Chapter II: Part 2

The regression line for _Procyon lotor_ in winter (Table 5) extrapolates to zero metabolism at 35.2 deg.C, which is below normal T_{b} (Figures 6, 7). This suggests that not all raccoons measured in winter minimized thermoregulatory metabolism or conductances at T_{a}'s below T_{lc} (Scholander et al., 1950b; McNab, 1980b). To assess this possibility, data for these animals were divided into three groups: (A) females with radio transmitters, (B) females without radio transmitters, and (C) males (Table 6). Regression equations of metabolism below T_{lc} were derived for each group, and based on extrapolated T_{b}'s at zero metabolism, only the two females with implanted radio transmitters (group A) minimized thermoregulatory metabolism and conductance. Had animals in groups B and C also minimized their thermal conductances, while retaining their measured metabolic rates, their rates of heat production would have been disproportionately higher than their rates of heat loss. Equation 4 predicts that under these conditions their body temperatures would have been elevated to 42.0 deg.C and 40.4 deg.C, respectively. Thus, in order to avoid such a large increase in body temperature, animals in groups B and C increased their thermal conductances in preference to lowering their metabolic rates. The regression equation of thermoregulatory metabolism for all winter animals (Table 5), therefore, overestimates minimum metabolic cost of temperature regulation below T_{lc}, and its slope underestimates C_{mw}. Consequently, the best estimate of C_{mw} for _Procyon lotor_ in winter is the value calculated for group A animals with Eq. 4 (0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}; Table 3), and the minimum cost of thermoregulatory metabolism at any T_{a} below T_{lc} is best estimated by substituting this value into Eq. 4 and solving for [.H]_{r}.

THERMOREGULATION AT HIGH TEMPERATURES

_Body Temperature_

In both summer and winter, T_{b}'s increased during metabolic measurements at T_{a}'s above T_{lc} (Figure 6). This response also was seen during metabolic measurements conducted on other procyonids (Mueller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Mueller and Rost, 1983; Chevalier, 1985).

_Summer_

During summer our data suggested that the upper critical temperature (T_{uc}) was higher than 35 deg.C. The lowest rates of oxygen consumption at T_{a} = 35 deg.C occurred after 1.5 to 2.5 hours of exposure to that temperature. Prolonged exposure to this temperature in summer did not make animals restless, and their rate of oxygen consumption was very stable throughout each measurement. Body temperature responses at T_{a} = 35 deg.C were recorded from two males and two females that had implanted radio transmitters. With the exception of one male, T_{b}'s were maintained near 38 deg.C (Figure 6). The one exception (a male) maintained its T_{b} at 39.3 deg.C. At T_{a} = 35 deg.C, summer males had rates of evaporative water loss that were lower than those of summer females (Figure 4). At this temperature, males dissipated 35% +-6% and females 56% +-18% of their metabolic heat via evaporative water loss. Thus, at T_{a} = 35 deg.C, males must have utilized modes of heat transfer other than evaporative cooling (convective and conductive heat transfer) to a greater extent than females.

_Winter_

Body temperature, evaporative water loss, and metabolic data indicated that, in winter, T_{uc} was very close to 35 deg.C. In winter, the lowest level of oxygen consumption was recorded during the first hour after the chamber had reached T_{a} = 35 deg.C. Unlike summer, animals became restless after the first hour at 35 deg.C, at which point their oxygen consumption increased and showed a high degree of variability. Body temperature responses at 35 deg.C were recorded from both females that had implanted radio transmitters. In one case, T_{b} rose from 37.9 deg.C at the end of the first hour to 40.5 deg.C by the end of the second hour, and as it did not show signs of leveling off, we terminated the experiment. We exposed that same animal to T_{a} = 35 deg.C one other time during winter. In that instance, its T_{b} rose to 40.0 deg.C during the first 30 minutes and was maintained at that level for three hours with no apparent distress. The other female elevated its T_{b} from 37.3 deg.C to 39.0 deg.C during the second hour at T_{a} = 35 deg.C and maintained its T_{b} at that level for two hours. Thus, during winter, prolonged exposure to T_{a} = 35 deg.C stimulated more of an increase in T_{b} than it did in summer. During winter, both males and females increased evaporative water loss at T_{a} = 35 deg.C (Figure 5) but only to the extent that they dissipated 35% +-10% of their metabolic heat production. Thus, even in winter, convective and conductive heat transfers were still the most important modes of heat loss at this temperature.

DAILY CYCLE OF BODY TEMPERATURE

The daily cycle of raccoon T_{b}'s during summer and winter are presented in Figure 7. In general, T_{b}'s showed a marked circadian cycle in phase with photoperiod. T_{b}'s rose above 38 deg.C for several hours each night but remained below 38 deg.C during daytime. During summer, with the exception of one female whose record was not typical (Figure 7), T_{b}'s rose above 38 deg.C shortly after sunset, whereas in winter T_{b}'s did not rise above 38 deg.C until several hours after sunset. Once T_{b} was elevated it usually remained so until just before or after sunrise (Figure 7). During summer, T_{b} was above 38 deg.C for 85% or more of the time between sunset and sunrise (87% for the female with the typical body temperature pattern, and 85% and 98% for males), whereas in winter it was elevated for only 47%-78% of the time between sunset and sunrise (47% and 61% for females, and 67% and 78% for males). During night, T_{b} would oscillate between 38 deg.C and about 39 deg.C, such that two peak values occurred. These peak values presumably corresponded to two periods of heightened nighttime activity. During summer, one of these peaks occurred before and the other after 24:00 hours, whereas in winter both peaks occurred after 24:00 hours. With the exception of one female in winter (Figure 7), the lowest T_{b} of the day for both sexes was near 37 deg.C, and this typically occurred during daytime (Figure 7).

