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 Ta's and solar radiation) than are nocturnal procyonids. Both coatis are more heat-tolerant than Potos flavus; their Tuc's are higher (33°C-35°C; [Table 7]), they can tolerate Ta's of 35°C without raising their Tb'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 Tuc is higher (35.5°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°C Bassariscus astutus is able to dissipate 100% of its resting metabolic heat via evaporative water loss, and at 45°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 Ḣb, should allow it to keep thermoregulatory water requirements to a minimum.
Procyon lotor.—Our data suggested that Tuc for Procyon lotor in winter was comparable to that for Bassariscus astutus (35°C), and that in summer it was even higher. When exposed to temperatures near the upper end of its Tn, Procyon lotor increased the gradient for passive heat loss with a controlled rise in Tb ([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 Tb 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.