During the late Miocene, late Pliocene, and Pleistocene, the Bering land bridge between North America and Asia formed periodically, offering an avenue for dispersal between northern continents (Darlington, 1963:366; Webb, 1985a). However, by the late Tertiary, northern continents had cooled to the extent that climate, with its attendant sharply defined vegetative zones, became the major factor limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a). Those Holarctic mammals that did cross the Bering land bridge in the late Tertiary were "cold-adapted" species associated with relatively cool, but not alpine, climates (Darlington, 1963:366; Ewer, 1973:369). Among carnivores this included some canids, ursids, mustelids, and felids (Darlington, 1963:393-395, 397; Webb, 1985a). Procyonids, however, did not cross the Bering land bridge into Asia, and Ewer (1973:369) ascribes this to their being an "essentially tropical group." Miocene radiation of procyonids occurred at a time when two of the four major climatic deteriorations (middle and late Miocene) were taking place (Webb, 1985a, 1985b). These deteriorations had the effect of cooling the middle latitudes to the extent that temperate forest forms began to appear in mid-latitude floras, along with a rapid influx of herbaceous plants (Barghoorn, 1953). The procyonid radiation did not penetrate beyond these climatically changing middle latitudes, which implies that these animals were "warm-adapted," and were, therefore, physiologically excluded from reaching the Bering land bridge. Today, three of the six genera and over half of the 18 species that comprise Procyonidae ([Table 1]; Wozencraft, 1989b) remain confined to tropical regions of North and South America (Hall and Kelson, 1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and Paradiso, 1983:977-985).
Typical Procyonids
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 Ḣb. 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 Ḣb's that are 40%-80% of the values predicted for them by the Kleiber (1961:206) equation. Lower than predicted Ḣb 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 Ḣb 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.