Comparison of Thermal Conductances
Procyon lotor versus Tropical Procyonids.—Cmwr 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 Cmw's that are similar on a mass specific basis to the value for Procyon lotor in winter. However, at their Tlc's, the thermal gradient sustained by these tropical animals is only about 11°C, whereas for Procyon lotor in winter it was 26.5°C. Examination of [Eq. 4] with respect to these thermal gradients suggests that tropical procyonids achieve such low Cmw's by virtue of their lower than predicted Ḣb'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 Ta's and solar radiation) than their nocturnal cousins. Values of Cmwr 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 Cmw's than their nocturnal tropical cousins. A high Cmw reduces the cost of thermoregulation in hot environments because it increases an animal's ability to lose excess heat passively. The higher Cmw'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 Cmw of these procyonids (Cmwr = 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 Ḣb, allows Bassariscus astutus to maintain Tb 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.