Part 10

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

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