_Background_
Basal metabolism represents the minimum energy required by a mammal to
maintain endothermy and basic homeostasis (Lusk, 1917:141; Kleiber,
1932, 1961:251; Benedict, 1938:191-215; Brody, 1945:59; Robbins,
1983:105-111). Mammals with lower than predicted [.H]_{b} maintain
endothermy and enjoy its attendant advantages at a discount, whereas
others, with rates that are higher than predicted, pay a premium
(Calder, 1987). Such variation in [.H]_{b} appears to be tied to
ecological circumstances rather than taxonomic affinities (Vogel,
1980; McNab, 1986a, 1988a, 1989), and depending on environmental
conditions, each rate provides an individual with various advantages
and limitations. During the course of evolution, therefore, each
species' [.H]_{b} evolves to provide it with the best match between
its energy requirements for continuous endothermy, its food supply,
and the thermal characteristics of its environment.
_Captive versus Wild Raccoons_
Male raccoons trapped in summer had higher [.H]_{b}'s than our captive
animals in any season (Table 2). The higher rate of metabolism of
these trapped males could have been due to the stress of captivity or
to the fact that "wild" animals actually may have higher metabolic
rates than those that have adjusted to captivity. If the latter is
true, then our data for captive animals underestimated the actual
energy cost of maintenance metabolism for _Procyon lotor_ in the wild.
At present, we have no way of determining which of these alternatives
is true.
_Seasonal Metabolism of Raccoons_
In some temperate-zone mammals, [.H]_{b} is elevated in winter, which
presumably increases their "cold-hardiness." Conversely, lower summer
metabolism is considered to be a mechanism that reduces the potential
for heat stress. Such seasonal variation in [.H]_{b} has been found in
several species: collard peccary, _Tayassu tajacu_ (Zervanos, 1975);
antelope jackrabbit, _Lepus alleni_ (Hinds, 1977); desert cottontail,
_Sylvilagus audubonii_ (Hinds, 1973); and, perhaps, cold-acclimatized
rat, _Rattus norvegicus_ (Hart and Heroux, 1963). Unlike these
species, our captive raccoons showed no seasonal variation in [.H]_{b}
(Table 2). Instead, raccoons achieved "cold-hardiness" in winter and
reduced their potential for heat stress in summer with a large
seasonal change in thermal conductance (Table 3).
TABLE 7.--Metabolic characteristics of several procyonid species.
---------------------+------------------------------------------------
|Body Basal[a] Minimum[b]
Species |mass metabolism conductance T_{b}[c]
|(g) ------------ ------------- -------------
| Meas H_{br} Meas C_{mwr} [alpha] [rho]
---------------------+------------------------------------------------
_Bassariscus astutus_| 865 0.43 0.68 0.0288[e] 0.85 37.6 23
_Procyon cancrivorus_|1160 0.40 0.69 0.0368[e] 1.25
_Potos flavus_ |2030 0.36 0.51
_Potos flavus_ |2400 0.32 0.65 38.1 36.0
_Potos flavus_ |2600 0.34 0.71 0.0200[f] 1.02
_Nasua nasua_ |3850 0.26 0.60 0.0200[f] 1.24 38.3 36.4
_Nasua nasua_ |4847 0.33 0.79 0.0238[e] 1.65 39.1 37.9
_Nasua narica_ |5554 0.25 0.62 0.0208[e] 1.55 38.9 37.4
_Nasua narica_ |4150 0.42 1.20 0.0341[e] 2.20
| 0.0224[g] 1.45
_Procyon lotor_ |
Summer |
Trapped male |4400 0.54 1.28
Captive male |4790 0.46 1.07 0.0256[f] 1.77 38.4 37.5
Captive female |4670 0.42 1.02 0.0256[f] 1.79 38.2 37.6
Winter |
Captive male |5340 0.47 1.17 38.6 37.6
Captive female |4490 0.46 1.10 0.0172[f] 1.15 38.3 37.3
---------------------+------------------------------------------------
---------------------+-----------------------------------------------
|
Species | T_{n}[d]
|---------------
| T_{lc} T_{uc} References
---------------------+-----------------------------------------------
_Bassariscus astutus_| 35.5 Chevalier (1985)
_Procyon cancrivorus_| 26 Scholander et al. (1950b, c)
_Potos flavus_ | McNab (1978a)
_Potos flavus_ | 23 30 Mueller and Kulzer (1977)
_Potos flavus_ | 23 33 Mueller and Rost (1983)
_Nasua nasua_ | 25 33 Chevillard-Hugot et al. (1980)
_Nasua nasua_ | 30 35 Mugaas et al. (in prep.)
_Nasua narica_ | 25 35
_Nasua narica_ | Scholander et al. (1950b, c)
|
_Procyon lotor_ | This study
Summer |
Trapped male | 20
Captive male | 20
Captive female | 25
Winter |
Captive male | 11
Captive female | 11
---------------------+-----------------------------------------------
[a] Meas is measured basal metabolism (mL O_{2}.g^{-1}.h^{-1}). H_{br}
is the ratio of measured to predicted basal metabolism where the
predicted value is calculated from [.H]_{b} = 3.42.m^{-.25}
(Kleiber, 1932, 1961:206) and m is body mass in grams.
[b] Meas is measured minimum thermal conductance
(mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}). C_{mwr} is the ratio of measured
to predicted minimum thermal conductance where the predicted value
is calculated from C_{m} = 1.0.m^{-0.5} (McNab and Morrison, 1963;
Herreid and Kessel, 1967), and m is body mass in grams.
[c] T_{b} is body temperature during the active ([alpha]) and rest
([rho]) phases of the daily cycle ( deg.C).