$Discussion$

BASAL METABOLIC RATE

_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).

1. _Potos flavus:_ (1) arboreal/frugivore, insectivore.

2. _Procyon cancrivorus:_ (1) semiaquatic/crustacivore,
molluscivore, insectivore, piscivore, carnivore.

3. _Nasua nasua:_ (1) terrestrial/insectivore, arachnidivore,
carnivore, frugivore.

4. _Bassariscus astutus:_ (1) terrestrial/carnivore, insectivore,
frugivore.

5. _Procyon lotor:_ (1) terrestrial/carnivore, granivore,
frugivore, insectivore; and (2) semiaquatic/crustacivore,
molluscivore, insectivore, piscivore, carnivore.

FOOD HABITS AND BASAL METABOLISM.--The most important foods in the diet of _Procyon lotor_ are vertebrates, nuts, seeds, and fruits (Table 9). These are the same foods that are eaten by those dietary specialists that have [.H]_{b}'s equivalent to, or higher than, values predicted for them by the Kleiber equation (McNab, 1986a). The most important foods in the diets of _Potos flavus_, _Procyon cancrivorus_, and _Nasua nasua_ are invertebrates and fruit (Table 9), and these foods are eaten by dietary specialists that have lower than predicted [.H]_{b}'s (McNab, 1986a). Major foods in the diet of _Bassariscus astutus_ are terrestrial vertebrates, insects, and fruit (Table 9). Dietary specialists that eat terrestrial vertebrates have higher than predicted [.H]_{b}'s, whereas those that feed on insects have [.H]_{b}'s that are lower than predicted (McNab, 1986a). Year-round utilization of vertebrates by _Bassariscus astutus_ suggests that it also should have a metabolic rate that is equivalent to or higher than predicted, rather than lower (McNab, 1986a). However, perhaps year-round inclusion of insects in its diet (Martin et al., 1951; Taylor, 1954; Wood, 1954; Toweill and Teer, 1977; Trapp, 1978), plus water- and energy-conserving advantages of a low metabolic rate, each exert a stronger selective influence on [.H]_{b} than do vertebrates in its diet.

SUMMARY.--The basal metabolic rate of these procyonids does appear to be influenced by diet. But, it is apparent from this family's evolutionary history and tropical origins that climate also has had a profound influence on its member's metabolism. The history of the family and the data presented here (Table 7) suggest that lower than predicted [.H]_{b} is a feature that evolved very early as the primary metabolic adjustment to a tropical climate. From this perspective, it could be argued that climate would have been the major selective force determining [.H]_{b}, whereas food habits would have had a secondary influence.

_Basal Metabolism and Intrinsic Rate of Natural Increase_

BACKGROUND.--McNab (1980a) suggested that if food is not restricted during an animal's reproductive period, the factor that will limit growth and reproduction will be the rate at which energy can be used in growth and development. Under these conditions, an increase in [.H]_{b} would actually increase r_{max} because it would provide a higher rate of biosynthesis, a faster growth rate, and a shorter generation time. Hennemann (1983) tested McNab's (1980a) premise and found a significant correlation between r_{max} and metabolic rate, independent of body size, for 44 mammal species. A low correlation coefficient for this relationship, however, indicated to him (Hennemann, 1983) that factors such as (1) food supply, (2) thermal characteristics of the environment, and (3) brain size also contribute toward shaping a species' reproductive potential, particularly when these factors strongly influence rates of biosynthesis or growth or for some reason alter generation time. Results of our estimates of r_{max} for procyonids are presented in Table 10.

_Procyon lotor._--This species had the highest [.H]_{b} and D_{d}, and also had the highest r_{max} (1.34; Table 10). Such a high r_{max} may infer that this trait evolved under conditions where food and temperature were not limiting to reproduction. Under these conditions selection could have favored those reproductive characteristics sensitive to a higher [.H]_{b} (biosynthesis, growth, and generation time; McNab, 1980a). _Procyon lotor_'s high reproductive potential is due to its early age of first female reproduction and its large litter size, characteristics that may reflect metabolically driven increases in both biosynthesis and growth.

_Bassariscus astutus._--This species has a low [.H]_{b} but an r_{max} that was 124% of expected (Table 10). This suggests that r_{max} evolved under conditions where food and temperature were not limiting to reproduction. Reduced litter size should restrict this species' reproductive potential and may be a reflection of its low [.H]_{b}. The factor that is responsible for increasing its reproductive potential, however, is its early age of first female reproduction. _Bassariscus astutus_ is the smallest of these procyonids, and even though it has a low [.H]_{b}, its small mass may contribute to its ability to reach adult size and sexual maturity in its first year. The high quality of its diet (a high proportion of small vertebrates; Table 9) also may be a factor that is permissive to early female reproduction. Thus, small body size and diet may be factors that have allowed this species to evolve a higher than expected reproductive potential in spite of its low [.H]_{b}.

_Nasua narica._--This species is one of the largest procyonids (Table 7), and it possesses characteristics that should limit its reproductive potential: lower than predicted [.H]_{b} (Table 7), a relatively low-quality diet (Kaufmann, 1962:182-198; Table 9), and delayed time of first reproduction (Table 10). In spite of this, _Nasua narica_ has a higher than expected r_{max} (111% of predicted; Table 10). The life history feature that enhances _Nasua narica_'s reproductive potential, and increases r_{max} beyond expected, is its large litter size. In this species females live in bands. Each year just before their young are born these bands break up, and each female seeks out a den for herself and her litter. Once the young are able to leave the den (approximately five weeks), bands reform. In this situation, females not only care for their own young but also for those of other females in the band (Kaufmann, 1962:157-159, 1982, 1987; Russell, 1983). This social structure may contribute to this species' ability to produce large litters and in this way increase its reproductive potential.

