Henry S. Fitch

“ Opossums, like other animals, obviously make various types of movements. Ordinarily one tends to keep within a relatively small area that is familiar to it and that satisfies all its ecological requirements. This constitutes its home range. Many other animals, including various mammals, are characterized by territoriality; individuals, pairs or groups occupy definite areas, defended as territories, to the exclusion of other members of their species. ”

“ On a natural area, the University of Kansas Natural History Reservation, in Douglas County, northeastern Kansas, the population of opossums was studied, chiefly by live-trapping, in the fall-winter-spring seasons of 1949-50, 1950-51 and 1951-52. The study area provided a varied habitat of elm-oak-hickory woodland, pastureland, and fallow fields. Opossums use all parts of it, but concentrate their activities in the woodland.
Opossums being mainly nocturnal were rarely seen in the daytime, except when caught in traps. Reactions to humans varied ”

“ An opossum being followed would almost invariably take refuge in a tree if it caught sight of the observer. Other information regarding the animal's habits was obtained from tracks in snow or soft soil and from the distribution and contents of scats. Carcasses of opossums which had fallen victim to predators were found on a few occasions and in some instances clues as to the identity of the predator were obtained. One hundred and seventeen opossums were live-trapped and handled a total of 276 times. ”

“ Undisturbed opossums were seen in the course of their normal activities on only a few occasions, and behavior is known to us mainly from the sign and from observations made on those that were live-trapped. Ordinarily those taken in live-traps were found curled up in deep sleep from which they did not arouse until touched or until the trap was moved or jarred. Reactions to humans varied greatly in individuals and was not necessarily correlated with age or sex. Adult males were uniformly hostile to the trapper and reacted with harsh, low growls, with back arched and hair bristling. ”

“ The opossum plays an important part in this process of change; being relatively large, numerous, and of omnivorous habits, it variously influences, directly and indirectly, the populations of its plant and animal associates, through a complex web of interrelationships. ”

“ Food is found by turning chips and leaves, and by poking and probing in chinks and crevices with its snout and paws. On a few occasions short, well worn trails made [Pg 318] by opossums were found, from dens to near-by feeding areas where grape tangles provided an abundant and readily available food source over periods of weeks. More often, an opossum follows no trail in its search for food, but seems to wander at random within its home range.
Figure 4. Quarter-mile-square areas on Reservation showing dates and successive sites of capture of individual opossums ”

“ In general, opossums are unsocial but not intolerant in their behavior. In the present study numerous individuals of both sexes and various sizes and ages were found to be occupying the same area simultaneously, with overlapping but no exact correspondence in home ranges. Occasionally two or more opossums may use the same den, but each goes its own way on its foraging and it seems that no sociability is involved. ”

“ Every part of the Reservation is used by opossums, but their activity is concentrated in the woodland, and all dens found were in woodland. Most parts of the fields are within 100 yards of the edge of the woodland and no point is more than 700 feet from the edge. Most of the opossums' foraging in fields was concentrated along the edge; otherwise they tended to follow creeks and gullies and they follow well worn trails more often than they do in the woods. Within the woodland, activity tended to be concentrated along the small streams, and along the rock ledges where den sites were plentiful. ”

“ In the winter of 1949-50, the area covered was most extensive, from 350 to 400 acres, and the numbers of opossums taken were correspondingly larger. In the 1950-51 season the area involved was approximately 220 acres, and in the 1951-52 season it was a little less than 200 acres. In view of the census figures obtained and the probable errors, it appears that the opossum [Pg 335] population in early autumn is about one to 20 acres, and that by late spring it is reduced to not much more than half that number. ”

“ During fall, wild fruits made up the greater part of the diet and were represented in almost every scat that was seen. Wild grape (Vitis vulpina) is an abundant woodland vine on the area and often forms dense tangles both in deep woods and in edge situations. [Pg 322] Grape was the most abundant single item, and a large number of scats consisted exclusively of grape seeds and skins. In November and December opossums could be trapped most effectively by making sets in or near grapevine tangles where the animals were attracted by the abundant ripe fruits. ”

“ It is unlikely that this dispersal of the young males is motivated either by rivalry and intolerance of larger males or by sexual drive. The dispersal occurs in late summer when there is no breeding activity, and when food is present in greatest abundance and variety. ”

