Adaptive Significance of the Variables
Basal Metabolic Rate and Intrinsic Rate of Natural Increase
Basal metabolic rate represents the minimum energy required by an animal to maintain basic homeostasis (Lusk, 1917:141; Kleiber, 1932, 1961:251; Benedict, 1938; Brody, 1945:59; Robbins, 1983:105-111). For mammals, Ḣbappears to be determined by complex interactions between their body size (Kleiber, 1932, 1961:206; Benedict, 1938; Brody, 1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; Calder, 1987), the climate in which they live (Scholander et al., 1950c; McNab and Morrison, 1963; Hulbert and Dawson, 1974; Shkolnik and Schmidt-Nielsen, 1976; McNab, 1979a; Vogel, 1980), their food habits (McNab, 1978a, 1978b, 1980a, 1983a, 1984a, 1986a, 1986b, 1988a, 1989), and their circadian period (Aschoff and Pohl, 1970; Prothero, 1984). Some species have higher mass-specific Ḣbthan others, and this variation appears to be tied to ecological circumstances rather than taxonomic affinities (McNab, 1988a, 1989). Basal metabolic rate is important ecologically because it serves as a measure of a species' minimum "obligatory" energy requirement, and under many circumstances, it represents the largest energy demand associated with a daily energy budget (King, 1974:38-55; McNab, 1980a; Mugaas and King, 1981:37-40). Recently it also has been implicated as a permissive factor with respect to rmaxof mammals (Hennemann, 1983; Lillegraven et al., 1987; Nicoll and Thompson, 1987; Thompson, 1987) via its direct effect on their rates of development and fecundity (McNab, 1980a, 1983a, 1986b; Hennemann, 1983; Schmitz and Lavigne, 1984; Glazier, 1985a, 1985b). The implication of this latter point is that those species with higher Ḣb's also have faster rates of development and greater fecundity and hence enjoy the competitive advantage of a higher rmax. Basal metabolism is, therefore, "a highly plastic character in the course of evolution" (McNab, 1988a:25) that has a profound influence on each species' life history.
Minimum Thermal Conductance
Whole-body resistance to passive heat transfer is equal to tissue resistance plus coat resistance. Within limits, these resistances can be altered; tissue resistance can be varied by changes in blood flow, whereas coat resistance can be changed by piloerection, molt, and behavior. When whole-body resistance is maximized (maximum tissue and coat resistances), passive heat transfer is minimized. The inverse of resistance is conductance; therefore, maximum whole-body resistance is the inverse of minimum thermal conductance (Cm). Minimum thermal conductance is readily derived from metabolic chamber data, and it is commonly used to describe an animal's capacity to minimize passive heat transfer. Minimum thermal conductance interacts with Ḣband body mass to set the maximum temperature differential a mammal can maintain without increasing its basal level of heat production. The low temperature in this differential is the lower critical temperature (Tlc).
Mass-specific Cmfor mammals is negatively correlated with body mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970, 1979b; Bradley and Deavers, 1980; Aschoff, 1981), and for any given mass its magnitude is 52% higher during the active, rather than the inactive, phase of the daily cycle (Aschoff, 1981). However, some mammals have Cm's that are higher or lower than would be predicted for them on the basis of body mass and circadian phase. Seasonalvariation in Cm(higher values during summer than winter) has been reported for many northern mammals that experience large annual variations in air temperature (Scholander et al., 1950a; Irving et al., 1955; Hart, 1956, 1957; Irving, 1972:165). Some tropical mammals with very thin fur coats, and others with nearly hairless bodies, have high Cm's (McNab, 1984a), as do burrowing mammals (McNab, 1966, 1979b, 1984a) and the kit fox,Vulpes macrotis(Golightly and Ohmart, 1983). Some small mammals with low basal metabolic rates tend to have lower than predicted Cm's: small marsupials (McNab, 1978a), heteromyid rodents (McNab, 1979a), several ant eaters (McNab, 1984a), the arctic hare,Lepus arcticus(Wang et al., 1973), the ringtail,Bassariscus astutus(Chevalier, 1985), and the fennec,Fennecus zerda(Noll-Banholzer, 1979). Thus, in spite of its mass dependence, Cmalso has been modified during the course of evolution by selective factors in the environment and by the animal's own metabolic characteristics.
