[Pg 307]

University of Kansas Publications
Museum of Natural History

Volume 7, No. 2, pp. 307-338, 5 figures in text

   August 24, 1953  

Ecology of the Opossum on a Natural Area
in Northeastern Kansas

BY

HENRY S. FITCH

AND

LEWIS L. SANDIDGE

University Of Kansas
Lawrence
1953

[Pg 308]

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson

Volume 7, No. 2, pp. 307-338, 5 figures in text
Published August 24, 1953

University of Kansas
Lawrence, Kansas

PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1953

24-7812

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. The coyote, racoon and red fox also occur on the area but each ranges widely, beyond the Reservation boundaries. With the passing nearly a century ago of the larger animals of the original fauna, the buffalo, elk, deer, antelope, wild turkey, gray wolf and others, lesser herbivores and carnivores including the opossum and animals of similar size fell heir to their key positions of predominance at the peak of the food pyramid. These smaller animals, however, exert less powerful effects in controlling the general aspect of the biotic community, and affect it in different directions. 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 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. Several excellent field- and laboratory-studies of the opossum have been published (Hartman, 1928, 1952; Lay, 1942; Reynolds, 1945; Wiseman and Hendrickson, 1950) and the life history of this remarkable marsupial is already well known. The purpose of our study, therefore, was to gain a better understanding of the ecological relationships of the opossum in the particular region represented by the study area. To accomplish this, we gathered data concerning the animal's responses to climate and varying weather conditions; its annual cycle of breeding, growth and activity, movements, principal food sources, numbers, population turnover, and natural enemies. Although we did gain a somewhat better understanding of the opossum's ecology, results are remarkably meager in proportion to the large amount of time expended. The hours of work daily in setting and tending a line of live-traps ordinarily were rewarded with only a few records, sometimes none. Comparable time and effort directed to the study of smaller and more abundant kinds of animals has been far more productive of data. Field work was carried on in parts of 1949, 1950, 1951 and 1952.

Figure 1. Map of the University of Kansas Natural History Reservation showing locations where opossums were live-trapped.

Because opossums are nocturnal and rarely seen in the course of their regular activities, the present study is based mainly on information gained by live-trapping them. Several different sizes of traps of the type described by Fitch (1951) were used. The most successful were 2' × 8" × 8" in dimensions although many of the larger ones were also used. They were constructed of hardware cloth having a half-inch mesh. Live-trapping was begun in October 1949 by Fitch with a line of about a dozen traps. In the following month Sandidge joined in the field work. The trapping was continued throughout the winter and spring of 1949-1950 and was resumed the following fall and more traps were added from time to time until a maximum line of approximately 60 was attained. Sandidge's participation ended in December, 1950. The live-trapping was continued on a reduced scale by Fitch through the winter and spring of 1951 and some was done sporadically in the fall, winter and spring of 1951 to 1952.

Traps were baited with a variety of foods such as carcasses of small vertebrates, meat scraps, canned dog food, ground horse meat and bacon grease. At each capture, sex, weight, and individual formula of the opossum, based on toe-clipping and ear-clipping (Fitch, 1952), were recorded. Also recorded was the exact site of capture as located in one of 84 divisions of the Reservation and estimated in feet from some named landmark. Notes on breeding condition, pelage, injuries, parasites and general appearance were also taken at the time of capture. For opossums caught in 1951 and 1952, the hind foot measurement was recorded.

Often, attempt was made to follow the released opossum to determine the direction and distance of its homeward travel but this was difficult because of brushy terrain and secretive habits of the animal. 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. Six of these were dead when first found in the traps. The remaining 111 were marked and released. In addition, 207 pouch-young carried by adult females were recorded and 115 of these were individually marked by toe-clipping. Some of the opossums that were marked while in the mother's pouch were subsequently recaptured when they were well-grown, independent young, or adults, affording information on growth and dispersal.

