Ecology of the Opossum on a Natural Area
in Northeastern Kansas
BY
HENRY S. FITCH and LEWIS L. SANDIDGE
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.