Mugaas, J. N., J. Seidensticker, and K. Mahlke-Johnson. Metabolic
Adaptation to Climate and Distribution of the Raccoon _Procyon lotor_
and Other Procyonidae. _Smithsonian Contributions to Zoology_, number
542, 34 pages, 8 figures, 12 tables, 1993.--Although the family
Procyonidae is largely a Neotropical group, the North American
raccoon, _Procyon lotor_, is more versatile in its use of climate, and
it is found in nearly every habitat from Panama to 60 deg.N in Canada. We
hypothesized that most contemporary procyonids have remained in tropic
and subtropic climates because they have retained the metabolic
characteristics of their warm-adapted ancestors, whereas _Procyon
lotor_ evolved a different set of adaptations that have enabled it to
generalize its use of habitats and climates. To test this hypothesis
we compared _Procyon lotor_ with several other procyonids
(_Bassariscus astutus_, _Nasua nasua_, _Nasua narica_, _Procyon
cancrivorus_, and _Potos flavus_) with respect to (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, where possible, (5) capacity for evaporative cooling
(E_{c}). We measured basal and thermoregulatory metabolism,
evaporative water loss, and body temperature of both sexes of _Procyon
lotor_ from north central Virginia, in summer and winter. Metabolic
data for other procyonids were from literature, as were dietary and
reproductive data for all species.
Procyon lotor differed from other procyonids in all five
variables. (1) _Procyon lotor_'s mass specific [.H]_{b} (0.46
mL O_{2}.g^{-1}.h^{-1}) was 1.45 to 1.86 times greater than values for
other procyonids. (2) Because of its annual molt, _Procyon lotor_'s
C_{mw} was about 49% higher in summer than winter, 0.0256 and 0.0172
mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, respectively. The ratio of measured to
predicted C_{mw} for _Procyon lotor_ in winter (1.15) was similar to
values calculated for _Potos flavus_ (1.02) and _Procyon cancrivorus_
(1.25). Values for other procyonids were higher than this, but less
than the value for _Procyon lotor_ (1.76) in summer. On a mass
specific basis, _Bassariscus astutus_ had the lowest C_{mw} with a
ratio of 0.85. (3) _Procyon lotor_ utilized three times as many food
categories as _Procyon cancrivorus_, _Nasua nasua_, and _Bassariscus
astutus_; about two times as many as _Nasua narica_; and nine times as
many as _Potos flavus_. (4) Intrinsic rate of natural increase
correlated positively with [.H]_{b}. _Procyon lotor_ had the highest
r_{max} (2.52 of expected) and _Potos flavus_ the lowest (0.48 of
expected). The other procyonids examined also had low [.H]_{b}, but
their r_{max}'s were higher than predicted (1.11-1.32 of expected).
Early age of first female reproduction, fairly large litter size, long
life span, high-quality diet, and, in one case, female social
organization all compensated for low [.H]_{b} and elevated r_{max}.
(5) Although data on the capacity for evaporative cooling were
incomplete, this variable appeared to be best developed in _Procyon
lotor_ and _Bassariscus astutus_, the two species that have been most
successful at including temperate climates in their distributions.
These five variables are functionally interrelated, and have
co-evolved in each species to form a unique adaptive unit that
regulates body temperature and energy balance throughout each annual
cycle. The first four variables were converted into normalized
dimensionless numbers, which were used to derive a composite score
that represented each species' adaptive unit. _Procyon lotor_ had the
highest composite score (1.47) and _Potos flavus_ the lowest (0.39).
Scores for the other procyonids were intermediate to these extremes
(0.64-0.79). There was a positive correlation between the number of
climates a species occupies and the magnitude of its composite score.
Linear regression of this relationship indicated that 89% of the
variance in climatic distribution was attributed to the composite
scores. Differences in metabolic adaptation, therefore, have played a
role in delimiting climatic distribution of these species.
It was clear that _Procyon lotor_ differed from the other procyonids
with respect to thermoregulatory ability, diet, and reproductive
potential. These differences have enabled it to become a highly
successful climate generalist, and its evolution of an [.H]_{b} that
is higher than the procyonid norm appears to be the cornerstone of its
success.
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