Part 7

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.

SUMMARY

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.

APPENDIX: LIST OF SYMBOLS

a potential age of females first producing young

b potential annual birth rate of female young

C_{a} conductance of air

C_{d} conductance of den walls

C_{m} minimum thermal conductance

C_{md} minimum dry thermal conductance

C_{mw} minimum wet thermal conductance

C_{mwr} ratio of measured to predicted minimum wet thermal conductance

C_{t} total conductance

D_{d} diversity of diet

D_{dr} ratio of food categories actually used by a species to the total number of food categories taken by all species tested

[.E] evaporative water loss

E_{c} ratio of evaporative heat lost to metabolic heat produced

[.E]_{eq} oxygen equivalent for heat lost by evaporation

[.H]_{b} basal metabolic rate

[.H]_{r} lowest resting metabolic rate at each temperature

H_{br} ratio of measured to predicted basal metabolic rate

m mass of animal

m_{w} mass of water

n potential age of females producing their final young

r_{max} intrinsic rate of natural increase

r_{maxe} expected intrinsic rate of natural increase

r_{maxr} ratio of calculated to expected intrinsic rate of natural increase

RQ respiratory quotient

T_{a} chamber air temperature

T_{b} body temperature

T_{lc} lower critical temperature

T_{n} thermoneutral zone

T_{uc} upper critical temperature

t time

[.V]_{a} rate of air flow through U-tubes

[.V]_{e} rate of air flow into metabolism chamber

[alpha] active phase of the daily cycle

[gamma] heat equivalent of oxygen

[lambda] heat of vaporization of water

[rho] rest phase of the daily cycle

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