Part 3

Diet

McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures of [.H]{b} from 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 [.H]{b}'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 [.H]{b}'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 [.H]{b}'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 [.H]_{b}, 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 r_{max}.

Our analysis demonstrated clear differences between Procyon lotor and other procyonids with respect to [.H]{b}, C{mw}, D{d}, and r{max}. The composite score calculated from these variables for Procyon lotor was 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 than Procyon lotor or Bassariscus astutus. It was clear, therefore, that with respect to its thermal physiology, Procyon lotor differed 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 of Procyon lotor's success as a climate generalist is its [.H]_{b}, 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 to conduct 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(R)) along with high-protein dog chow (Purina(R)). When available, fresh fruit also was added to their diet. Water was always provided ad libitum.

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 cold temperatures. 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 (T{a}) in the metabolism chamber was regulated with a Yellow Springs Instrument model 74 temperature controller. T{a} was 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(R) and Ascarite(R) 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-tubes containing Drierite(R). The aliquot then passed through a second series of U-tubes containing Ascarite(R) before entering the oxygen analysis system. Evaporative water loss was calculated using Eq. 1

where [.E] is evaporative water loss (mg·g^{-1}·h^{-1}), m{w} is mass of water collected (mg), [.V]{e} is rate of air flow into the chamber (3.0 L/min), [.V]_{a} is rate of air flow through the U-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