Thermoregulation at High Temperatures
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 Ta'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 Tuc is at 30°C to 33°C ([Table 7]; Müller and Kulzer, 1977; Müller and Rost, 1983). It begins to pant at about 30°C, but its efforts at evaporative cooling are very ineffective. At 33°C Potos flavus can dissipate 33% of its metabolic heat via evaporative water loss, but at 35°C the efficiency of this mechanism falls to 20% (Müller and Rost, 1983). Consequently, when exposed to Ta's above 33°C, any kind of excitement causes its Tb to rise rapidly in an uncontrolled manner (Müller and Kulzer, 1977; Müller and Rost, 1983). These animals rely on their nocturnal and arboreal habits to keep them out of situations that could lead to hyperthermia (Müller and Kulzer, 1977; Müller and Rost, 1983).