_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 deg.C in January and 23.3 deg.C in July
(Crockett, 1972). Light:dark (L:D) periods for the latitude of CRC
(48 deg.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)[1]) along with
high-protein dog chow (Purina(R)). 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 deg.C intervals from
-10 deg.C to 35 deg.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 deg.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 deg.C warmer than the first.
The metabolism chamber was constructed from galvanized sheet metal
(77.5 x 45.5 x 51.0 cm = 180 liters) and was painted black inside.
Within the chamber, the animal was held in a cage (71 x 39 x 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 deg.C at temperatures below
freezing, and to +-0.5 deg.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 deg.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
[.E] = (m_{w}.[.V]_{e})/([.V]_{a}.t.m) 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 deg.C at 30-minute
intervals. The daily cycle of body temperature of these raccoons also
was measured once a month.