TABLE 10.--Intrinsic rate of natural increase (r_{max}) of several
procyonids. (a = potential age of females producing first young;
b = potential annual birth rate of female young (= average litter
size/2; average litter size was calculated from the published range
of litter sizes for each species); n = potential age of females
producing their final young; r_{maxe} = intrinsic rate of natural
increase expected from body mass (Hennemann, 1983); r_{maxr} = ratio
of calculated to expected intrinsic rate of natural increase
(r_{max}/r_{maxe}).)

---------------------+------------------------------------------------
|
Species |Body mass a b n r r [a] r [b]
| (g) max maxe maxr
---------------------+------------------------------------------------
_Procyon lotor_ | 4940 0.83 2.25 16 1.34 0.53 2.52
|
|
|
_Bassariscus astutus_| 900 0.83 1.50 14 1.02 0.82 1.24
|
|
|
_Nasua narica_ | 3900 2.50 2.25 14 0.62 0.56 1.11
|
_Nasua nasua_ | 3850
_Procyon cancrivorus_| 1160 0.83 1.50 15 1.02[c] 0.77 1.32
| 1.75 0.65[c] 0.84
_Potos flavus_ | 2490 1.75 0.50 12 0.30 0.63 0.48
|
_Bassaricyon gabbii_ | 1600 1.75 0.50 15 0.32 0.71 0.45
|
---------------------+------------------------------------------------

---------------------+------------------------------------------------
|
Species | References
---------------------+------------------------------------------------
_Procyon lotor_ | Dunn and Chapman (1983); Eisenberg (1981:489);
| Kaufmann (1987); Lotze and Anderson (1979);
| Nowak and Paradiso (1983:981); Sanderson
| (1987); Stains (1956:28-31); This study
_Bassariscus astutus_| Kaufmann (1982, 1987); Nowak and Paradiso
| (1983:979, 980); Poglayen-Neuwall and
| Poglayen-Neuwall (1980); Poglayen-Neuwall
| and Toweill (1988); Russell (1983)
_Nasua narica_ | Kaufmann (1982, 1987); Nowak and Paradiso
| (1983:983); Sanderson (1983)
_Nasua nasua_ | Chevillard-Hugot et al. (1980)
_Procyon cancrivorus_| Crandall (1964:312); Poglayen-Neuwall (1987)
|
_Potos flavus_ | Ford and Hoffmann (1988); Nowak and Paradiso
| (1983:984)
_Bassaricyon gabbii_ | Eisenberg (1981:489); Nowak and Paradiso
| (1983:985)
---------------------+------------------------------------------------

[a] r_{maxe} = 4.9.m^{0.2622}, where m is body mass in grams.

[b] Regression of r_{max} on body mass (m). Assume r_{max} = 1.02 for
_Procyon cancrivorus_: r_{max} = 0.00005.m + 0.623; R = 0.19;
R squared = 0.03; Regression of r_{maxr} (Table 10) on H_{br} (Table 7);
assume _Nasua nasua_ has the same r_{maxr} as _Nasua narica_:
r_{maxr} = 3.35.H_{br} - 1.11; R = 0.93; R squared = 0.86.

[c] Estimate based on females reproducing in their first (a = 0.83) or
second (a = 1.75) year.

_Nasua nasua._--Unfortunately, there is not enough reproductive data to allow calculation of r_{max} for _Nasua nasua_ (Table 10), therefore, it is not possible to compare the reproductive potential of this South American coati with its North American relative, _Nasua narica_. Given its low [.H]_{b} and relatively low-quality diet of fruit and terrestrial invertebrates (Table 9), however, r_{max} of _Nasua nasua_ may be very similar to that of _Nasua narica_.

_Procyon cancrivorus._--The age of first female reproduction for _Procyon cancrivorus_ has not been reported. However, if one assumes females can reproduce in their first year, r_{max} for _Procyon cancrivorus_ would be 1.02 (132% of expected; Table 10). If, on the other hand, first female reproduction is delayed until the second year, r_{max} would be 0.65 (84% of predicted; Table 10). _Procyon cancrivorus_ has a low [.H]_{b}, reduced litter size, and small body mass. Its low [.H]_{b} may limit litter size, but as with _Bassariscus astutus_, the quality of its diet (a high percentage of small vertebrates; Table 9) and its small body size may make it possible for females to reproduce in their first year and thus increase the species' reproductive potential. This reasoning would argue that _Procyon cancrivorus_ probably enjoys higher, rather than lower, than expected r_{max}.

_Potos flavus._--In addition to a low [.H]_{b}, this species possesses other characteristics that limit its reproductive potential: low-quality diet, delayed reproduction, and birth of a single young each year. Because there does not appear to be any other feature of its life history that can counteract the influence of these factors, r_{max} in _Potos flavus_ has evolved to be only 48% of expected (0.30; Table 10). Its close relative, the olingo, _Bassaricyon gabbii_, appears to share the same condition (Table 10).

