McNab (1988a) contends that basal metabolism is a highly plastic character in evolution, and he has amply shown that ecologically uniform species are more apt to share common metabolic rates than taxonomically allied species from drastically different environments (McNab, 1984a, 1986a, 1986b, 1988a). Procyonids represent a taxonomically allied group that shared a common ecological situation for millions of years; consequently, members of this family might be expected to show some uniformity in their [.H]_{b}. Basal and thermoregulatory metabolism of several procyonids have been measured: kinkajou, _Potos flavus_ (Müller and Kulzer, 1977; McNab, 1978a; Müller and Rost, 1983), coatis, _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al., in prep.), and _Nasua narica_ (Scholander et al., 1950c; Mugaas et al., in prep.), ringtail, _Bassariscus astutus_ (Chevalier, 1985), and crab-eating raccoon, _Procyon cancrivorus_ (Scholander et al., 1950c). In general, these species have [.H]_{b}'s that are 40%-80% of the values predicted for them by the Kleiber (1961:206) equation. Lower than predicted [.H]_{b} is viewed as an energy-saving adaptation for procyonids living in relatively stable tropical climates (Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, 1983). This implies that lower than predicted [.H]_{b} is a general procyonid condition and that it represents a characteristic that evolved in response to the family's long association with tropical and subtropical forest environments. _The Atypical Procyonid_ Although most procyonids are found in only tropical to subtropical climates, the North American raccoon, _Procyon lotor_, (Figure 1) has a much broader distribution that extends from tropical Panama (8°N) to southern Canada. In Alberta, Canada, its range reaches the edge of the Hudsonian Life Zone at 60°N (for distribution maps see Hall and Kelson, 1959:878-897, and Poglayen-Neuwall, 1975). Range extensions and an increase in numbers have been noted in Canada and in parts of the United States since the 19th century (Lotze and Anderson, 1979; Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). Thus, _Procyon lotor_ is more complex ecologically than other procyonids, particularly when one takes into account its highly generalized food habits (Hamilton, 1936; Stuewer, 1943; Stains, 1956:39-51; Greenwood, 1981) and the wide range of habitat types (forest, prairie, desert, mountain, coastal marsh, freshwater marsh) and climates (tropical to north temperate) in which it is successful (Whitney and Underwood, 1952:1; Hall and Kelson, 1959:885; Lotze and Anderson, 1979; Kaufmann, 1982). On this basis it is clear that _Procyon lotor_ has deviated from the typical procyonid portrait and has become the consummate generalist of the Procyonidae. [Illustration: FIGURE 1.--North American raccoon, _Procyon lotor_.] _The Hypothesis_