Capacity for Evaporative Cooling
Latent heat loss occurs as a result of evaporation from the respiratory tract and through the skin, and except under conditions of heat stress, it "is a liability in thermal and osmotic homeostasis" (Calder and King, 1974:302). Ec, defined as the ratio of evaporative heat lost to metabolic heat produced, can be used to quantify thermoregulatory effectiveness of evaporative cooling and to make comparisons of heat tolerance between species. Thermoregulatory effectiveness of latent heat loss is not just a function of the rate of evaporative water loss but also of the rate of metabolic heat production (Lasiewski and Seymour, 1972). For example, a low metabolic rate minimizes endogenous heat load and thus conserves water, whereas the opposite is true of high metabolic rates (Lasiewski and Seymour, 1972). Some mammals that live in arid regions have evolved low metabolic rates and thus capitalize on this relationship to reduce their thermoregulatory water requirement (McNab and Morrison, 1963; McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979). What is evident, therefore, is that an animal's capacity for increasing latent heat loss must evolve together with its Ḣband Cmin response to specific environmental demands.
Diet
McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures of Ḣbfrom the Kleiber (1961:206) "norm" are highly correlated with diet and independent of phylogenetic relationships. McNab's analysis indicates that for mammals that feed on invertebrates, those species with body mass less than 100 g have Ḣb's that are equal to or greater than values predicted by the Kleiber equation, whereas those with body mass greater than 100 g have metabolic rates that are lower than predicted. Grazers, vertebrate eaters, nut eaters, and terrestrial frugivores also have Ḣb's that are equal to or greater than predicted, whereas insectivorous bats, arboreal folivores, arboreal frugivores, and terrestrial folivores all have rates that are lower than predicted. McNab (1986a) found animals with mixed diets harder to categorize, but in general he predicted that their Ḣb's would be related to (1) a food item that is constantly available throughout the year, (2) a food item that is most available during the worst conditions of the year, or (3) a mix of foods available during the worst time of the year. Although these correlations do not establish cause and effect between food habits and Ḣb, McNab's analysis does make it clear that the relationship between these variables has very real consequences for an animal's physiology, ecology, and evolution.
Experimental Design and Summary
In this investigation we measured basal and thermoregulatory metabolism, evaporative water loss, and body temperature of raccoons from north central Virginia. Measurements were conducted on both sexes in summer and winter to determine how season and sex influenced these variables. We then compared the data for this widely distributed generalist with data from literature for its ecologically more restricted relatives. Dietary data for all species were taken from literature, as were reproductive data for calculation of rmax.
Our analysis demonstrated clear differences betweenProcyon lotorand other procyonids with respect to Ḣb, Cmw, Dd, and rmax. The composite score calculated from these variables forProcyon lotorwas much higher than those derived for other species, and there was a positive correlation between the number of climates a species occupies and the magnitude of its composite score. Data on evaporative water loss, although not complete for all species, suggested that tropical and subtropical procyonids have less capacity for evaporative cooling thanProcyon lotororBassariscus astutus. It was clear, therefore, that with respect to its thermal physiology,Procyon lotordiffered markedly from other procyonids, and we contend that these differences have allowed this species to become a highly successful climate generalist and to expand its distribution into many different habitats and climates. Our analysis also suggested that the cornerstone ofProcyon lotor's success as a climate generalist is its Ḣb, which is higher than the procyonid norm.
