Part 5

After these losses in the spring of 1950, trap lines were checked daily. However, in October, 1950, further mortality in traps resulted in the loss of three or more opossums. All three of these were rat-sized young of second litters. These young lacked the abundant supply of fat characteristic of larger opossums in fall, and seemingly were unable to withstand exposure to chilly nights. Such susceptibility to cold might result in heavy mortality in retarded second-litter young when cold weather of autumn is unseasonably early or is unusually severe.

Natural enemies of the opossum on the area include the red-tailed hawk, horned owl and coyote. Because of the opossum's nocturnal habits it is rarely exposed to hawk predation. Food habits of the coyote on the area have not yet been investigated. Numerous instances of horned owl predation on opossums have been recorded in the literature. On January 15, 1950, an owl attacked an opossum caught in a live-trap. The trap was found overturned, and a few feet away were entrails and a quantity of opossum hair where the animal was eaten. Low vegetation in the vicinity had many fine down feathers of the owl clinging to it. On December 24, 1950, the carcass of a small adult opossum was found in a pasture near the edge of the woods. The head and tail were intact, but otherwise little more remained than the spinal column, girdles and larger limb bones. White excreta of a large bird beside the carcass indicated predation by a raptor, probably a horned owl.

SUMMARY

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; some were indifferent, some feigned death, others merely tried to escape, and some defended themselves vigorously, snarling and snapping.

No evidence of territorial behavior was found in the opossum. Many individuals of both sexes and various sizes, occurred together on the same area. Successive captures of individuals revealed the usual extent of home ranges, which averaged approximately 50 acres, and tended to a circular or broadly oval shape. No significant difference in size of home ranges between males and females, or between adults and well-grown young, was found. Of 115 young marked by toe-clipping while still in the females' pouches, 15 were recaptured after periods of months. All but two of these recaptured young were females which had settled down within a few hundred feet of the locations where they were born. The young males seem to wander much more extensively than do the females.

Feeding habits were investigated by field examination of scats found mainly in fall and winter. These consisted mainly of wild fruits, especially grape, blackberry, wild crabapple, wild plum, and hackberry. Crayfish was the most important animal food. No comparable data for spring or summer were obtained because scats deteriorate rapidly in warm weather and were seldom found then. Clues as to the summer food were gained from sign. On many occasions opossums disturbed live-traps set for small animals, to obtain the voles, mice, skinks, or insects caught in them. Evidence of opossum activity such as digging and scratching was frequently noticed at the edges of rocks and in crevices, where such prey as skinks, narrow-mouthed toads, beetles, spiders and centipedes seek shelter. One opossum was observed to catch and kill a young cottontail.

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. By late April and May some opossums were emaciated and in critical condition.

The entire population of opossums, including the majority less than a year old, breeds in February, and litters are born mainly in the first half of March. The young develop rapidly in the female's pouch, and become independent in late May, and there is a second breeding season with young born mainly in the first half of June. By the onset of cool fall weather, young born in early spring have grown so that most are as large as small adults. The young born in early summer are still less than half-grown. 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.

In each of three different winters, the largest age group in the population of opossums was that of the newly matured young born in early spring. The old adults were the next most numerous group, and the second-litter young born in early summer were the least numerous. The figures obtained from live-trapping indicate an annual population turnover of approximately 95 per cent, with some 70 per cent eliminated by various mortality factors and replaced by young, the remaining 25 per cent shifting to new areas, with compensatory shifts of individuals replacing them.

The various mortality factors which regulate the numbers of opossums are not well known, and even less is known regarding the relative importance of the factors. Food supply and weather are obviously of major importance and closely interrelated in their effect on the population. One large adult opossum that was trapped seemed to be dying from disease and was scarcely able to stand; but others caught near-by before and after were unaffected. The horned owl is perhaps the most important natural enemy of the opossum on the Reservation, and instances of owl predation on opossums were noted.

LITERATURE CITED

FITCH, H. S.

1950. A new style live-trap for small mammals. Jour. Mamm., 31:364-365.

1952. The University of Kansas Natural History Reservation. Univ. Kansas Mus. Nat. Hist., Misc. Publ., 4:1-38, 4 pls.

HALL, E. R., and KELSON, K. R.

1952. Comments on the taxonomy and geographic distribution of some North American marsupials, insectivores and carnivores. Univ. Kansas Publ., Mus. Nat. Hist., 5:319-341.

HARTMAN, C. G.

1923. Breeding habits, development and birth of the opossum. Smithsonian Report 1921:347-363.

1928. The breeding season of the opossum (Didelphis virginiana) and the rate of intrauterine and postnatal development. Jour. Morph. and Physiol., 46:143-215.

1952. Possums. Univ. of Texas Press, Austin. xvi + 174 pp.

LAY, D. W.

1942. Ecology of the opossum in eastern Texas. Jour. Mamm., 23:147-159.

LEONARD, A. B., and GOBLE, R. C.

1952. Mollusca of the University of Kansas Natural History Reservation. Univ. Kansas Sci. Bull., 34:1013-1055.

REYNOLDS, H. C.

1945. Some aspects of the life history and ecology of the opossum in central Missouri. Jour. Mamm., 26:361-379.

SANDIDGE, L. L.