Part 5

+------------------------------------------------ | 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_