Acknowledgments
The authors would like to thank John Eisenberg and Devra Kleiman for their support and encouragement throughout the study. This investigation was supported by research grants from the West Virginia School of Osteopathic Medicine (WVSOM), and Friends of the National Zoo (FONZ). Logistic support was provided by the National Zoological Park's Conservation and Research Center (CRC), and the departments of Mammalogy and Zoological Research. Our ability toconduct physiological research at CRC was made possible by the thoughtful support and encouragement provided by Chris Wemmer. His excellent staff at CRC, especially Jack Williams, Junior Allison, and Red McDaniel, were very helpful in providing hospitality and logistical support to the senior author and his family during their various visits to the Center. The assistance of several people at the National Zoo also is gratefully acknowledged: Mitch Bush and Lyndsay Phillips not only provided veterinary support throughout the investigation, but also performed surgical procedures required to implant temperature-sensitive radio transmitters in several raccoons; Olav Oftedal made his laboratory available to us at various times and loaned us equipment to use at CRC; Miles Roberts and his staff provided care for our captive raccoons in the Department of Zoological Research during various parts of the investigation. Greg Sanders and Ken Halama, supported by FONZ assistantships, cared for our captive raccoons at CRC, provided assistance in the laboratory whenever needed, and were an invaluable source of aid. Their friendship and help is gratefully acknowledged. Ellen Broudy and Andy Meyer, supported by WVSOM and a student work study grant, respectively, provided assistance in the laboratory. David Brown, John Eisenberg, Mary Etta Hight, Brian McNab, Steve Thompson, and W. Chris Wozencraft critically reviewed various phases of the manuscript and provided many helpful suggestions. We deeply appreciate the work of Jean B. McConville, whose beneficial editorial suggestions helped us improve several early versions of the manuscript. We also gratefully acknowledge Diane M. Tyler, our editor at the Smithsonian Institution Press, whose expertise helped us mold the manuscript into its final form. Jill Mellon and Sriyanie Miththalapa, supported by FONZ traineeships, assisted in measuring the daily cycle of body temperature in raccoons. The Virginia Commission of Game and Inland Fisheries gave us permission to use wild-caught raccoons in this project.
Materials and Methods
Live-trapping
Raccoons were caught from May 1980 through December 1984 on a trapping grid of 30 to 35 stations (one or two "live traps" per station) that covered about one-third of the National Zoological Park's Conservation and Research Center (CRC) near Front Royal, Virginia (Seidensticker et al., 1988; Hallett et al., 1991). Animals were trapped during 10 consecutive days each month, and in this five-year interval 407 raccoons were captured and marked with tattoos and ear tags. All captured animals were individualized with respect to age, reproductive status, physical condition, parasite load, and mass and body dimensions. These data characterized the structure and dynamics of the raccoon population at CRC and provided information on the annual cycle of fattening for raccoons in north central Virginia.
Animals used for metabolic measurements were captured at CRC about 1.5 km south of the trapping grid and thus were genetically representative of the area. Six males were captured and measured during the summer of 1983. These animals were kept isolated for a week before being measured and were released later that summer at the site of their capture. The other seven animals used in our study were from the collection of the National Zoological Park and all of them had their origins at CRC.
Metabolic Studies
Basal and Thermoregulatory Metabolism
Metabolic measurements, conducted at CRC, were carried out on eight males during July and August 1983, on four females and three males from November 1983 through March 1984, and on four females during June and July 1984.
Raccoons were housed throughout the study such that they were constantly exposed to a natural cycle of temperature and photoperiod. Weather records for the Front Royal area indicate that average temperatures are around -0.5°C in January and 23.3°C in July (Crockett, 1972). Light:dark (L:D) periods for the latitude of CRC (48°55'N; United States Department of the Interior Geological Survey, 1972), calculated from duration of daylight tables (List, 1971:506-512), were 14.9:9.1 and 9.4:14.6 hours L:D for summer and winter solstices, respectively, and 12.2:11.8 hours L:D for vernal and autumnal equinoxes.
Our animals were fed a measured amount of food daily, and they usually ate most of what was provided. Occasionally these animals would eat very little or none of their ration, and on some days they would eat all that was given to them. We fed them either feline diet (ground horse meat) or canned mackerel (Star-kist®[1]) along with high-protein dog chow (Purina®). When available, fresh fruit also was added to their diet. Water was always provided ad libitum.
