_Procyonid Origins_
The major carnivore radiations took place about 40 million years
before present (MYBP) in the late Eocene and early Oligocene (Ewer,
1973:363; Wayne et al., 1989). Between 30 and 40 MYBP, a progenitor
split into the ursid and procyonid lineages, which evolved into
present-day bears, pandas, and raccoons (Wayne et al., 1989). The
taxonomic relatedness of pandas to bears and raccoons has been tested
extensively and a number of authors have summarized current thinking
on the problem (Martin, 1989; Wayne et al., 1989; Wozencraft, 1989a,
1989b; Decker and Wozencraft, 1991). Davis (1964:322-327) and others
(Leone and Wiens, 1956; Todd and Pressman, 1968; Sarich, 1976; O'Brien
et al., 1985) place the giant panda, _Ailuropoda melanoleuca_, with
the ursids. The taxonomic status of the red panda, _Ailurus fulgens_,
appears to be less certain. Some current investigations align the red
panda with bears (Segall, 1943; Todd and Pressman, 1968; Hunt, 1974;
Ginsburg, 1982; Wozencraft, 1984:56-110; 1989a), whereas others place
them intermediate to procyonids and bears (Wurster and Benirschke,
1968; Sarich, 1976; O'Brien et al., 1985), or in close relationship to
the giant panda (Tagle et al., 1986).
The procyonid radiation took place in North America and produced forms
that were mostly arboreal and omnivorous (Eisenberg, 1981:122; Martin,
1989). The center of this diversification occurred in Middle America
(Baskin, 1982; Webb, 1985b) during the Miocene (Darlington, 1963:367;
Webb, 1985b). Fossil procyonids from the late Miocene are represented
in Florida, California, Texas, Nebraska, Kansas, and South Dakota
(Baskin, 1982; Martin, 1989) and include such genera as _Bassariscus_,
_Arctonasua_, _Cyonasua_, _Paranasua_, _Nasua_, and _Procyon_ (Baskin,
1982; Webb, 1985b). During the Miocene procyonids underwent a modest
radiation within tropical and subtropical climates of North America's
central and middle latitudes. _Cyonasua_, which has close affinities
to _Arctonasua_ (Baskin, 1982), appears in tropical South America in
the late Miocene and immigrated there either by rafting across the
Bolivar Trough or by island-hopping through the Antilles archipelagoes
(Marshall et al., 1982; Marshall, 1988). Thus, procyonids were found
on both continents prior to formation of the Panamanian land bridge
(Darlington, 1963:367, 395; Marshall et al., 1982; Marshall, 1988).
Origins of _Bassaricyon_ and _Potos_ are obscure but probably occurred
in tropical rainforests of Middle America (Baskin, 1982; Webb, 1985b).
A subsequent Pleistocene dispersal carried several modern genera
(Table 1) across the Panamanian land bridge into South America (Webb,
1985b). _Bassariscus_ and _Bassaricyon_ represent the most primitive
genera in Procyoninae and Potosinae subfamilies, respectively (Table 1;
Wozencraft, 1989a; Decker and Wozencraft, 1991).
In the early Tertiary, mid-latitudes of North America were much warmer
than they are now, but not fully tropical, and temperate deciduous
forests, associated with strongly seasonal climates, occurred only in
the far north (Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589,
590). Major climatic deteriorations, with their attendant cooling of
northern continents, occurred during the Eo-Oligocene transition, in
the middle Miocene, at the end of the Miocene, and at about 3 MYBP
(late Pliocene). This last deterioration corresponds with closure of
the Panamanian isthmus (Berggren, 1982; Webb, 1985a). Climatic
deterioration went on at an accelerating rate during the late
Tertiary, with glacial conditions developing at the poles by the
mid-Pliocene (Barghoorn, 1953). Therefore, throughout the Tertiary, as
continents cooled, northern climate zones moved toward the tropics
(Barghoorn, 1953; Colbert, 1953; Darlington, 1963:589, 590, 594, 595;
Webb, 1985a).
TABLE 1.--Classification of recent Procyonidae after Wozencraft
(1989a) and Decker and Wozencraft (1991). Information in
parenthesis indicates general geographic distribution (modified from
Kortlucke and Ramirez-Pulido (1982) and Poglayen-Neuwall (1975)):
S.A. = South America; C.A. = Central America; M. = Mexico;
U.S. = United States; C. = Canada. Lower case letters preceding
geographic areas signify north (n), south (s), and west (w).