The habitats of the Reservation have been described briefly by Fitch (1952) and by Leonard and Goble (1952). More than half the area consists of steep wooded slopes with mixed second growth forest, consisting of elm, hickory, oak, walnut, ash, honey locust, hackberry and osage orange, in about that order of abundance, with thickets of blackberry, crabapple, wild plum and grape. Fallow fields and pastures of the upland and valley floors alternate with the woodland. The varied habitat provides numerous different food sources. Along the edges of the hilltops there is a nearly continuous limestone outcrop with a lower outcrop paralleling it. 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.

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. Throughout the annual cycle, and from year to year, there were minor shifts in areas of concentrated activity depending on seasonal changes in food sources such as thickets of wild plum, crabapple, blackberry and grape, with fruits ripening at slightly different times of year. The areas adjoining the Reservation offer somewhat similar habitat conditions, part woodland, part pasture land and some cultivated fields with corn or other crops which provide food sources for the opossum.

Under original conditions the area that is now the Reservation probably was marginal habitat for opossums, consisting mainly of open grassland with trees in small and scattered clumps, if indeed they were present at all. There has been steady encroachment of shrubs and trees, originally chiefly confined to near-by bottomlands such as those of the Kaw and Wakarusa valleys. Concurrently, the original hardwood forest of the bottomlands has mostly disappeared, and the land has been taken over for intensive agricultural use. The new upland forest provides a habitat different in many respects from the original bottomland forest. The species composition, in trees and other plants, is somewhat different, with more xeric types, especially on steep south slopes. Logs and large old hollow trees are scarce. The lack of such potential den sites is compensated for by the abundance of holes and crevices along hilltop rock ledges.

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. Although many adult females and young of both sexes were similarly hostile in behavior, others were not. Some cowered silently in the trap. Others showed hardly any uneasiness. A small proportion of them feigned death when handled or even before they were touched. Feigning was especially frequent in response to clipping of toes and ears when the animal was marked. In some that were handled, the feigning reaction was weak or incomplete, the animal arising almost immediately after collapsing or beginning to collapse in the feint.

Those that feigned death usually maintained the deception for not more than two or three minutes after a person had moved away out of sight. The opossum first raised its head and sniffed, listened, and looked about cautiously for a short time, with body and limbs still relaxed in the feigning posture. Failing to detect any sign of danger, it gradually shifted to a sitting position, and then to a standing one, from which it began moving away with many short pauses at first, and then more rapidly.

Upon being released, some opossums scrambled for shelter immediately; others stood their ground defiantly with back arched, hair bristling and fangs bared. One that was put on the defensive would usually maintain its stance for less than a minute if not further disturbed by movements of the trapper. It would then slowly turn its head and begin walking away with deliberate gliding movements, often pausing abruptly in the middle of its stride with one or two feet off the ground in a pose reminiscent of that of a bird dog making its "point." After moving away a few yards, it would gradually accelerate its pace in a scramble for shelter, but an occasional individual moved away unhurriedly, even foraging as it went.

On the few occasions when opossums were seen at night, their relative alertness and speed of movement contrasted with the sluggishness and seeming stupidity of those observed in daylight. Several were seen on roads in the beam of automobile headlights. These were quick to escape, running into thick roadside vegetation or woods to elude pursuit. Others were found in woodland, with the aid of a powerful flashlight as the investigator moved about on foot. They did not permit close approach, and escaped by running. One hid in a blackberry thicket. Several that were chased climbed trees when hard pressed. One that was overtaken, and others that were shaken out of trees and caught, showed fight, standing on the defensive, and slashing at the pursuer with a rapidity and vigor never encountered in those removed from traps in the daytime.

Figure 3. Half-mile-square area on Reservation, showing dates and successive sites of capture of an old adult male in upper half of map and an adult female in lower half.

Nocturnal tendencies of the opossum were emphasized by the infrequency with which undisturbed individuals were seen in the daytime. In more than a thousand days of field work on the Reservation, opossums were found out on only four occasions. These