[1]The use of product brand names in this publication is not intended as an endorsement of the products by the Smithsonian Institution.
Measurements were conducted during the raccoons' daily inactive period (sunrise to sunset) in both summer and winter. Oxygen consumption was measured in a flow-through metabolism chamber at 5°C intervals from -10°C to 35°C. Animals were held at each temperature until the lowest rate of oxygen consumption had been obtained and maintained for at least 15 minutes. During each determination, oxygen consumption was monitored for 30 minutes to one hour beyond a suspected minimum value to see if an even lower reading could be obtained. Raccoons attained minimum levels of oxygen consumption more quickly at warm (>10°C) than at coldtemperatures. Depending on the temperature, therefore, each measurement took from two to five hours to complete. On days when two measurements could be completed, the second trial was always at a temperature 10°C warmer than the first.
The metabolism chamber was constructed from galvanized sheet metal (77.5 × 45.5 × 51.0 cm = 180 liters) and was painted black inside. Within the chamber, the animal was held in a cage (71 × 39 × 33 cm) constructed from turkey wire that also was painted black. This cage prevented the raccoons from coming into contact with the walls of the chamber, yet it was large enough to allow them to stand and freely move about. The bottom of the cage was 11 cm above the chamber floor, which was covered to a depth of one cm with mineral oil to trap urine and feces.
During measurements, the metabolism chamber was placed in a controlled-temperature cabinet (modified Montgomery Ward model 8969 freezer). Air temperature (Ta) in the metabolism chamber was regulated with a Yellow Springs Instrument model 74 temperature controller. Tawas controlled to ± 1.0°C at temperatures below freezing, and to ± 0.5°C at temperatures above freezing. The chamber air and wall temperatures were recorded continuously (Linseis model LS-64 recorder) during each experiment, and, except during temperature changes, they were always within 0.5°C of each other.
Columns of Drierite®and Ascarite®removed water vapor and carbon dioxide, respectively, from air entering and leaving the chamber. Dry carbon-dioxide-free room air was pumped into the chamber (Gilman model 13152 pressure/vacuum pump) at a rate of 3.0 L/min (Gilmont model K3203-20 flow meter). Downstream from the chemical absorbents, an aliquot (0.1 L/min) of dry carbon-dioxide-free air was drawn off the chamber exhaust line and analyzed for oxygen content (Applied Electrochemistry model S-3A oxygen analyzer, model 22M analysis cell, and model R-1 flow control). All gas values were corrected to standard temperature and pressure for dry gas. Oxygen consumption was calculated from the difference in oxygen content between inlet and outlet air using Eq. 8 of Depocas and Hart (1957).
Each raccoon was fasted for at least 12 hours before oxygen consumption measurements began. At the start and end of each metabolic trial the animal was weighed to the nearest 10 g (Doctors Infant Scale, Detecto Scales, Inc., Brooklyn, N.Y., U.S.A.). The body mass used in calculating minimum oxygen consumption and evaporative water loss was estimated from timed extrapolations of the difference between starting and ending weights, and the time at which these variables were measured.
Evaporative Water Loss
During metabolic measurements at temperatures above freezing, evaporative water loss was determined gravimetrically. Upstream from the chemical columns, an aliquot of air (0.1 L/min) was drawn off the exhaust line and diverted for a timed interval through a series of preweighed (0.1 mg)U-tube-tubes containing Drierite®. The aliquot then passed through a second series ofU-tube-tubes containing Ascarite®before entering the oxygen analysis system. Evaporative water loss was calculated usingEq. 1
where Ė is evaporative water loss (mg·g-1·h-1), mwis mass of water collected (mg),.Veis rate of air flow into the chamber (3.0 L/min),.Vais the rate of air flow through theU-tube-tubes (0.1 L/min), t is length of the timed interval (h), and m is the estimated mass of the raccoon at the time of sampling (g).
