_Procyon lotor_ is the only extant procyonid with high [.H]_{ b}. _Procyon cancrivorus_ is its congeneric counterpart in Central and South America (Table 1), and the two species are sympatric in Panama and Costa Rica. However, in terms of its metabolism, thermal conductance, molt, diversity of diet, r_{ max}, and climatic distribution, _Procyon cancrivorus_ shares more in common with other procyonids than it does with _Procyon lotor_ (Tables 7, 11, 12; Figure 8). This suggests that metabolically _Procyon lotor_ portrays a divergent line of this genus that arose as the result of a series of mutations that gave rise to different metabolic characteristics. This view is in keeping with a recent phylogenetic analysis of this family that shows the genus _Procyon_ to be highly derived (Decker and Wozencraft, 1991). Consequently, it would be instructive and would add to our knowledge of the evolution of climatic adaptation to know more about the genetic relatedness of these two species as well as their historical relationship.
Genus _Procyon_ appears in the fossil record (Hemphillian and Blancan ages; Baskin, 1982) prior to Pleistocene glaciations. During the Pleistocene, there were four different glacial advances and retreats in a relatively short time period (the first appearing little more than a million years ago; Darlington, 1963:578-596; Webb, 1985a; Marshall, 1988). Glacial retreats created pulses of time during which subtropic and temperate climates advanced toward the poles into areas with large seasonal differences in light/dark cycles, whereas glacial advances pushed these climates southward into areas having smaller seasonal differences in light/dark cycles (Raven and Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988). Those members of the genus _Procyon_ caught in these wide latitudinal fluctuations would have experienced conditions favorable to continued selection for characteristics conducive to physiologic adaptation to a wide range of climatic conditions. _Procyon lotor_ is the only member of its genus to have survived this selective process, and as we have seen, it does possess traits that adapt it to a wide range of climatic conditions. Primary among these is its higher [.H]_{ b}, which provides it with advantages not shared with other procyonids (see earlier discussion). Three other adaptations also have had a profound influence on _Procyon lotor_'s ability to generalize its use of climate: (1) the increased insulative quality of its pelt coupled with its sharply defined molt cycle, which allows for a large annual change in thermal conductance; (2) its annual cycle of fat storage; and (3) a diverse high-quality diet. The first two of these adaptations required evolution of neuroendocrine pathways capable of responding to time-dependent environmental cues such as changing day length, changing temperature, etc. Such conditions would have been available as selective stimuli in high-latitude forests and savannas of interglacial periods. _Procyon lotor_'s elevated basal metabolic rate would have increased its overall energy requirement, and it makes good intuitive sense, therefore, that evolution during the Pleistocene also would have favored selection of a diverse diet containing many items of high nutritive value.