Part 6

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°C for several hours each night but remained below 38°C during daytime. During summer, with the exception of one female whose record was not typical (Figure 7), T{b}'s rose above 38°C shortly after sunset, whereas in winter T{b}'s did not rise above 38°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°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°C and about 39°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°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 Minimum Species |mass metabolism conductance T{b} |(g) ------------ ------------- ------------- | Meas H{br} Meas C{mwr} [alpha] ---------------------+------------------------------------------------ Bassariscus astutus| 865 0.43 0.68 0.0288 0.85 37.6 23 Procyon cancrivorus|1160 0.40 0.69 0.0368 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 1.02 Nasua nasua |3850 0.26 0.60 0.0200 1.24 38.3 36.4 Nasua nasua |4847 0.33 0.79 0.0238 1.65 39.1 37.9 Nasua narica |5554 0.25 0.62 0.0208 1.55 38.9 37.4 Nasua narica |4150 0.42 1.20 0.0341 2.20 | 0.0224 1.45 Procyon lotor_ | Summer | Trapped male |4400 0.54 1.28 Captive male |4790 0.46 1.07 0.0256 1.77 38.4 37.5 Captive female |4670 0.42 1.02 0.0256 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 1.15 38.3 37.3 ---------------------+------------------------------------------------

---------------------+----------------------------------------------- | Species | T{n} |--------------- | 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 Müller and Kulzer (1977) Potos flavus | 23 33 Müller 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 ---------------------+-----------------------------------------------

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.

Meas is measured minimum thermal conductance (mL O{2}·g^{-1}·h^{-1}·°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.

T_{b} is body temperature during the active ([alpha]) and rest () phases of the daily cycle (°C).

T{n} is the thermoneutral zone as defined by the lower (T{lc}) and upper (T_{uc}) critical temperatures (°C).

Conductance calculated as the slope of the line describing oxygen consumption at temperatures below the lower critical temperature.

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.

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; R² = 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).

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.