison, 1963; Herreid and
Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff,
1981). Therefore, to make comparisons between species of various sizes,
we scaled out body mass by expressing C_{mw} as the ratio of measured
to predicted values (C_{mwr}; Table 7). These ratios were used to make
comparisons of heat-transfer characteristics between species that
occupy different habitats or climates.
_Effect of Molt on Thermal Conductance_
In summer, T_{lc}'s of male and female _Procyon lotor_ (Figure 2) were
very similar to those of other procyonids (22 deg.C-26 deg.C; Table 7). In
winter, T_{lc} of both sexes shifted downward to 11 deg.C (Figure 3). This
seasonal shift in T_{lc} occurred as the result of a seasonal change
in minimum thermal conductance (Table 3). For many northern mammals, a
seasonal change in thermal conductance is partly mediated via cyclic
changes in the insulative quality of their pelt (Scholander et al.,
1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165).
_Procyon lotor_ begins to shed its heavy winter coat about the time
its young are born. Molt progresses through summer and by late August
the new coat is complete (Stuewer, 1942). During its summer molt,
_Procyon lotor_'s C_{mw} increased by about 49% over the value for
female raccoons in winter (Table 3). In summer, therefore, it had the
highest mass specific C_{mw} of those procyonids considered
(C_{mwr} = 1.77 and 1.79; Table 7). An increase in thermal conductance
facilitates passive heat loss for temperate and arctic species, and
this serves as an important thermoregulatory adaptation during warm
summer months (Scholander et al., 1950c; Irving et al., 1955;
Hart, 1956, 1957; Irving, 1972:165). This adaptation is particularly
important to those temperate- and arctic-zone species (including
raccoons) whose [.H]_{b}'s do not decrease during summer (Irving et al.,
1955). From August on, the fur of _Procyon lotor_ becomes increasingly
longer and heavier, with peak, or prime, condition occurring in late
fall and early winter (Stuewer, 1942). Minimum conductance of our
captive raccoons was lowest in winter (C_{mwr} = 1.15) when their pelts
were in prime condition (Tables 3, 7). Because "primeness" of raccoon
pelts varies geographically, thicker pelts being associated with colder
climates (Goldman, 1950:21; Whitney and Underwood, 1952:24-41), the
degree of seasonal change in C_{mw} must also vary geographically.
The only other procyonid for which a seasonal molt has been described
is _Bassariscus astutus_. Molt in this species extends from late summer
to late fall (Toweill and Toweill, 1978). How molt effects thermal
conductance in _Bassariscus astutus_ is not known because metabolic
data for this species (Table 7) apparently were collected only when
their pelts were in prime condition (Chevalier, 1985).
Goldman (1950:20) reports that _Procyon cancrivorus_ does not have a
seasonal molt. Like other tropical procyonids, _Procyon cancrivorus_
lives in an environment that has the following characteristics: high
even temperatures throughout the year (1 deg.C-13 deg.C difference in monthly
mean temperature), a greater range in temperature between day and
night than in mean monthly temperature throughout the year, uniform
lengths of day and night, seasonal variation in rainfall, and lowest
temperatures during the rainy season(s) (Kendeigh, 1961:340). In such
a stable environment there would be no advantage to a sharply defined
seasonal molt cycle that could place an animal in thermoregulatory
jeopardy by increasing its thermal conductance. This would be
particularly true for animals like tropical procyonids that have lower
than predicted [.H]_{b}'s but that maintain typical eutherian body
temperatures (Table 7). Consequently, molt in all tropical procyonids
may either be prolonged or continuous. This is a feature of their
biology that needs to be examined in more detail.
_Comparison of Thermal Conductances_
_Procyon lotor_ VERSUS TROPICAL PROCYONIDS.--C_{mwr} for _Procyon
lotor_ in winter was 1.15, which is similar to the values for _Potos
flavus_ and _Procyon cancrivorus_, 1.02 and 1.25, respectively
(Table 7). These two tropical species, therefore, have C_{mw}'s that
are similar on a mass specific basis to the value for _Procyon lotor_
in winter. However, at their T_{lc}'s, the thermal gradient sustained
by these tropical animals is only about 11 deg.C, whereas for _Procyon
lotor_ in winter it was 26.5 deg.C. Examination of Eq. 4 with respect to
these thermal gradients suggests that tropical procyonids achieve such
low C_{mw}'s by virtue of their lower than predicted [.H]_{b}'s rather
than by having pelts that are exceptionally good insulators. In fact,
the insulation afforded by the pelts of these tropical procyonids is
about the same as that of the 50 g arctic lemming, _Dicrostonyx
groenlandicus rubricatus_, whose coat has an insulative value that is
about half that of the hare, _Lepus americanus_, red fox, _Vulpes fulva
alascensis_, and pine martin, _Martes americana_, animals comparable in
size to these procyonids (Scholander et al., 1950a). Therefore, pelts
of these tropical procyonids do not have the same insulative value as
the prime winter coat of _Procyon lotor_.
_Nasua narica_ and _Nasua nasua_ have tropical and subtropical
distributions and they are the only procyonids that are diurnal
(Kaufmann, 1962:103-105, 1982, 1987). Because they are active during
the day they experience a more extreme thermal environment (higher
T_{a}'s and solar radiation) than their nocturnal cousins. Values of
C_{mwr} for _Nasua narica_ (1.45 and 1.55) and _Nasua nasua_ (1.24 and
1.65) are higher than those for _Procyon cancrivorus_ or _Potos flavus_
(Table 7). Thus, these coatis have higher mass specific C_{mw}'s than
their nocturnal tropical cousins. A high C_{mw} reduces the cost of
thermoregulation in hot environments because it increases an animal's
ability to lose excess heat passively. The higher C_{mw}'s of these
coatis serve as an adaptation that contributes to the success of their
diurnal life style as well as their ability to expand their habitat use
to areas with less thermal stability, such as oak and pine woodlands
and deserts.
_Bassariscus astutus._--This species has the lowest mass specific
C_{mw} of these procyonids (C_{mwr} = 0.85; Table 7), which indicates
that its pelt has a greater insulative value than the coats of _Potos
flavus_, _Procyon cancrivorus_, _Nasua nasua_, or _Nasua narica_. This,
coupled with a lower than predicted [.H]_{b}, allows _Bassariscus
astutus_ to maintain T_{b} with less energy expenditure than is
possible for any other procyonid of comparable size; and this
combination of adaptations provides _Bassariscus astutus_ with a
distinct energy advantage in environments that have low productivity
(Wang et al., 1973). The evolution of a pelt that provides better
insulation must be considered an important contributing factor for
the spread of this species into desert regions of the western United
States.