Our analysis has illustrated that within Procyonidae there are two
distinct modes of metabolic adaptation to climate. One is typified by
those species with low [.H]_{b}'s (_Bassariscus astutus_, _Nasua
nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Potos flavus_),
and the other by _Procyon lotor_ with its higher [.H]_{b}. Those with
low [.H]_{b}'s have more restricted geographic distributions, and,
with the exception of _Bassariscus astutus_, they are all confined to
tropical and subtropical areas. The fossil history of this family
indicates that it had its origins in tropical forests of North and
Central America. This indicates that those procyonids whose
distributions are still primarily restricted to tropical forests share
many of the metabolic adaptations characteristic of their ancestors.
We speculate, therefore, that ancestral procyonids had a lower than
predicted [.H]_{b}, a pelt with modest to poor insulative quality,
good thermogenic ability but poor heat tolerance, modest to poor
capacity for evaporative cooling, no well-defined molt cycle, no
cyclic period of fattening, nocturnal habits, and a modestly diverse
diet of high-enough quality to provide for an average reproductive
potential. Although this pedigree contributed to the success of this
family in tropical and subtropical forests, it limited the ability of
its members to expand their distributions into cooler, less stable
climates. Viewed in this perspective, _Procyon lotor_'s high basal
metabolic rate, extraordinarily diverse diet, well-defined cyclic
changes in fat content and thermal conductance, high level of heat
tolerance, high capacity for evaporative cooling, and high
reproductive potential all stand out in sharp contrast to the
condition described for other procyonids. This suggests that the North
American raccoon represents culmination of a divergent evolutionary
event that has given this species the ability to break out of the old
procyonid mold and carry the family into new habitats and climates.