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


 RESULTS

 Activity and Movement


 Opossums were strictly nocturnal (Figs. 1, 2). Figure 1 shows the distribution of
the onset and end of nightly activity from August to December in relation to sunset
and sunrise and minimum nightly temperatures. Usually opossums emerged about one
hour after sunset and were active for a varying period of time before reentering a den.
However, some animals became active only later at night. Leaving the den late was
more common in cool nights. Minimum temperature at nights when opossums
emerged from dens after 2200 h averaged 8.0C (n=12) compared to 13.2C of all
considered nights. The time activity ended varied more strongly than the time of the
onset of activity. The scatter increased in November and December as average nightly
temperatures declined and the variation in temperature increased. Average nightly
activity ( S.D.) from August to December was 8.01 h ( 1.67), 8.81 h ( 1.47), 8.54 h
( 1.24), 7.85 h ( 3.13) and 8.22 h ( 3.66) and did not differ significantly among
months (Kruskal-Wallis test, df=4, H=4.28, p=0.37).
 Maximum activity is reached between 2000 h and 0200 h (Fig. 2), when on
average up to 97.4% of the monitored animals were active (cold nights excluded).
 Individual nightly activity varied greatly in duration and tended to be slightly,
but not significantly longer in females than in males (Table 1, t145=0.97, ns). Activity
was neither correlated with home range size (r=0.01, n=147) nor with age (r=-0.03) or
body size (r=-0.04), but was clearly influenced by temperature (Fig. 3). Activity
increased linearly with temperature up to about 80C (y=2.561+0.828x, n=33,
r2=0.571, p<0.0001). Above this threshold activity was less temperature-dependent,
but tended to decrease with increasing temperature (y=11.053-0.136x, n=114,
r2=0.129, p<0.0001). Based on the former equation, activity is predicted to cease
altogether at nightly minimum temperatures of -3C. Nightly inactivity was recorded
in only one case (minimum temperature 0C), but several additional cases occurred
during the mating season in January at nightly lows of -7 to +6C.
 Distances moved per hour and per night varied widely, but were significantly
greater in males than in females (t140=3.90 and 2.53, respectively, p<0.01 and <0.05,
Table 1). Maximum recorded movements within an hour reached 950 m in males and
790 m in females. Maximum distance traveled per night was nearly 4 km in females
and 4.7 km in males. A partial correlation analysis showed that distance moved per
hour in males was positively associated with home range size and minimum nightly
temperature, whereas in females only body size exerted a significant positive effect
(Table 2). Distance traveled per night is a function of the duration of activity and of
average distances moved per hour and is therefore influenced by the same factors that
influence activity and average hourly movement (Table 2). Thus males with large
home ranges tended to move farther than males with small home ranges and