Order CARNIVORA Bowdich, 1821
Suborder CANIFORMIA Kretzoi, 1945
Family PROCYONIDAE Gray, 1825
Subfamily POTOSINAE Trouessart, 1904
Genus _Potos_ E. Geoffroy and G. Cuvier, 1795
_P. flavus_ (S.A., C.A., M.)
Genus _Bassaricyon_ Allen, 1876
_B. alleni_[a] (S.A.)
_B. beddardi_[a] (S.A.)
_B. gabbii_[a] (nS.A., C.A.)
_B. lasius_[a] (C.A.)
_B. pauli_[a] (C.A.)
Subfamily PROCYONINAE Gray, 1825
Genus _Bassariscus_ Coues, 1887
_B. astutus_ (M., wU.S.)
_B. sumichrasti_ (C.A., M.)
Genus _Nasua_ Storr, 1780
_N. narica_[b] (nS.A., C.A., M., swU.S.)
_N. nasua_[b] (S.A., sC.A.)
Genus _Nasuella_ Hollister, 1915
_N. olivacea_ (S.A.)
Genus _Procyon_ Storr, 1780
_P. cancrivorus_ (S.A., sC.A.)
_P. gloveralleni_[c] (Barbados)
_P. insularis_[c] (Maria Madre Is., Maria Magdalene Is.)
_P. lotor_[c] (C.A., M., U.S., sC.)
_P. maynardi_[c] (Bahamas, New Providence Is.)
_P. minor_[c] (Guadeloupe Is.)
_P. pygmaeus_[c] (M., Quintana Roo, Cozumel Is.)
[a] The several named forms of _Bassaricyon_ are a single
species, _Bassaricyon gabbii_ (Wozencraft, 1989a).
[b] These are considered conspecific in some current
taxonomies (Kortlucke and Ramirez-Pulido, 1982); however,
the scheme followed here maintains them as separate species
(Decker, 1991).
[c] Several named forms of _Procyon_ are a single species,
_Procyon lotor_ (Wozencraft, 1989a).
During the late Miocene, late Pliocene, and Pleistocene, the Bering
land bridge between North America and Asia formed periodically,
offering an avenue for dispersal between northern continents
(Darlington, 1963:366; Webb, 1985a). However, by the late Tertiary,
northern continents had cooled to the extent that climate, with its
attendant sharply defined vegetative zones, became the major factor
limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a).
Those Holarctic mammals that did cross the Bering land bridge in the
late Tertiary were "cold-adapted" species associated with relatively
cool, but not alpine, climates (Darlington, 1963:366; Ewer, 1973:369).
Among carnivores this included some canids, ursids, mustelids, and
felids (Darlington, 1963:393-395, 397; Webb, 1985a). Procyonids,
however, did not cross the Bering land bridge into Asia, and Ewer
(1973:369) ascribes this to their being an "essentially tropical
group." Miocene radiation of procyonids occurred at a time when two of
the four major climatic deteriorations (middle and late Miocene) were
taking place (Webb, 1985a, 1985b). These deteriorations had the effect
of cooling the middle latitudes to the extent that temperate forest
forms began to appear in mid-latitude floras, along with a rapid
influx of herbaceous plants (Barghoorn, 1953). The procyonid radiation
did not penetrate beyond these climatically changing middle latitudes,
which implies that these animals were "warm-adapted," and were,
therefore, physiologically excluded from reaching the Bering land
bridge. Today, three of the six genera and over half of the 18 species
that comprise Procyonidae (Table 1; Wozencraft, 1989b) remain confined
to tropical regions of North and South America (Hall and Kelson,
1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido,
1982; Nowak and Paradiso, 1983:977-985).
