Chapter III: Part 3

_Bassariscus astutus_, the other species with low [.H]_{b}, is found in three climates, which indicates that it has greater ecological flexibility than _Nasua nasua_, _Nasua narica_, or _Procyon cancrivorus_. D_{dr} and r_{maxr} are comparable for these four species (Table 12). This suggests that the greater ecological flexibility of _Bassariscus astutus_ is derived largely from its greater cold tolerance. _Bassariscus astutus_ has a more insulative pelt than these other procyonids (C_{mwr} = 0.85; Table 7), so its H_{br}/C_{mwr} ratio is higher (0.80; Table 12). This, and its greater capacity for evaporative cooling (Chevalier, 1985), allows _Bassariscus astutus_ to take advantage of a wider range of thermal environments than these other species. However, even with its higher H_{br}/C_{mwr} ratio, the composite score for _Bassariscus astutus_ is not much different than those for _Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ (Table 12). Consequently, _Bassariscus astutus_ is found in more climates than would be predicted for it on the basis of its composite score (Figure 8). This suggests that either the H_{br}/C_{mwr} ratio carries greater weight in determining distribution than is reflected in this analysis, or as has been described for some other species (Bartholomew, 1958, 1987), _Bassariscus astutus_ may extend its distribution farther than expected via use of its behavior. In either case, for procyonids with low [.H]_{b}, _Bassariscus astutus_ represents the pinnacle of adaptation for climate generalization.

EVOLUTION OF METABOLIC ADAPTATIONS

_Evolution of Low Basal Metabolic Rate_

A radiation of frugivorous and omnivorous Procyoninae (Table 1) occurred in the middle and late Miocene of North America. It included origins of such terrestrial genera as _Cyonasua_, _Nasua_, and _Procyon_ (Webb, 1985b). The earliest procyonid genus to find its way to South America was _Cyonasua_, an omnivorous carnivore that presumably split, along with its sister genus _Arctonasua_, from a common North American ancestor (Baskin, 1982; Webb, 1985b). _Cyonasua_, about the size of present-day raccoons, was adapted to a wide range of habitats and was probably comparable to modern raccoons with respect to the breadth of its feeding habits (Webb, 1985b; Marshall, 1988). Because North American _Arctonasua_ was about the same size as _Cyonasua_ (Webb, 1985b) and shared a number of characters with it (Baskin, 1982), we speculate that it also may have had similar habits and occupied similar climates and habitats. _Bassariscus_, another member of Procyoninae, had an even earlier origin in tropical North America (Webb, 1985b). The origin of the small arboreal forms _Potos_ and _Bassaricyon_ (subfamily Potosinae) is obscure but is thought to have occurred in the rainforests of Central America (Webb, 1985b). What were the metabolic capabilities of these early procyonids? We do not know, but for several million years, from middle to late Miocene, procyonids lived in tropical and subtropical forests of Central and North America (Webb, 1985b; Marshall, 1988). Then, in the Pleistocene, several modern forms crossed the Panamanian land bridge into similar habitats and climates in South America; but none of them appear to have spread far enough northward to have crossed the Bering land bridge.

Several million years exposure to a tropical environment, with its continuous high temperatures and modest range of thermal extremes, would have favored selection of metabolic and thermoregulatory traits that would minimize energy requirements: a lower than predicted basal metabolic rate, a prolonged or continuous molt resulting in very little annual change in minimum thermal conductance, and a modest capacity for evaporative cooling. In addition, we would expect selection to have favored a diverse diet, good reproductive potential, and behavioral flexibility to utilize a variety of habitats within these climates. Our analysis has shown that such characteristics are the norm for extant members of this family living in tropical and subtropical climates, and we speculate that these traits also were common to early procyonids and served to restrict them to these climates. Our speculation is supported by the fact that their known fossil history from the Miocene is confined to geographic areas that had tropical and subtropical climates.