Body Temperature
Veterinarians at the National Zoological Park surgically implanted calibrated temperature-sensitive radio transmitters (Telonics, Inc., Mesa, AZ, U.S.A.) into abdominal cavities of two female and two male raccoons. Transmitter pulse periods were monitored with a digital processor (Telonics TDP-2) coupled to a receiver (Telonics TR-2-164/166). During some metabolic measurements, body temperatures of these animals were recorded to the nearest 0.1°C at 30-minute intervals. The daily cycle of body temperature of these raccoons also was measured once a month.
Calibrations
Calorimeter
At the conclusion of these experiments, the accuracy of our calorimetry apparatus was tested by burning an ethanol lamp in the metabolism chamber. During these tests a CO2analyzer was incorporated into the system (Beckman, LB-2). Results demonstrated that we measured 84% of the oxygen consumed by the lamp as well as 84% of the water and CO2it produced; standard deviation = ± 2.6, ± 5.0, and ± 3.6, respectively (n = 27). Average respiratory quotient (RQ) calculated from these data was O.657 ± 0.008 (n = 27), which is 99.5% of that predicted (0.66). McNab (1988b) reports that the accuracy of open-flow indirect calorimetry systems, such as ours, depends on the rate of air flow through the animal chamber. If flow rates are too low, there is inadequate mixing of air within the chamber, and the rate of oxygen consumption, as calculated from the difference in oxygen content of air flowing into and out of the chamber (Depocas and Hart, 1957), is underestimated. At some critical rate of air flow, which is unique to each combination of chamber and animal, this situation changes such that measured rates of oxygen consumption become independent of any further increase in flow rate (McNab, 1988b). In recent tests of our system, where we burned the ethanol lamp at a variety of chamber flow rates, the efficiencyof measurement increased linearly as flow rate increased, and the critical rate of air flow was about 6.7 L/min. This appeared to explain why a flow rate of 3.0 L/min underestimated oxygen consumption of the ethanol lamp.
Our earlier tests of the efficiency of our system indicated that although we underestimated actual oxygen consumption of the ethanol lamp, we did so with a fair degree of precision; probably because flow rates were closely controlled. During our metabolic measurements, chamber flow rates also were closely controlled at 3.0 L/min, and we believe, therefore, that these measurements also were carried out with a high degree of precision. Consequently, all measured values of oxygen consumption and water production were considered to be 84% of their actual value and were adjusted to 100% before being included in this report.
Body Temperature Transmitters
The calibration of all temperature-sensitive radio transmitters drifted over time. Transmitters were calibrated before they were surgically implanted and again after they were removed from the animals. Although the drift of each transmitter was unique, it was also linear (S. Tomkiewicz, Telonics, Inc., pers. com.). All body temperature measurements were corrected from timed extrapolations of the difference between starting and ending calibrations.
Statistical Methods
Values of oxygen consumption, evaporative water loss, and body temperature were plotted as a function of chamber air temperature. Linear regressions of oxygen consumption at temperatures below the thermoneutral zone (Tn), and evaporative water loss at temperatures above freezing, were determined with the SAS (1982) GLM procedure. Lower critical temperature (Tlc) was determined graphically from intersection of the line representing Ḣband the regression line representing oxygen consumption below Tn. Slopes and intercepts of regression lines, as well as other mean values, were compared witht-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). Unless indicated otherwise, data are expressed as mean ± standard deviation (s.d.).
Estimating Intrinsic Rate of Natural Increase
We employed the method first described by Cole (1954) to calculate rmax:
where a is potential age of females first producing young, b is potential annual birth rate of female young, and n is potential age of females producing their final young. After life-history data were substituted intoEq. 2, rmaxwas determined by trial and error substitution (Hennemann, 1983).
Because rmaxrepresents the genetically fixed, physiologically determined maximum possible rate of increase, data on earliest possible age of female reproduction, highest possible birth rate of female young, and longest possible female reproductive life span were used for a, b, and n, respectively. Calculated values, therefore, represent physiologically possible, not ecologically possible, intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984; McNab, 1984b). Values of n were derived from longevity records for captive animals, and as these were all large values of similar duration (14-16 years), they had very little effect on rmax. All species considered have one litter per year, and because their sex ratios at birth are about 50:50, variation in b was due to differences in litter size. Therefore, age of first reproduction and litter size had the greatest effect on rmax. Intrinsic rate of increase scales to body mass (Fenchel, 1974), and we removed this effect by comparing each calculated rmaxwith the value expected (rmaxe) on the basis of body mass (Hennemann, 1983).