SUMMARY.--This brief survey illustrates that, with the exception of _Potos flavus_, procyonids tend to have values of r_{max} that are higher than those predicted for them on the basis of mass (Table 10). Regression analysis indicates that, within the family, body mass accounts for only a small amount (3%) of the variation in r_{max}, whereas the positive slope of the correlation between r_{maxr} and H_{br} (R = 0.93) suggests that low metabolism has a limiting effect on r_{max} (see Table 10, footnote f). The implication here is that low [.H]_{b} would be associated with a lower rate of biosynthesis, a slower growth rate, and a longer generation time. Procyonids with low [.H]_{b} but higher than expected r_{max} must possess other traits that serve to offset the effects of low metabolism. Our survey indicates that the following features compensate for low [.H]_{b} and help increase r_{max}: (1) a high-quality diet may make biosynthesis and growth more efficient, thus optimizing the time element associated with each of these processes; (2) larger litter sizes and cooperation in care of the young may increase survivorship in spite of a slower growth rate; and (3) an early age of first reproduction, a long reproductive life span, and moderate-size litters (two to four young) may in the long run add as many individuals to the population as a shortened generation time. Our survey also suggests that, at the other extreme, factors such as a low-quality diet, reduced litter size, absence of cooperative care of the young, delayed age of first reproduction, and shortened reproductive life span all serve to decrease r_{max}. Thus, it is obvious that diet, litter size, social structure, reproductive strategy, and reproductive life span can operate synergistically with [.H]_{b} to magnify its influence on r_{max} (as with _Procyon lotor_ and _Potos flavus_), or they can function in opposition to [.H]_{b} to change the direction of its influence on r_{max} (as with _Bassariscus astutus_, _Procyon cancrivorus_, _Nasua narica_, and perhaps _Nasua nasua_).

_Basal Metabolism and Climatic Distribution_

_Procyon lotor._--The evolution of a higher [.H]_{b} (Tables 7, 8) may have been the physiological cornerstone that enabled _Procyon lotor_ to break out of the mold being exploited by other procyonids and to generalize its use of habitats and climates. Once this basic physiological change was in place, selection for appropriate alterations in thermal conductance, capacity for evaporative cooling, diversity of diet, and energy storage would have provided this species with the suite of adaptations needed to extend its distribution into other habitats and climates. Support for this concept follows from the fact that high levels of [.H]_{b} are associated with (1) cold-hardiness in mammals that live in cold-temperate and arctic climates (Scholander et al., 1950c; Irving et al., 1955; Irving, 1972:115, 116; Shield, 1972; Vogel, 1980; Golightly and Ohmart, 1983); (2) the ability to utilize a wide variety of food resources and to occupy a large number of different environments and habitats (McNab, 1980a); and (3) a high intrinsic rate of natural increase (McNab, 1980a; Hennemann, 1983; Lillegraven et al., 1987; Nicoll and Thompson, 1987; Thompson, 1987).

OTHER PROCYONIDS.--Other procyonids (_Potos flavus_, _Procyon cancrivorus_, _Nasua narica_, and _Nasua nasua_) have lower than predicted [.H]_{b}'s (Table 7), a characteristic that is considered to be an energy-saving adaptation for those that live in relatively stable tropical and subtropical habitats (Mueller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Mueller and Rost, 1983). However, _Bassariscus astutus_ is found in tropical, subtropical, and temperate climates. This species is found from tropical Mexico to temperate regions of the western United States (Kaufmann, 1982, 1987; Nowak and Paradiso, 1983:979). In the northern part of its distribution, _Bassariscus astutus_ lives in habitats that are unstable (arid regions), that are low in productivity, and that characteristically have marked seasonal changes in temperature. Its lower than predicted [.H]_{b} could be an important water-conserving adaptation at times when temperatures are high (McNab and Morrison, 1963; McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979) and an important energy-conserving mechanism when cold weather may limit food availability and hunting time (Scholander et al., 1950c; Wang et al., 1973). As will be seen later, _Bassariscus astutus_ is unique among procyonids with lower than predicted [.H]_{b}'s in that it also has a lower than predicted C_{mw} (Table 7). This allows it to use less energy than expected for thermoregulation at low temperatures. Another species with a similar set of adaptations (lower than predicted [.H]_{b} and C_{mw}) is the arctic hare, _Lepus arcticus_ (Wang et al., 1973), which lives in one of the coldest and least-productive regions on earth. Wang et al. (1973) suggest that this combination of adaptations allows _Lepus arcticus_ to better match its energy requirements to the low productivity of its environment. A similar relationship may hold for _Bassariscus astutus_, particularly in colder arid portions of its distribution, and may be the reason that it, but not other procyonids with low [.H]_{b}'s, has been able to inhabit temperate climates.

MINIMUM THERMAL CONDUCTANCE

_Background_

Thermal conductance is a measure of the ease with which heat is passively transferred to or from a body through its tissues and pelt. Within T_{n}, a mammal is able to vary its thermal conductance over a wide range of values by changing heat transfer characteristics of both of these layers. Minimum thermal conductance occurs when total heat transfer through these layers is reduced to its lowest possible rate. This minimum value, which is the reciprocal of maximum resistance, occurs, theoretically, but not always practically (see McNab, 1988b), at the animal's T_{lc} and is best estimated under standard conditions in a metabolism chamber (McNab, 1980b; Aschoff, 1981). Minimum thermal conductance scales to body mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff, 1981). Therefore, to make comparisons between species of various sizes, we scaled out body mass by expressing C_{mw} as the ratio of measured to predicted values (C_{mwr}; Table 7). These ratios were used to make comparisons of heat-transfer characteristics between species that occupy different habitats or climates.

_Effect of Molt on Thermal Conductance_

In summer, T_{lc}'s of male and female _Procyon lotor_ (Figure 2) were very similar to those of other procyonids (22 deg.C-26 deg.C; Table 7). In winter, T_{lc} of both sexes shifted downward to 11 deg.C (Figure 3). This seasonal shift in T_{lc} occurred as the result of a seasonal change in minimum thermal conductance (Table 3). For many northern mammals, a seasonal change in thermal conductance is partly mediated via cyclic changes in the insulative quality of their pelt (Scholander et al., 1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165).

