BULLETIN FLORIDA MUSEUM NATURAL HISTORY VOL 38, PT. 11(6)


 Decreasing foraging activity at low temperatures is likely to be the consequence
of increased energy expenditure and, at the same time, decreased foraging success,
particularly concerning invertebrates and ectotherm vertebrates, leading to a negative
energy balance at some low temperature. Increasing energetic costs already at
moderately cold temperatures are suggested by the fact that opossums are not well
adapted to the cold. Their pelage is sparse and has relatively poor insulative properties
(Scholander et al. 1950 cited in McManus 1971; Hsu et al. 1988). Opossums start
showing behavioral thermoregulation (e.g. shivering) already at around 10C
(McManus 1969; Dills 1972), and they are unable to maintain average body
temperatures below 30C (McManus 1969). In north-central Florida, the generally mild
winter temperatures certainly do not seriously limit the opossum's energy intake, but
the favorable conditions may allow the animals to reduce activity already at less cold
temperatures compared to opossums in more northern parts of their range with longer
lasting freezing temperatures.
 The decrease of activity with increasing temperatures above 80C is most likely
related to other factors associated with temperature. However, average duration of
nightly activity did not differ among months, excluding the possibility that decreasing
activity at higher temperatures was a seasonal effect. Perhaps it was greater food
abundance at higher temperatures which allowed opossums to reduce activity in warm
nights.
 The time of the onset of nightly activity did not show much individual variation,
as most animals left their dens within about an hour. However, a few individuals
started activity distinctly later at night, and the onset of activity shifted seasonally
according to the time of sunset. Due to varying individual duration of activity, the
temporal distribution of the end of activity varied to a much greater degree than did
the distribution of the onset. In November and December, apparently as a consequence
of the much greater variation in nightly temperatures, including low values close to
freezing, the patterns of activity clearly deviated from that in August to October.
However, despite similar temperature regime, opossums behaved differently in the two
months. In November, the pattern of the onset of activity corresponded to that of the
earlier months, but increased individual variation in the duration of activity led to a
great scatter in the time activity ended. Average nightly activity, however, was only
slightly lower than in the previous months, because the long nights allowed opossums
in mild nights to stay active for up to 13 hours. In December, on the other hand,
increased individual variation in activity was expressed primarily as variation in the
onset of activity.
 Other factors beside temperature that may be responsible for the individual
 variation in nightly activity may include the physical condition of the animals,
 previous and current weather conditions, and foraging success. Opossums seemed to
 stay active longer than expected following periods of cold nights. Individual
 differences also seemed to exist, as it was often the same animal that stayed active the
 longest, and as opossums sometimes left and entered dens in the same order on each of
 the four nights of consecutive tracking.