[d] T_{n} is the thermoneutral zone as defined by the lower (T_{lc})
and upper (T_{uc}) critical temperatures ( deg.C).
[e] Conductance calculated as the slope of the line describing oxygen
consumption at temperatures below the lower critical temperature.
[f] Conductance calculated from C_{mw} = [.H]_{r}/(T_{b} - T_{a}),
where [.H]_{r} is resting metabolic rate at temperatures below
T_{lc}, and other symbols are as described elsewhere.
[g] Inactive-phase thermal conductance: estimated from Scholander et
al. (1950b), assuming that active-phase thermal conductance is 52%
higher than values determined during the inactive phase (Aschoff,
1981).
_Comparison of Procyon lotor with Other Procyonids_
_Procyon lotor_ has a much higher mass-specific [.H]_{b} than other
procyonids (Table 7). To quantify the magnitude of this difference,
we compared the measured value for _Procyon lotor_ with one calculated
for it from a mass-specific least-squares regression equation (Eq. 6;
R squared = 0.78) derived from data for those procyonids with lower than
predicted [.H]_{b}: _Potos flavus_, _Procyon cancrivorus_,
_Nasua nasua_, _Nasua narica_, and _Bassariscus astutus_ (Table 7).
[.H]_{b} = 2.39.m^{-0.25} Eq. 6
[.H]_{b} in Eq. 6 is basal metabolism (mL O_{2}.g^{-1}.h^{-1}) and m
is body mass (g). Measured values of [.H]_{b} for _Procyon lotor_ were
1.45 to 1.86 times greater than those predicted for it by Eq. 6
(Table 8).
TABLE 8.--Basal metabolism (mL O_{2}.g^{-1}.h^{-1}) of _Procyon
lotor_ as predicted by Eq. 6 ([.H]_{b} = 2.39.m^{-0.25}). Body
masses, used to calculate predicted values, and measured values
were taken from Table 7.
----------------+--------------------------------
Season and sex | Predicted Measured/Predicted
----------------+--------------------------------
Summer |
Trapped male | 0.29 1.86
Captive male | 0.29 1.59
Captive female| 0.29 1.45
Winter |
Captive male | 0.28 1.68
Captive female| 0.29 1.59
----------------+--------------------------------
_Influence of Diet on Basal Metabolism_
BACKGROUND.--With respect to [.H]_{b}, McNab (1986a:1) maintains that
"the influence of climate is confounded with the influence of food
habits," and that departures from the Kleiber (1961) "norm" are best
correlated with diet. Although this does appear to be the case for
diet specialists, the analysis is not so clear-cut for omnivorous
species (McNab, 1986a). His analysis also indicates that an animal's
"behavior" (i.e., whether it is terrestrial, arboreal, subterranean,
aquatic, etc.), secondarily modifies the influence of food habits on
[.H]_{b}. For example, terrestrial frugivores have [.H]_{b}'s that are
very near predicted values, whereas arboreal frugivores have rates
that are much lower than predicted (McNab, 1986a).
TABLE 9.--Food habits of some Procyonids. References for foods were
as follows: _Potos flavus_, _Procyon cancrivorus_, and _Nasua nasua_
taken from Bisbal (1986); _Nasua narica_ taken from Kaufmann
(1962:182-198); _Bassariscus astutus_ taken from Martin et al.
(1951), Taylor (1954), Wood (1954), Toweill and Teer (1977), and
Trapp (1978); _Procyon lotor_ taken from Hamilton (1936), Stuewer
(1943:218-220), Stains (1956:39-51), and Greenwood (1981). Symbols
represent either qualitative (#) or quantitative (+,|) assessments
of feeding habits: # indicates that the animal was observed eating
the food; + and | represent volume and frequency, respectively, of
food utilization. No attempt was made to account for seasonal
variation in the use of these foods.
+ <20% by volume when found. | 1%-19% frequency of occurrence.
++ >20% by volume when found. || 20%-50% frequency of occurrence.
||| >50% frequency of occurrence.
-----------+--------------------------------------------------------------
|_Potos_ _Procyon_ _Nasua_ _Nasua_ _Bassariscus_ _Procyon_
Food |_flavus_ _cancrivorus_ _nasua_ _narica_ _astutus_ _lotor_
-----------+--------------------------------------------------------------
Mammalia | + | # ++ ||| ++ ||
Aves | ++ | + ||
Birds' eggs| |||
Reptilia | + | + ||| # + | + |
Amphibia | + | # + |
Pices | ++ || ++ ||
Insecta |++ | + ||| ++ ||| # + || ++ ||
Arachnida | ++ ||| # + | + |
Chilopoda | ++ |||
Diplopoda | # + |
Crustacea | ++ ||| # ++ |||
Mollusca | + || # + ||
Annelida | # + |
Nuts | ++ ||
Grains | ++ ||
Buds | + |
Fruit |++ ||| ++ # || ++ |||
Leaves | + |
Grass | + |
-----------+--------------------------------------------------------------
FOOD HABITS OF PROCYONIDS.--Food habits of six procyonids for which
metabolic data are available are presented in Table 9. All six species
clearly have mixed diets. Compared to other species, _Procyon lotor_
is highly catholic in its diet, taking food from almost twice as
many categories as _Nasua narica_, three times as many as _Procyon
cancrivorus_, _Nasua nasua_, and _Bassariscus astutus_, and nine times
as many as _Potos flavus_.
For those species for which food habit data are quantified, we used
Eisenberg's (1981:247-251) substrate/feeding matrix method, where
"substrate" is analogous to McNab's (1986a) "behavior," to construct
the following feeding categories that are based on the major food
groups utilized by each species (Table 9).