_Typical Procyonids_
McNab (1988a) contends that basal metabolism is a highly plastic
character in evolution, and he has amply shown that ecologically
uniform species are more apt to share common metabolic rates than
taxonomically allied species from drastically different environments
(McNab, 1984a, 1986a, 1986b, 1988a). Procyonids represent a
taxonomically allied group that shared a common ecological situation
for millions of years; consequently, members of this family might be
expected to show some uniformity in their [.H]_{b}. Basal and
thermoregulatory metabolism of several procyonids have been measured:
kinkajou, _Potos flavus_ (Mueller and Kulzer, 1977; McNab, 1978a;
Mueller and Rost, 1983), coatis, _Nasua nasua_ (Chevillard-Hugot et
al., 1980; Mugaas et al., in prep.), and _Nasua narica_ (Scholander et
al., 1950c; Mugaas et al., in prep.), ringtail, _Bassariscus astutus_
(Chevalier, 1985), and crab-eating raccoon, _Procyon cancrivorus_
(Scholander et al., 1950c). In general, these species have [.H]_{b}'s
that are 40%-80% of the values predicted for them by the Kleiber
(1961:206) equation. Lower than predicted [.H]_{b} is viewed as an
energy-saving adaptation for procyonids living in relatively stable
tropical climates (Mueller and Kulzer, 1977; Chevillard-Hugot et al.,
1980; Mueller and Rost, 1983). This implies that lower than predicted
[.H]_{b} is a general procyonid condition and that it represents a
characteristic that evolved in response to the family's long
association with tropical and subtropical forest environments.
_The Atypical Procyonid_
Although most procyonids are found in only tropical to subtropical
climates, the North American raccoon, _Procyon lotor_, (Figure 1) has
a much broader distribution that extends from tropical Panama (8 deg.N) to
southern Canada. In Alberta, Canada, its range reaches the edge of the
Hudsonian Life Zone at 60 deg.N (for distribution maps see Hall and
Kelson, 1959:878-897, and Poglayen-Neuwall, 1975). Range extensions
and an increase in numbers have been noted in Canada and in parts of
the United States since the 19th century (Lotze and Anderson, 1979;
Kaufmann, 1982; Nowak and Paradiso, 1983:977-985). Thus, _Procyon
lotor_ is more complex ecologically than other procyonids,
particularly when one takes into account its highly generalized food
habits (Hamilton, 1936; Stuewer, 1943; Stains, 1956:39-51; Greenwood,
1981) and the wide range of habitat types (forest, prairie, desert,
mountain, coastal marsh, freshwater marsh) and climates (tropical to
north temperate) in which it is successful (Whitney and Underwood,
1952:1; Hall and Kelson, 1959:885; Lotze and Anderson, 1979;
Kaufmann, 1982). On this basis it is clear that _Procyon lotor_ has
deviated from the typical procyonid portrait and has become the
consummate generalist of the Procyonidae.
[Illustration: FIGURE 1.--North American raccoon, _Procyon lotor_.]
_The Hypothesis_
Our general hypothesis was that whereas most contemporary procyonids
have retained the metabolic characteristics of their warm-adapted
ancestors, _Procyon lotor_ possesses a different set of adaptations,
which either evolved as characteristics unique to this species or were
acquired from its ancestral stock. In either case, its unique
adaptations have given _Procyon lotor_ the physiological flexibility
to generalize its use of habitats and climates and expand its
geographic distribution to a much greater extent than other
procyonids.
_Hypothesis Testing_
We tested our hypothesis by comparing _Procyon lotor_ with several
other procyonids (_Bassariscus astutus_, _Nasua nasua_, _Nasua
narica_, _Procyon cancrivorus_, and _Potos flavus_) on the basis of
their (1) basal metabolic rate ([.H]_{b}), (2) minimum wet thermal
conductance (C_{mw}), (3) diversity of diet (D_{d}), (4) intrinsic
rate of natural increase (r_{max}), and, when data were available, (5)
capacity for evaporative cooling (E_{c}). In a genetic sense each one
of these variables is a complex adaptive characteristic, expression of
which is determined by the interaction of several genes (Prosser,
1986:110-165). Experience has shown that a given species will express
each one of these variables in a specific manner that is relevant to
its mass, physiology, behavior, and environmental circumstance. Thus,
different expressions of these variables may represent specific
climatic adaptations (Prosser, 1986:16) that have been selected-for by
evolutionary process. Because these variables are interrelated with
respect to regulation of body temperature and energy balance, they
have co-evolved in each species to form an adaptive unit. For each
species, measured and calculated values for the first four variables
were converted into dimensionless numbers and used to derive a
composite score that represented its adaptive unit. Climatic
distributions of these species were then compared relative to their
composite scores.