_Procyon lotor_ begins to shed its heavy winter coat about the time its young are born. Molt progresses through summer and by late August the new coat is complete (Stuewer, 1942). During its summer molt, _Procyon lotor_'s C_{mw} increased by about 49% over the value for female raccoons in winter (Table 3). In summer, therefore, it had the highest mass specific C_{mw} of those procyonids considered (C_{mwr} = 1.77 and 1.79; Table 7). An increase in thermal conductance facilitates passive heat loss for temperate and arctic species, and this serves as an important thermoregulatory adaptation during warm summer months (Scholander et al., 1950c; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165). This adaptation is particularly important to those temperate- and arctic-zone species (including raccoons) whose [.H]_{b}'s do not decrease during summer (Irving et al., 1955). From August on, the fur of _Procyon lotor_ becomes increasingly longer and heavier, with peak, or prime, condition occurring in late fall and early winter (Stuewer, 1942). Minimum conductance of our captive raccoons was lowest in winter (C_{mwr} = 1.15) when their pelts were in prime condition (Tables 3, 7). Because "primeness" of raccoon pelts varies geographically, thicker pelts being associated with colder climates (Goldman, 1950:21; Whitney and Underwood, 1952:24-41), the degree of seasonal change in C_{mw} must also vary geographically.

The only other procyonid for which a seasonal molt has been described is _Bassariscus astutus_. Molt in this species extends from late summer to late fall (Toweill and Toweill, 1978). How molt effects thermal conductance in _Bassariscus astutus_ is not known because metabolic data for this species (Table 7) apparently were collected only when their pelts were in prime condition (Chevalier, 1985).

Goldman (1950:20) reports that _Procyon cancrivorus_ does not have a seasonal molt. Like other tropical procyonids, _Procyon cancrivorus_ lives in an environment that has the following characteristics: high even temperatures throughout the year (1 deg.C-13 deg.C difference in monthly mean temperature), a greater range in temperature between day and night than in mean monthly temperature throughout the year, uniform lengths of day and night, seasonal variation in rainfall, and lowest temperatures during the rainy season(s) (Kendeigh, 1961:340). In such a stable environment there would be no advantage to a sharply defined seasonal molt cycle that could place an animal in thermoregulatory jeopardy by increasing its thermal conductance. This would be particularly true for animals like tropical procyonids that have lower than predicted [.H]_{b}'s but that maintain typical eutherian body temperatures (Table 7). Consequently, molt in all tropical procyonids may either be prolonged or continuous. This is a feature of their biology that needs to be examined in more detail.

_Comparison of Thermal Conductances_

_Procyon lotor_ VERSUS TROPICAL PROCYONIDS.--C_{mwr} for _Procyon lotor_ in winter was 1.15, which is similar to the values for _Potos flavus_ and _Procyon cancrivorus_, 1.02 and 1.25, respectively (Table 7). These two tropical species, therefore, have C_{mw}'s that are similar on a mass specific basis to the value for _Procyon lotor_ in winter. However, at their T_{lc}'s, the thermal gradient sustained by these tropical animals is only about 11 deg.C, whereas for _Procyon lotor_ in winter it was 26.5 deg.C. Examination of Eq. 4 with respect to these thermal gradients suggests that tropical procyonids achieve such low C_{mw}'s by virtue of their lower than predicted [.H]_{b}'s rather than by having pelts that are exceptionally good insulators. In fact, the insulation afforded by the pelts of these tropical procyonids is about the same as that of the 50 g arctic lemming, _Dicrostonyx groenlandicus rubricatus_, whose coat has an insulative value that is about half that of the hare, _Lepus americanus_, red fox, _Vulpes fulva alascensis_, and pine martin, _Martes americana_, animals comparable in size to these procyonids (Scholander et al., 1950a). Therefore, pelts of these tropical procyonids do not have the same insulative value as the prime winter coat of _Procyon lotor_.

_Nasua narica_ and _Nasua nasua_ have tropical and subtropical distributions and they are the only procyonids that are diurnal (Kaufmann, 1962:103-105, 1982, 1987). Because they are active during the day they experience a more extreme thermal environment (higher T_{a}'s and solar radiation) than their nocturnal cousins. Values of C_{mwr} for _Nasua narica_ (1.45 and 1.55) and _Nasua nasua_ (1.24 and 1.65) are higher than those for _Procyon cancrivorus_ or _Potos flavus_ (Table 7). Thus, these coatis have higher mass specific C_{mw}'s than their nocturnal tropical cousins. A high C_{mw} reduces the cost of thermoregulation in hot environments because it increases an animal's ability to lose excess heat passively. The higher C_{mw}'s of these coatis serve as an adaptation that contributes to the success of their diurnal life style as well as their ability to expand their habitat use to areas with less thermal stability, such as oak and pine woodlands and deserts.

_Bassariscus astutus._--This species has the lowest mass specific C_{mw} of these procyonids (C_{mwr} = 0.85; Table 7), which indicates that its pelt has a greater insulative value than the coats of _Potos flavus_, _Procyon cancrivorus_, _Nasua nasua_, or _Nasua narica_. This, coupled with a lower than predicted [.H]_{b}, allows _Bassariscus astutus_ to maintain T_{b} with less energy expenditure than is possible for any other procyonid of comparable size; and this combination of adaptations provides _Bassariscus astutus_ with a distinct energy advantage in environments that have low productivity (Wang et al., 1973). The evolution of a pelt that provides better insulation must be considered an important contributing factor for the spread of this species into desert regions of the western United States.

