Metabolic Adaptation to Climate and Distribution of the Raccoon Procyon Lotor and Other Procyonidae

by John N. Mugaas; Kathleen P. Mahlke-Johnson; John Seidensticker

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°C-26°C; [Table 7]). In winter, T_lc of both sexes shifted downward to 11°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 Ḣ_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°C-13°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 Ḣ_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.