Comparison of Adaptive Units
Dimensionless numbers for each of the four variables used in calculating composite scores were derived as follows. Ratios of measured to predicted values were used for basal metabolism (Hbr) and minimum wet thermal conductance (Cmwr). Thermoregulatory ability at low temperatures is closely related to the ratio Hbr/Cmwr(McNab, 1966). This ratio was used, therefore, to gauge each species' cold tolerance. For Ddwe used the ratio of food categories actually used by a species to the total number of food categories taken by all species tested (Ddr). The ratio of calculated to expected intrinsic rates of natural increase was used to derive rmaxr. Composite scores were calculated as
The correlation between number of climates these species occupy and their composite scores was tested by linear regression.
Results
Body Mass
According to monthly live-trapping records, the body mass of free-ranging female raccoons increased from 3.6 ± 0.6 kg during summer to 5.6 ± 0.8 kg in early winter, and the mass of free-ranging males increased from 4.0 ± 0.5 to 6.7 ± 0.9 kg during the same interval. These seasonal changes in body mass were due to fluctuations in the amount of body fat and represent a mechanism for storing energy during fall for use in winter. In summer, captive and trapped male and captive female raccoons had the same body mass (4.73 ± 0.61, 4.41 ± 0.70, and 4.67± 0.88 kg, respectively,Table 2). Mass of captive females did not change between seasons, whereas captive males were heavier in winter than summer (p<0.005;Table 2). This seasonal change in mass of our captive males was of a much smaller magnitude (0.6 kg) than that observed for wild males (2.7 kg). During winter, captive males (5.34 ± 1.39 kg) were heavier than captive females (4.49 ± 0.98 kg; p<0.005;Table 2). Thus, our captive animals maintained a body mass throughout the year that was intermediate to the range of values found for wild raccoons in the same area.
Table 2.—Body mass in kg and basal metabolism(mL O2·kg-0.75·h-1)ofProcyon lotorin summer and winter (s.d. = standard deviation and n = number of observations).
Basal Metabolic Rate
Within thermoneutrality, Ḣb(mL O2·g-1·h-1)was 0.54 ± 0.09 for trapped males in summer, 0.46 ± 0.07 for captive males in summer, 0.42 ± 0.07 for captive females in summer, 0.47 ± 0.06 for captive males in winter, and 0.46 ± 0.10 for captive females in winter (Figures 2,3). Ratios of these measured values to those predicted by the Kleiber (1932, 1961:206) equation are 1.28, 1.12, 1.02, 1.17, and 1.09, respectively. To minimize the effect of body size (Mellen, 1963) and to facilitate comparisons between sexes and seasons and between captive and trapped animals, basal metabolism also was calculated as a function of metabolic body size(mL O2·kg-0.75·h-1;Table 2). Based on this analysis, trapped summer males had a higher basal metabolism than captive males (p<0.025) or females (p<0.005) in either season (Table 2). There was no difference in basal metabolism between captive males and females in either summer or winter, and there was no seasonal difference in their basal metabolic rates (Table 2).
Minimum Thermal Conductance
Minimum wet and dry thermal conductances were calculated using Eqs. 4 and 5
where Cmwis wet and Cmdis dry conductance(mL O2·g-1·h-1·°C-1);Ḣris the lowest resting metabolic rate measured at each temperature(mL O2·g-1·h-1);Ėeqis oxygen equivalent for heat lost by evaporation [Ėeq= mL O2·g-1·h-1= Ė·λ/γ, where Ė is evaporative water loss (mg·g-1·h-1), λ is heat of vaporization for water (2.43 J/mg), and γ is heat equivalent for oxygen (20.097 J/mL)]; Tbis body temperature (°C); and Tais chamber air temperature (°C). Only data from animals equipped with temperature-sensitive radio transmitters were used for these calculations.