“ Opossums were found to have little inclination to follow beaten trails, either their own or those of other animals. A foraging opossum moved about in an extremely circuitous and erratic route, seldom taking more than a few steps without a change of direction, and frequently crossing its own course in a series of loops, some only a few feet or a few inches in diameter. ”

“ On the 590-acre University of Kansas Natural History Reservation where our study was made, the opossum, Didelphis marsupialis virginiana Kerr, is the largest predatory animal having a permanently resident population. ”

“ Flat rocks a few inches in diameter frequently have been found flipped over; larger flat rocks and those solidly anchored in the ground often have been found partly undermined by opossums scratching away the loose dirt at their edges. Flat rocks similar to those found disturbed by opossums are the favorite resting places of the skinks, which, in cold or wet weather, are sluggish when beneath such shelters; this is especially true of female skinks that are nesting. The shape and size of some of the excavations suggested predation on skink nests. ”

“ These rock ledges, well distributed throughout the area, provide an abundance of den sites and most of the opossums definitely trailed to a home base were found to be utilizing dens in the rock ledges. Two small creeks on the area have some water for most of the year. As compared with wooded bottomland of larger stream courses in Douglas County and those counties adjoining it, the Reservation area probably supports a relatively low population density of opossums. "Sign" has been found in much greater abundance in near-by areas supporting a heavier woodland. ”

“ The number of active opossums in the region of the study reaches a peak in late summer and early fall, when second litters of young have grown large enough to become independent. At this season the population contains a high proportion of young of the year. During the ensuing months of fall and winter there is a steady decrease in numbers, through various mortality factors, with no replacement until young are born about the first week of March. ”

“ November, 1952, 2 scats. Grape in both, making up all of one and about 90% of the other.
Sandidge (loc. cit.) found remains of cottontail rabbit in some of the stomachs he examined, but followed Reynolds (1945) in regarding these as carrion since the opossum was considered to be too inefficient a predator to catch and kill cottontails—prey approximating its own size and much superior in speed. Adult cottontails seem to be secure from opossum predation under ordinary circumstances. However, the opossum obtains some of its food by raiding the nests of small animals, including those of rabbits. ”

“ FEEDING HABITS
The feeding habits of the opossum in Douglas County, northeastern Kansas, have been discussed by Sandidge (1953) . His data were obtained from stomach analysis of specimens caught in steel traps. ”

“ After November, young cease to gain, or gain slowly and irregularly through the winter and spring and adults tend to decline in weight, as food becomes scarce and frequent fasting is enforced by cold or stormy weather. ”

“ However, it seems that the young of the summer litters must be subject to other unusual and selective mortality factors which eliminate most of them by fall. That such factors vary from year to year is indicated by the changing ratio of summer-born young to other opossums in each of the three winter seasons when trapping was carried on. ”

“ Foraging consists of a succession of tests of such objects, as the animal moves from one to another. Opossums may habitually follow intermittent creeks or gullies or even roads when these provide better foraging than does the adjoining habitat. ”

“ Shifts occur frequently, contributing to population turnover, which may result in almost complete replacement of individuals in the course of a year's time, on an area of less than a square mile.
DISPERSAL OF YOUNG
One hundred and fifteen small young of 14 different litters were marked while still attached to the mother's teats in the pouches. A fairly high rate of mortality probably is normal in the small dependent young and further mortality probably resulted from the deleterious effects of examining and handling them and the females that carried them. ”

“ The young of the second litter are less successful than those of the first litter and make up only a small part of the breeding population the following year. In 28 litters of young the average was 7.4, but probably some of these litters had already sustained losses. ”

“ By late March most of the females are carrying young in their pouches, and those which do not have young, have their pouches enlarged and vascularized for accommodation of the young. Presumably such females have already borne young and then lost them. Nearly all the litters seen in the latter half of March had young that were much larger than at birth. ”

“ The over-all ecology is greatly altered. The flora and fauna both are undergoing successional changes which will continue for a long time and probably will culminate in a biotic community much different from the original climax. ”

“ The opossums trapped ranged in weight from 126 grams to 5000 grams but most weighed between 1000 and 2000 grams. After being trapped and marked by toe-clipping, animals usually lost weight, up to as much as 18 per cent of the original weight. Food scarcity and enforced fasting in cold weather caused a weight loss from November until the arrival of warm spring weather. ”