Kathleen P. Mahlke-Johnson

“ 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. ”

“ This reasoning would argue that Procyon cancrivorus probably enjoys higher, rather than lower, than expected rmax.
Potos flavus.—In addition to a low Ḣb, 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, rmax in Potos flavus has evolved to be only 48% of expected (0.30; Table 10) . ”

“ 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 Ḣb would actually increase rmax 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 rmax and metabolic rate, independent of body size, for 44 mammal species. ”

“ Male raccoons trapped in summer had higher Ḣb'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. ”

“ 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 (Cmw) is the only barrier to heat transfer with the external environment. ”

“ Evolution of Metabolic Adaptations
Evolution of Low Basal Metabolic Rate
A radiation of frugivorous and omnivorous Procyoninae (Table 1) occurred in the middle and late Miocene of North America. It included origins of such terrestrial genera as Cyonasua, Nasua, and Procyon (Webb, 1985b) . The earliest procyonid genus to find its way to South America was Cyonasua, an omnivorous carnivore that presumably split, along with its sister genus Arctonasua, from a common North American ancestor (Baskin, 1982; Webb, 1985b) . ”

“ All animals listed have about the same Tlc and Tb, 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 Ta = 0°C, it would be difficult for them to acquire enough food each day to maintain endothermy. ”

“ What were the metabolic capabilities of these early procyonids? We do not know, but for several million years, from middle to late Miocene, procyonids lived in tropical and subtropical forests of Central and North America (Webb, 1985b; Marshall, 1988) . Then, in the Pleistocene, several modern forms crossed the Panamanian land bridge into similar habitats and climates in South America; but none of them appear to have spread far enough northward to have crossed the Bering land bridge. ”

“ Our survey indicates that the following features compensate for low Ḣb and help increase rmax: (1) a high-quality diet may make biosynthesis and growth more efficient, thus optimizing the time element associated [Pg 21] 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. ”

“ 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. ”

“ Body temperature, evaporative water loss, and metabolic data indicated that, in winter, Tuc was very close to 35°C. In winter, the lowest level of oxygen consumption was recorded during the first hour after the chamber had reached Ta = 35°C. Unlike summer, animals became restless after the first hour at 35°C, at which point their oxygen consumption increased and showed a high degree of variability. Body temperature responses at 35°C were recorded from both females that had implanted radio transmitters. ”

“ During the course of evolution, therefore, each species' Ḣb 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. ”

“ 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. ”

“ This suggests that rmax 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 Ḣb. 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 Ḣb, its small mass may contribute to its ability to reach adult size and sexual maturity in its first year. ”

“ Other Procyonids.—Other procyonids (Potos flavus, Procyon cancrivorus, Nasua narica, and Nasua nasua) have lower than predicted Ḣb'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 (Müller and Kulzer, 1977; Chevillard-Hugot et al., 1980; Müller and Rost, 1983) . However, Bassariscus astutus is found in tropical, subtropical, and temperate climates. ”

“ 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. ”

“ Procyon lotor's elevated basal metabolic rate would have increased its overall energy requirement, and it makes good intuitive sense, therefore, that evolution during the Pleistocene also would have favored selection of a diverse diet containing many items of high nutritive value. ”

“ Thus, different expressions of these variables may represent specific climatic adaptations (Prosser, 1986:16) that have been selected-for by evolutionary process. Because these variables are interrelated with respect to regulation of body temperature and energy balance, they have co-evolved in each species to form an adaptive unit. For each species, measured and calculated values for the first four variables were converted into dimensionless numbers and used to derive a composite score that represented its adaptive unit. ”

“ Several named forms of Procyon are a single species, Procyon lotor (Wozencraft, 1989a) .
During the late Miocene, late Pliocene, and Pleistocene, the Bering land bridge between North America and Asia formed periodically, offering an avenue for dispersal between northern continents (Darlington, 1963:366; Webb, 1985a) . However, by the late Tertiary, northern continents had cooled to the extent that climate, with its attendant sharply defined vegetative zones, became the major factor limiting dispersal by this route (Darlington, 1963:366; Webb, 1985a) . ”

“ 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 Ḣb than do vertebrates in its diet. ”

“ 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 Ta and 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) . ”

“ 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. ”

“ 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 Ḣb and Cm in response to specific environmental demands. ”

“ At this temperature, males dissipated 35% ± 6% and females 56% ± 18% of their metabolic heat via evaporative water loss. Thus, at Ta = 35°C, males must have utilized modes of heat transfer other than evaporative cooling (convective and conductive heat transfer) to a greater extent than females. ”

“ 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 Ḣb maintain endothermy and enjoy its attendant advantages at a discount, whereas others, with rates that are higher than predicted, pay a premium [Pg 17] (Calder, 1987) . ”

“ Because rmax represents 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. ”

“ Because they are active during the day they experience a more extreme thermal environment (higher Ta's and solar radiation) than their nocturnal cousins. Values of Cmwr 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 Cmw's than their nocturnal tropical cousins. A high Cmw reduces the cost of thermoregulation in hot environments because it increases an animal's ability to lose excess heat passively. ”