Basal Metabolism and Climatic Distribution
Procyon lotor.—The evolution of a higher Ḣb ([Tables 7],[ 8]) may have been the physiological cornerstone that enabled Procyon lotor to break out of the mold being exploited by other procyonids and to generalize its use of habitats and climates. Once this basic physiological change was in place, selection for appropriate alterations in thermal conductance, capacity for evaporative cooling, diversity of diet, and energy storage would have provided this species with the suite of adaptations needed to extend its distribution into other habitats and climates. Support for this concept follows from the fact that high levels of Ḣb are associated with (1) cold-hardiness in mammals that live in cold-temperate and arctic climates (Scholander et al., 1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972; Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to utilize a wide variety of food resources and to occupy a large number of different environments and habitats (McNab, 1980a); and (3) a high intrinsic rate of natural increase (McNab, 1980a; Hennemann, 1983; Lillegraven et al., 1987; Nicoll and Thompson, 1987; Thompson, 1987).
Other Procyonids.—Other procyonids (Potos flavus, Procyon cancrivorus, Nasua narica, and Nasua nasua) have lower than predicted Ḣb's ([Table 7]), a characteristic that is considered to be an energy-saving adaptation for those that live in relatively stable tropical and subtropical habitats (Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, 1983). However, Bassariscus astutus is found in tropical, subtropical, and temperate climates. This species is found from tropical Mexico to temperate regions of the western United States (Kaufmann, 1982, 1987; Nowak and Paradiso, 1983:979). In the northern part of its distribution, Bassariscus astutus lives in habitats that are unstable (arid regions), that are low in productivity, and that characteristically have marked seasonal changes in temperature. Its lower than predicted Ḣb could be an important water-conserving adaptation at times when temperatures are high (McNab and Morrison, 1963; McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979) and an important energy-conserving mechanism when cold weather may limit food availability and hunting time (Scholander et al., 1950c; Wang et al., 1973). As will be seen later, Bassariscus astutus is unique among procyonids with lower than predicted Ḣb's in that it also has a lower than predicted Cmw ([Table 7]). This allows it to use less energy than expected for thermoregulation at low temperatures. Another species with a similar set of adaptations (lower than predicted Ḣb and Cmw) is the arctic hare, Lepus arcticus (Wang et al., 1973), which lives in one of the coldest and least-productive regions on earth. Wang et al. (1973) suggest that this combination of adaptations allows Lepus arcticus to better match its energy requirements to the low productivity of its environment. A similar relationship may hold for Bassariscus astutus, particularly in colder arid portions of its distribution, and may be the reason that it, but not other procyonids with low Ḣb's, has been able to inhabit temperate climates.