THERMOREGULATION AND USE OF STORED FAT AT LOW TEMPERATURES

_Background_

THERMOREGULATION.--At temperatures below a mammal's T_{n}, heat loss exceeds [.H]_{b}. To maintain T_{b} under these conditions, metabolic rate must be increased (Eq. 4). _Procyon lotor_ in summer during its annual molt (Table 5; Figure 2), _Bassariscus astutus_ (Chevalier, 1985), _Nasua nasua_ (Chevillard-Hugot et al., 1980; Mugaas et al., in prep.), _Nasua narica_ (Scholander et al., 1950b; Mugaas et al., in prep.), and _Potos flavus_ (Mueller and Kulzer, 1977; Mueller and Rost, 1983) all are able to elevate their metabolic rates by 130% above basal when they are exposed to T_{a} = 0 deg.C. _Procyon cancrivorus_ responds to 0 deg.C with an increase in metabolic rate of 257% above basal (Scholander et al., 1950b). All animals listed have about the same T_{lc} and T_{b}, so the temperature differential producing this response is about the same for each species. Metabolic ability to defend body temperature against low ambient temperatures, therefore, is well developed in these procyonids. Such large increases in metabolic rate are energetically expensive, and if these animals were routinely exposed to T_{a} = 0 deg.C, it would be difficult for them to acquire enough food each day to maintain endothermy. Raccoons in winter pelage, however, need only elevate their metabolic rate by 47% above basal to maintain endothermy at T_{a} = 0 deg.C (Table 5; Figure 3). Each year at the completion of its molt, the raccoon's highly insulative pelt is renewed. This lowers their T_{lc} by 9 deg.C to 15 deg.C below that measured for them in summer (Figure 3) and decreases their cost of thermoregulation at low temperatures. The increased insulative capacity of their pelt is one of the primary adaptations that has allowed _Procyon lotor_ to extend its distribution into cold climates.

STORED FAT.--Cyclic fattening is an integral and important part of a raccoon's annual cycle (Mugaas and Seidensticker, ms); however, it has not been reported for other procyonids. During winter in parts of the United States and Canada, raccoons are confined to their dens for variable periods of time (days to months) depending on the severity of the weather (Stuewer, 1943:223-225; Whitney and Underwood, 1952:108-116; Sharp and Sharp, 1956; Mech et al., 1968; Schneider et al., 1971). During this confinement, they do not hibernate but rather enter a state of "dormancy" and become inactive. While dormant they remain endothermic (T_{b} > 35 deg.C; Thorkelson, 1972:87-90) and derive most of their energy requirement from fat reserves accumulated during fall. The rate at which fat stores are consumed during winter dormancy depends on the thermoregulatory requirement imposed on them by local weather conditions, the insulative quality of their pelt, and any advantage they may gain by seeking shelter in a den.

_Thermal Model of the Raccoon and Its Den_

Heat transfer between an animal and its environment is a function of the interaction of its body temperature and thermal conductance with various environmental variables (air temperature, wind speed, vapor pressure, and thermal radiation). When a raccoon is outside its den, its thermal conductance (C_{mw}) is the only barrier to heat transfer with the external environment. However, when it enters a tree den, a raccoon imposes two other thermal barriers between itself and the external environment: (1) conductance of the air space between its fur and the den's walls (C_{a}) and (2) conductance of the den's walls (C_{d}; Thorkelson, 1972:59-63; Thorkelson and Maxwell, 1974). Thorkelson and Maxwell (1974) modeled heat transfer of a simulated raccoon (a water-filled aluminum cylinder equipped with a heater and covered with a raccoon pelt) in a closed tree den. In their system, 65% of resistance to heat flux was attributable to the pelt, whereas the remainder (35%) was due to C_{a} and C_{d}. Because resistance is the inverse of conductance, and resistances for the raccoon and its den are arranged in series, we can estimate total conductance (C_{t}) of this system with Eq. 7.

1/C_{t} = 1/C_{mw} + 1/C_{a} + 1/C_{d} Eq. 7

Minimum thermal conductance C_{mw} for raccoons in winter was 0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1} (Table 3). Based on Thorkelson and Maxwell's (1974) model we let 1/C_{mw} = 0.65(1/C_{t}) = 1/0.0172 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, and 1/C_{a} + 1/C_{d} = 0.35(1/C_{t}). Substituting these values into Eq. 7 and solving for C_{t} yields 0.0112 mL O_{2}.g^{-1}.h^{-1}. deg.C^{-1}, a value that is 35% lower than that of the animal alone. Substituting this value and the value for basal metabolism of winter raccoons (0.47 mL O_{2}.g^{-1}.h^{-1}; Table 7) into Eq. 4 and solving for (T_{b} - T_{a}) yields a new temperature differential of 42 deg.C. Therefore, by using tree dens, raccoons in north central Virginia, with T_{b} = 37 deg.C (Figure 7), could effectively reduce their T_{lc} from 11 deg.C to -5 deg.C and markedly reduce their metabolic cost of thermoregulation.

_Metabolic Advantage of the Den_

Given prevailing winter temperatures in north central Virginia (see "Materials and Methods"), adult raccoons in that area should be able to sustain endothermy most of the time they are in their dens by simply maintaining [.H]_{b}. Depending on the mass of their stored fat, they could remain in their dens for several weeks without eating (Mugaas and Seidensticker, ms). The thermal advantage of a den could be further enhanced during colder temperatures if two or more raccoons occupied it at the same time and huddled together, and/or if these animals could reduce C_{mw} even more by lowering T_{b} and cooling their extremities. Although we do not have any data to verify the second mechanism, there are many accounts in natural history literature that document raccoons occupying dens together (Lotze and Anderson, 1979). This habit could be particularly important for the young of the year and may be one reason why they often continue to den with their mothers during winter (Lotze and Anderson, 1979; Seidensticker et al., 1988). Raccoons that live in colder climates, such as Minnesota, undoubtedly obtain the same advantage from a den as Virginia animals, but because of their greater body mass, longer fur, and potentially lower C_{mw}, T_{lc} of a Minnesota raccoon in a den could be even lower than what we calculated for Virginia raccoons. Therefore, when they are in their dens, raccoons living in very cold climates also may be able to maintain homeothermy with a basal level of metabolism.

