RYSER: VIRGINIA OPOSSUMS (DIDELPHIS VIRGINIANA) IN FLORIDA


 Spatial Organization


 The opossum is not necessarily nomadic, as suggested by Hunsaker (1977),
Gardner (1982), and others. Under stable environmental conditions, the opossum's
spatial organization is characterized by fairly even spacing among individuals and at
least seasonal stability of the spatial structure. There are sexual differences, however,
with females having smaller home ranges and tending to more exclusive use and
greater stability of home ranges than males. The impression of earlier capture-
recapture studies of an unstable spatial structure may have been, among others, the
result of the males' expansion of home ranges during, and frequent shifting of home
ranges after mating seasons (Ryser 1992), which could cause males to be trapped
several kilometers from their usual area of activity.
 The data from this study suggest that opossums have a highly flexible strategy of
space use. On one hand, females exploit much of their home range exclusively, which
allows the exploitation of predictable resources with minimal competition. On the
other hand, mutual tolerance allows the opossum, as an omnivorous, opportunistic
feeder, to jointly exploit high quality areas or clumped food supplies, such as fruiting
trees, amphibian breeding ponds, or animal carcasses. In May 1987, an amphibian
breeding pond was visited by four opossums almost every night. At close distance,
however, overt aggressive behavior prevailed. Opossums also frequently shifted
foraging areas within home ranges after days or weeks, probably to react to temporal
and spatial changes in the distribution and abundance of food. Although I could not
compare seasonal home ranges due to a lack of adequate data, there seemed to be no
obvious shifts of home range related to particular seasons. Except for cases of
aggression at common feeding places, there was no evidence for physical interactions
among neighboring individuals, suggesting that exclusive areas in female ranges
were maintained by mutual avoidance. A similar spatial system was found by Charles-
Dominique (1983) in several South American didelphids and by Sunquist et al. (1987)
in Didelphis marsupialis.
 Female reproductive success in polygynous mammals such as the opossum is
usually limited by access to adequate resources, such as food and shelter (Trivers
1972). Male reproductive success, on the other hand, is largely limited by access to
mates (Ryser 1992). Male spatial distribution is therefore expected to be influenced not
only by ecological factors, but also by the mating system and the distribution of
females. The sexual difference in space use, which has also been observed in many
other mammals (Harestad and Bunnell 1979), therefore, most likely is related to a
great degree to the male's reproductive strategy, which includes covering numerous
female home ranges during mating seasons (Ryser 1992). The notion that sex
hormones influence greater space use in males is supported by the fact that a male
castrated prior to maturity exhibited a home range size (MCP 68.6 ha) typical of a
female. Some of the sexual difference in home range sizes, however, is also expected