Table 3.—Minimum wet and dry thermal conductances(mL O2·g-1·h-1·°C-1)ofProcyon lotorin summer and winter. Means of values were calculated from equations 3 and 4 (s.d. = standard deviation and n = number of observations).
summer - oxygen consumpsion vs air tempFigure 2.—Relationship between oxygen consumption and chamber air temperature for raccoons in summer: captive females, open circles; captive males, closed circles; trapped males, open squares. Sloping lines represent regressions of oxygen consumption on chamber air temperature, and horizontal lines, basal metabolism.
Figure 2.—Relationship between oxygen consumption and chamber air temperature for raccoons in summer: captive females, open circles; captive males, closed circles; trapped males, open squares. Sloping lines represent regressions of oxygen consumption on chamber air temperature, and horizontal lines, basal metabolism.
winter - oxygen consumpsion vs air tempFigure 3.—Relationship between oxygen consumption and chamber air temperature for raccoons in winter: captive females, open circles; captive males, closed circles. Solid sloping line represents regression of oxygen consumption on chamber air temperature for males and females, and the horizontal line, basal metabolism for males and females.
Figure 3.—Relationship between oxygen consumption and chamber air temperature for raccoons in winter: captive females, open circles; captive males, closed circles. Solid sloping line represents regression of oxygen consumption on chamber air temperature for males and females, and the horizontal line, basal metabolism for males and females.
Cmwwas calculated for each season from metabolic measurements made at all air temperatures below Tlc(Table 3). Because evaporative water loss was not measured at temperatures below freezing, Cmdwas calculated only from metabolic determinations made at air temperatures between Tlcand 0°C. There was no difference between males and females in summer for either Cmwor Cmd(mL O2·g-1·h-1·°C-1).Data for each sex were combined to give a summer average of 0.0256 ± 0.0028 for Cmw, and 0.0246 ± 0.0019 for Cmd(Table 3). These summer conductances were 49% higher (p<0.005) than those calculated for winter females (0.0172 ± 0.0023, and 0.0161 ± 0.0027 for Cmwand Cmd, respectively;Table 3). Cmwand Cmdwere not different from each other in either summer or winter, which indicated that in both seasons evaporative water loss contributed very little to heat dissipation at temperatures below Tn. Comparisons of thermal conductances calculated on the basis of metabolic body size (Mellen, 1963) gave the same results.
Evaporative Water Loss
Evaporative water loss increased as chamber temperature increased in both summer and winter (Figures 4,5). In summer, the pattern of increase was different for females and males. Polynomial regressions for trapped and captive males produced equations that describe a concave relationship between Taand evaporative water loss, whereas the equation for females describes a sigmoid curve (Table 4;Figure 4). For females, water loss increased rapidly at temperatures above 25°C (Figure 4). The intercepts and coefficients of the X, X2, and X3terms of the polynomial regression equations (Table 4) were compared (t-tests) to determine if they differed from each other. The coefficients in the equation for trapped males differed from those for captive females in the X2(p<0.05) and X3(p<0.025) terms. The intercept and coefficients of the equation for captive males, however, were not different from those for either captive females or trapped males. Although this lack of difference is understandable in the case of trapped males, where the shape of the two curves is similar (concave), it is not so clear for the sigmoid curve of captive females (Figure 4). Perhaps the lack of difference in this case is simply due to the small number of observations available for captive males (n = 10;Table 4). Nonetheless, in summer at 35°C, both captive and trapped males relied less on evaporative cooling than did captive females (Figure 4).
In winter, males and females had similar rates of evaporative water loss across the full range of temperatures tested (Figure 5). Therefore, data for both sexes were combined. The intercept and coefficients of this equation (Table 4) did not differ from those for summer females, but they did differ from those in the regression for trapped males in the X2(p<0.05) and X3(p<0.025) terms. As was the case for females in summer, rates of water loss for winter animals increased most rapidly at temperatures above 25°C (Figure 5).