THERMOREGULATION AT HIGH TEMPERATURES

_Background_

In hot environments mammals depend on behavior to minimize their thermal load (escape to shaded or cooler microclimates, use posture and orientation to wind and sun, restrict activity, become nocturnal, etc.) and on evaporative water loss to rid themselves of excess heat. With regard to evaporative heat loss, Calder and King (1974:326) arbitrarily subdivided the response to various T_{a}'s as follows: "(1) cool temperatures at which water loss should be minimized, both to reduce heat loss and as an adaptation to terrestriality; (2) an intermediate temperature range wherein evaporation is gradually increased as dry heat losses are proportionately reduced with smaller thermal gradients; and (3) warm to hot temperatures at which evaporation must be actively increased to dispose of metabolic and exogenous heat loads." Some mammals are able to thermoregulate very well at high ambient temperatures via panting or sweating, whereas others have a very limited capacity. Hence, there is no general approach to calculating evaporative water loss under these conditions (Campbell, 1977:85). However, the ratio of evaporative heat lost to metabolic heat produced can be used to quantify a species' capacity for evaporative cooling and to make comparisons between species.

_Comparison of Procyonid Responses to Heat Stress_

_Potos flavus._--This species lives in Neotropical forests of Central and South America. It is nocturnal, arboreal in habit, and appears to be the most heat-sensitive of these procyonids. Its T_{uc} is at 30 deg.C to 33 deg.C (Table 7; Mueller and Kulzer, 1977; Mueller and Rost, 1983). It begins to pant at about 30 deg.C, but its efforts at evaporative cooling are very ineffective. At 33 deg.C _Potos flavus_ can dissipate 33% of its metabolic heat via evaporative water loss, but at 35 deg.C the efficiency of this mechanism falls to 20% (Mueller and Rost, 1983). Consequently, when exposed to T_{a}'s above 33 deg.C, any kind of excitement causes its T_{b} to rise rapidly in an uncontrolled manner (Mueller and Kulzer, 1977; Mueller and Rost, 1983). These animals rely on their nocturnal and arboreal habits to keep them out of situations that could lead to hyperthermia (Mueller and Kulzer, 1977; Mueller and Rost, 1983).

_Nasua nasua_ and _Nasua narica_.--_Nasua nasua_ is abundant in tropical and subtropical South America, whereas _Nasua narica_ occupies the same climates in North America from southern Arizona and New Mexico south through Panama and on into Colombia and Ecuador (Hall and Kelson, 1959:892; Ewer, 1973:391, 392; Poglayen-Neuwall, 1975). Both coatis are diurnal and forage primarily on the ground (Kaufmann, 1962:185-188, 1987; Poglayen-Neuwall, 1975; Nowak and Paradiso, 1983:982), consequently they are exposed to a more severe thermal environment while active (higher T_{a}'s and solar radiation) than are nocturnal procyonids. Both coatis are more heat-tolerant than _Potos flavus_; their T_{uc}'s are higher (33 deg.C-35 deg.C; Table 7), they can tolerate T_{a}'s of 35 deg.C without raising their T_{b}'s (Chevillard-Hugot et al., 1980; Mugaas et al., in prep.), and they have a greater capacity for evaporative cooling than _Potos flavus_ (Mugaas et al., in prep.). The greater heat tolerance of these coatis is compatible with their diurnal habits and widespread distribution in a variety of forest habitats in both tropical and subtropical areas of the western hemisphere.

_Bassariscus astutus._--In addition to living in Neotropical forests of Mexico, _Bassariscus astutus_ also flourishes in hot arid climates, and it has extended its range much farther north than _Nasua narica_ (Hall and Kelson, 1959:881,892; Poglayen-Neuwall, 1975; Kaufmann, 1982). Its T_{uc} is higher (35.5 deg.C; Table 7) than that of _Potos flavus_, but it is comparable to those of _Nasua nasua_ and _Nasua narica_. Its capacity for evaporative cooling is well developed; at 40 deg.C _Bassariscus astutus_ is able to dissipate 100% of its resting metabolic heat via evaporative water loss, and at 45 deg.C it is able to dissipate 172% (Chevalier, 1985). In spite of its great capacity for evaporative cooling, this species is nocturnal, a habit that, along with its low [.H]_{b}, should allow it to keep thermoregulatory water requirements to a minimum.

_Procyon lotor._--Our data suggested that T_{uc} for _Procyon lotor_ in winter was comparable to that for _Bassariscus astutus_ (35 deg.C), and that in summer it was even higher. When exposed to temperatures near the upper end of its T_{n}, _Procyon lotor_ increased the gradient for passive heat loss with a controlled rise in T_{b} (Figure 6). In summer its capacity for passive heat loss was enhanced by the molt of its heavy winter fur. _Procyon lotor_'s capacity for evaporative cooling also appeared to be well developed, although our animals were not heated to the point that evaporative cooling was fully expressed (Figures 4, 5). However, _Procyon lotor_ is nocturnal, and this may allow it to eliminate, or at least reduce, the need for evaporative cooling, even in hot climates. Thus, _Procyon lotor_ appears to be well equipped physiologically and behaviorally to cope with thermal demands of hot environments in its distribution.

_Procyon cancrivorus._--Unfortunately, data for the crab-eating raccoon are not complete enough at high temperatures to include it in this survey.

SUMMARY.--This comparison demonstrates that capacity for evaporative cooling, tolerance of an elevated T_{b} to enhance passive heat loss, and behavioral avoidance of thermal stress are the primary methods used by procyonids to thermoregulate at high temperatures. _Procyon lotor_ and _Bassariscus astutus_, whose distributions extend into temperate regions, have developed these abilities to a greater extent than other procyonids. _Potos flavus_, whose distribution is confined to lowland tropical forests, has the least ability in this regard. _Nasua nasua_ and _Nasua narica_ appear to have thermoregulatory abilities that are intermediate to those of _Bassariscus astutus_ and _Potos flavus_. This suggests that ancestral procyonids may have had poor to modest ability to thermoregulate at high temperatures, a condition that would have limited their ability to leave the thermal stability afforded by tropical forests. Dispersal into temperate climates, therefore, required not only increased cold tolerance but also selective enhancement of those mechanisms used in thermoregulation at high temperatures.

