_Background_
In hot environments mammals depend on behavior to minimize their
thermal load (escape to shaded or cooler microclimates, use posture
and orientation to wind and sun, restrict activity, become nocturnal,
etc.) and on evaporative water loss to rid themselves of excess heat.
With regard to evaporative heat loss, Calder and King (1974:326)
arbitrarily subdivided the response to various T_{a}'s as follows:
"(1) cool temperatures at which water loss should be minimized, both
to reduce heat loss and as an adaptation to terrestriality; (2) an
intermediate temperature range wherein evaporation is gradually
increased as dry heat losses are proportionately reduced with smaller
thermal gradients; and (3) warm to hot temperatures at which
evaporation must be actively increased to dispose of metabolic and
exogenous heat loads." Some mammals are able to thermoregulate very
well at high ambient temperatures via panting or sweating, whereas
others have a very limited capacity. Hence, there is no general
approach to calculating evaporative water loss under these conditions
(Campbell, 1977:85). However, the ratio of evaporative heat lost to
metabolic heat produced can be used to quantify a species' capacity
for evaporative cooling and to make comparisons between species.
_Comparison of Procyonid Responses to Heat Stress_
_Potos flavus._--This species lives in Neotropical forests of Central
and South America. It is nocturnal, arboreal in habit, and appears to
be the most heat-sensitive of these procyonids. Its T_{uc} is at 30 deg.C
to 33 deg.C (Table 7; Mueller and Kulzer, 1977; Mueller and Rost, 1983). It
begins to pant at about 30 deg.C, but its efforts at evaporative cooling
are very ineffective. At 33 deg.C _Potos flavus_ can dissipate 33% of its
metabolic heat via evaporative water loss, but at 35 deg.C the efficiency
of this mechanism falls to 20% (Mueller and Rost, 1983). Consequently,
when exposed to T_{a}'s above 33 deg.C, any kind of excitement causes its
T_{b} to rise rapidly in an uncontrolled manner (Mueller and Kulzer,
1977; Mueller and Rost, 1983). These animals rely on their nocturnal
and arboreal habits to keep them out of situations that could lead to
hyperthermia (Mueller and Kulzer, 1977; Mueller and Rost, 1983).
_Nasua nasua_ and _Nasua narica_.--_Nasua nasua_ is abundant in
tropical and subtropical South America, whereas _Nasua narica_ occupies
the same climates in North America from southern Arizona and New Mexico
south through Panama and on into Colombia and Ecuador (Hall and Kelson,
1959:892; Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis
are diurnal and forage primarily on the ground (Kaufmann, 1962:185-188,
1987; Poglayen-Neuwall, 1975; Nowak and Paradiso, 1983:982),
consequently they are exposed to a more severe thermal environment
while active (higher T_{a}'s and solar radiation) than are nocturnal
procyonids. Both coatis are more heat-tolerant than _Potos flavus_;
their T_{uc}'s are higher (33 deg.C-35 deg.C; Table 7), they can tolerate
T_{a}'s of 35 deg.C without raising their T_{b}'s (Chevillard-Hugot et al.,
1980; Mugaas et al., in prep.), and they have a greater capacity for
evaporative cooling than _Potos flavus_ (Mugaas et al., in prep.). The
greater heat tolerance of these coatis is compatible with their diurnal
habits and widespread distribution in a variety of forest habitats in
both tropical and subtropical areas of the western hemisphere.
_Bassariscus astutus._--In addition to living in Neotropical forests of
Mexico, _Bassariscus astutus_ also flourishes in hot arid climates, and
it has extended its range much farther north than _Nasua narica_ (Hall
and Kelson, 1959:881,892; Poglayen-Neuwall, 1975; Kaufmann, 1982). Its
T_{uc} is higher (35.5 deg.C; Table 7) than that of _Potos flavus_, but it
is comparable to those of _Nasua nasua_ and _Nasua narica_. Its capacity
for evaporative cooling is well developed; at 40 deg.C _Bassariscus astutus_
is able to dissipate 100% of its resting metabolic heat via evaporative
water loss, and at 45 deg.C it is able to dissipate 172% (Chevalier, 1985).
In spite of its great capacity for evaporative cooling, this species is
nocturnal, a habit that, along with its low [.H]_{b}, should allow it
to keep thermoregulatory water requirements to a minimum.
_Procyon lotor._--Our data suggested that T_{uc} for _Procyon lotor_ in
winter was comparable to that for _Bassariscus astutus_ (35 deg.C), and
that in summer it was even higher. When exposed to temperatures near
the upper end of its T_{n}, _Procyon lotor_ increased the gradient for
passive heat loss with a controlled rise in T_{b} (Figure 6). In summer
its capacity for passive heat loss was enhanced by the molt of its
heavy winter fur. _Procyon lotor_'s capacity for evaporative cooling
also appeared to be well developed, although our animals were not
heated to the point that evaporative cooling was fully expressed
(Figures 4, 5). However, _Procyon lotor_ is nocturnal, and this may
allow it to eliminate, or at least reduce, the need for evaporative
cooling, even in hot climates. Thus, _Procyon lotor_ appears to be
well equipped physiologically and behaviorally to cope with thermal
demands of hot environments in its distribution.
_Procyon cancrivorus._--Unfortunately, data for the crab-eating
raccoon are not complete enough at high temperatures to include it in
this survey.
SUMMARY.--This comparison demonstrates that capacity for evaporative
cooling, tolerance of an elevated T_{b} to enhance passive heat loss,
and behavioral avoidance of thermal stress are the primary methods used
by procyonids to thermoregulate at high temperatures. _Procyon lotor_
and _Bassariscus astutus_, whose distributions extend into temperate
regions, have developed these abilities to a greater extent than other
procyonids. _Potos flavus_, whose distribution is confined to
lowland tropical forests, has the least ability in this regard. _Nasua
nasua_ and _Nasua narica_ appear to have thermoregulatory abilities
that are intermediate to those of _Bassariscus astutus_ and _Potos
flavus_. This suggests that ancestral procyonids may have had poor to
modest ability to thermoregulate at high temperatures, a condition that
would have limited their ability to leave the thermal stability
afforded by tropical forests. Dispersal into temperate climates,
therefore, required not only increased cold tolerance but also
selective enhancement of those mechanisms used in thermoregulation at
high temperatures.
TABLE 11.--Distribution by climate of selected procyonid species.
-----------------------+---------------------------------------------
| Mild[a] Cold[b]
Species | Tropics Subtropics temperate temperate
-----------------------+---------------------------------------------
_Procyon lotor_ | + + + +
_Bassariscus astutus_ | + + +
_Nasua nasua_ | + +
_Nasua narica_ | + +
_Procyon cancrivorus_ | + +
_Potos flavus_ | +
-----------------------+---------------------------------------------
[a] Extends from the subtropics north to the northern limit of
_Bassariscus astutus_' distribution (Hall and Kelson, 1959:881),
which approximates the 10 deg.C isotherm for average annual
temperature in the United States (Kincer, 1941).
[b] Extends northward from the 10 deg.C isotherm for average annual
temperature in the United States.