Later on, during Pleistocene glaciations, tropical and subtropical forests shrank, savannas expanded, and temperate climate was pushed toward equatorial regions. The opposite occurred during interglacial periods (Raven and Axelrod, 1975; Webb, 1977, 1978; Marshall, 1988). Consequently, mid-latitudes experienced alternating periods of temperate and tropical, or at least subtropical, climate change. Selection of characteristics that would have adapted a species with low [.H]_{b} to temperate as well as tropic or subtropic climates could have occurred in mid-latitudes at the temperate edge of these tropical advances and retreats. Our analysis indicates that, for this purpose, selection would have favored lower than predicted thermal conductance, seasonal molt, increased capacity for evaporative cooling, increased tolerance of elevated T_{b}, increased flexibility of thermoregulatory behavior, food habits that provided for year-round access to a high-quality diet in all three climates, and a higher than predicted r_{max}.

_Bassariscus astutus_ is the only species with low [.H]_{b} that has all these characteristics, and it is the only one of them that has added temperate climate to its distribution (Table 11). This suggests that _Bassariscus astutus_ is a species that evolved away from the norm for procyonids with low [.H]_{b}, toward characteristics that allowed it to become more of a climate generalist. _Potos flavus_, with its dietary specialization, low tolerance to high temperatures, and arboreal mode of existence, has become a highly specialized species totally dependent on tropical forests for its survival. As such, it also represents a species that has evolved away from the procyonid norm and portrays the extreme in climate specialization. Olingos, _Bassaricyon gabbii_ (Table 1), may be similar to _Potos flavus_ in this respect (see also Table 10). This suggests that of the extant procyonids, _Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ have retained metabolic and behavioral characteristics that are closest to those of their Miocene ancestors.

_Evolution of High Basal Metabolic Rate_

Between the time that _Cyonasua_ appeared and the Panamanian land bridge was established in the upper Pliocene (4 to 5 million years ago), northern climates continued their gradual cooling. This, along with ongoing elevation of the continents and continuous modification of their mountain ranges, served to shrink the tropical forest and create pockets of climatic instability within it and on its edges (Darlington, 1963:578-596; Marshall, 1988). In areas of instability, selection would have favored traits that provided for a broader range of thermal tolerance: higher [.H]_{b}, improved insulative quality of pelt, a more sharply defined molt cycle, improved capacity for evaporative cooling, greater D_{d}, and higher r_{max}. Consequently, by the upper Pliocene, two metabolically distinct groups of procyonids could have been established: those species with low [.H]_{b} living in climatically stable forests and those with higher [.H]_{b} living in unstable tropical, subtropical, and perhaps temperate climates.

_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.

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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* * * * * * *

SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION

Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the institution, Joseph Henry outlined a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with _Smithsonian Contributions to Knowledge_ in 1848 and continuing with the following active series:

_Smithsonian Contributions to Anthropology_
_Smithsonian Contributions to Botany_
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_Smithsonian Contributions to the Marine Sciences_
_Smithsonian Contributions to Paleobiology_
_Smithsonian Contributions to Zoology_
_Smithsonian Folklife Studies_
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_Smithsonian Studies in History and Technology_

In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world of science and scholarship. The publications are distributed by mailing lists to libraries, universities, and similar institutions throughout the world.

Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject to its own review for format and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover.

Robert McC. Adams
_Secretary_
Smithsonian Institution

* * * * * * *

TRANSCRIBER'S NOTES

With the exception of the typographical corrections listed below and some minor changes that may have been made in moving tables or illustrations so that they are rejoined, the text presented is that published in the original printed media. Also, the second instance of t-tests was changed to _t_-tests as the letter "t" is usually italicized by statisticians.

Emphasis Notation

_Text_ = Italics

$Text$ = Bold

a_{b} = a with subscript b

a^{b} = a with superscript b

Typographical Corrections

Page ii, Instituion's => Institution's
Page 1, linages => lineages
Page 4, consumate => consummate
Page 21, Table 10, footnote f => Table 10, footnote b
Page 26, Nassua => Nasua
Page 31, Incoporated => Incorporated
Page 34, Gettleman => Gittleman

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