TABLE 11.--Distribution by climate of selected procyonid species.

-----------------------+---------------------------------------------
| Mild[a] Cold[b]
Species | Tropics Subtropics temperate temperate
-----------------------+---------------------------------------------
_Procyon lotor_ | + + + +
_Bassariscus astutus_ | + + +
_Nasua nasua_ | + +
_Nasua narica_ | + +
_Procyon cancrivorus_ | + +
_Potos flavus_ | +
-----------------------+---------------------------------------------

[a] Extends from the subtropics north to the northern limit of
_Bassariscus astutus_' distribution (Hall and Kelson, 1959:881),
which approximates the 10 deg.C isotherm for average annual
temperature in the United States (Kincer, 1941).

[b] Extends northward from the 10 deg.C isotherm for average annual
temperature in the United States.

COMPOSITE SCORES OF ADAPTIVE UNITS AND GEOGRAPHIC DISTRIBUTION

In Table 11, procyonid species are arranged in descending order with respect to the number of major climates that are included in their geographic distributions (Hall and Kelson, 1959:878-897; Poglayen-Neuwall, 1975; Kortlucke and Ramirez-Pulido, 1982; Nowak and Paradiso, 1983:977-985). Composite scores ranged from a high of 1.47 for _Procyon lotor_ to a low of 0.39 for _Potos flavus_, whereas _Nasua nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Bassariscus astutus_ had intermediate values ranging from 0.64 to 0.79 (Table 12). Figure 8 demonstrates that there is a direct relationship between the number of climates these species occupy and their composite scores. Regression analysis (Y = 2.68.X + 0.24; where Y is number of climates, and X is composite score) demonstrates a high degree of correlation between these variables (R = 0.94) and indicates that 89% of the variance in distribution can be explained by composite scores. The various combinations of adaptations expressed by these species do, therefore, play a role in delimiting their climatic (latitudinal) distributions.

_Procyon lotor's_ normalized scores were higher in all categories than those of other procyonids. _Procyon lotor_, therefore, possesses those traits that have allowed it to become the premier climate generalist of the procyonid family. As an adaptive unit, these traits provide _Procyon lotor_ with the physiological and behavioral flexibility required to take full advantage of a wide range of climates and habitats, and its distribution verifies that it has done so. Even so, it is probably not fair to assume that this species represents a perfect physiological match with climate over its entire distribution. _Procyon lotor_ is, in many respects, still a forest-dwelling species, and its ability to expand its distribution into other habitats such as prairie and desert may well be due, in part, to its use of behavior to take advantage of favorable microclimates in otherwise hostile environments (Bartholomew, 1958, 1987). This feature of _Procyon lotor's_ biology needs to be further examined.

TABLE 12.--Normalized and composite scores for selected procyonids.
(H_{br} = ratio of measured to predicted basal metabolism (Table 7),
C_{mwr} = ratio of measured to predicted minimum thermal conductance
(Table 7), D_{dr} = ratio of food categories actually utilized by
each species to total food categories eaten by all six species
(calculated from Table 9), r_{maxr} = ratio of calculated to
expected r_{max} (Table 10).)

----------------------+----------------------------------------------
| Normalized scores
Species |---------------------------- Composite[a]
|H_{br}/C_{mwr} D_{dr} r_{maxr} score
----------------------+----------------------------------------------
_Procyon lotor_ | 0.95 0.95 2.52 1.47
_Bassariscus astutus_ | 0.80 0.33 1.24 0.79
_Nasua nasua_ | 0.48 0.33 1.11[b] 0.64
_Nasua nasua_ | 0.48 0.33 1.11[b] 0.64
_Nasua narica_ | 0.40 0.53 1.11 0.68
_Procyon cancrivorus_ | 0.55 0.33 1.32 0.73
_Potos flavus_ | 0.60 0.11 0.48 0.39
----------------------+----------------------------------------------

[a] Composite score = [(H_{br}/C_{mwr}) + D_{dr} + r_{maxr}]/3.

[b] Value calculated for _Nasua narica_ (Table 10) and used with the
assumption that it must be similar to the value for _Nasua nasua_.

All five species with low [.H]_{b}'s have composite scores less than 1.0 (Table 12; Figure 8). Four of these five, _Nasua nasua_, _Nasua narica_, _Procyon cancrivorus_, and _Potos flavus_, have H_{br}/C_{mwr} ratios that are 0.6 or less, which indicates they are the least cold-tolerant procyonids (McNab, 1966). These four species also are confined to either tropic, or tropic and subtropic climates (Table 11). This suggests that these species share a common thermoregulatory adaptation that represents a specialization to these climates. Attendant with this adaptation, however, is a high cost of thermoregulation at temperatures below their T_{lc}, and this must be an important factor in limiting their distributions to tropic and subtropic climates. Differences in their distributions within these climates, therefore, must hinge more on differences in their D_{dr} and r_{maxr} values than on differences in their H_{br}/C_{mwr} ratios. This is supported by the fact that _Potos flavus_, which has the lowest D_{dr} and r_{maxr} values, is confined to a single climate, whereas _Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ each possess larger D_{dr} and r_{maxr} values and are found in two climates. Thus, _Potos flavus_, with its highly specialized diet and low reproductive potential, is the most ecologically specialized of these procyonids, and its distribution is limited to the single climate that can provide its requirements. _Nasua nasua_, _Nasua narica_, and _Procyon cancrivorus_ are less specialized and thus show more ecological flexibility in their distributions.