Part 3

September, 1951, 16 scats. Grape in seven (all or most of 5 scats and small percentages of 2 others); cherry in seven (all or most of 5 scats and small percentages of 2 others); crayfish in seven (all or most of 5 and small percentages of 2 others); rabbit in two, making up most of both; insects (grasshopper, and large black beetle) in two making up small percentages.

October, 1951, 8 scats. Hackberry in three, making up nearly all of them; grape in two (all of 1 and most of the other); wild plum in one (100%); mast (acorn?) in one, making up 100%; crayfish in one making up about half; fox squirrel in one making up the remainder of the scat containing crayfish; rabbit in one making up a small percentage.

November, 1951, 12 scats. Hackberry in five, making up all or most of four and a small part of the fifth; grape in five, making up all or most of four and a small part of the fifth; wild crabapple in three, making up all of two and most of the third; and cottontail in one, making up all of it.

January, 1952, 3 scats. Hackberry in all, making up all of two and most of the third; copperhead (scales of medium-sized adult) making up a fraction of the third scat. Pile of more than a dozen scats not individually separable, nearly all consisted mainly or entirely of hackberry fruits estimated at 2000; other contents chiefly crabapple and corn.

September, 1952, 8 scats. Grape in all, making up all of six and 90% of the seventh, and about 20% of the eighth; wild plum seeds in one making up 40%; blue feathers, evidently of a jay, in one, making up a trace; carabid beetles in one making up a trace.

October, 1952, about 14 scats, two separate (both consisting exclusively of grape) and the remainder mixed in two approximately equal piles, one pile consisting of grape, except for small quantity of fine fur; second pile consisting mainly of grape (about 90%) with small percentages of yellowjackets (Vespula, about 6 individuals, all in one scat), toe bones and fur of cottontail rabbit; a few scales of immature copperhead; and a snail.

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. At the Reservation, on May 21, 1951, at 9:00 P. M., distressed squealing of a rabbit was heard in high brome grass. Investigation revealed that a large male opossum had killed a young cottontail, weighing approximately 150 grams, and had started to eat it. This young rabbit, about the minimum size of young wandering outside the nest, evidently was pounced upon as it hid beneath the high grass.

Live-traps for mice, in lines or grids of 100 or more, often were set on the Reservation, and predators, including opossums, disturbed them on many occasions. Attacks sometimes resulted in release and escape of the trapped animal, and in other instances resulted in its being caught and eaten. In many instances identity of the predator could not be determined, but it is believed that such attacks by the opossum were relatively infrequent and inefficient. Steel traps set beside the mouse traps after consistent raids, to catch or discourage the predator, caught opossums on several occasions. These opossums usually had overturned mouse traps without opening them and when the trapped mouse was missing from the trap no evidence of its having been eaten was obtained. On other occasions raccoons were caught in the steel traps, and their raids were characterized by systematic and dextrous opening of the mouse traps and, frequently, by predation on the small mammals inside them.

Wire funnel traps set for reptiles along rock ledges also were often disturbed by predators, mainly skunks and opossums, both of which were caught on several occasions, when steel traps were used as a protective measure. The opossums often were attracted to the funnel traps by large insects such as camel crickets, grasshoppers and beetles, but also by trapped lizards including the skinks (Eumeces fasciatus and E. obsoletus) and the racerunner (Cnemidophorus sexlineatus). Both Sandidge (1953) and Reynolds (1945) recorded the five-lined skink (E. fasciatus) in opossum stomachs. On the Reservation this common lizard probably is one of the most frequent items of vertebrate prey of the opossum. 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. Other possible food sources in the same situation, in loose soil beneath flat rocks, include narrow-mouthed toads, lycosid spiders, beetles (mainly carabids such as Pasimachus and Brachinus) and occasionally, snails, centipedes and millipedes.

A pond, a little more than an acre in size, was a focal area for opossums and more were caught there than on any other part of the Reservation. Opossums that were trapped and marked on other parts of the Reservation were likely to be caught here sooner or later. Tracks in the mud showed that the edge was patrolled almost nightly by one or more opossums and this activity was especially noticeable when the pond was drying. Frogs were obviously the chief attraction inducing the opossums to forage there. Of the 8 kinds of frogs and toads breeding at the pond, the bullfrog (Rana catesbeiana), leopard frog (Rana pipiens) and cricket frog (Acris gryllus) were most abundant, throughout the season and especially when drying occurred. All three probably are important foods of the opossum locally.

WEIGHTS

Opossums were weighed in the field, with small spring scales of 2000-gram capacity, graduated in 25-gram intervals. Weights recorded were accurate within a margin of about 10 grams. After other data were recorded, the opossum was offered the hook at the base of the scale, and usually bit and held fast. Then it could be suspended off the ground and a reading taken.

When the same opossum was trapped two or more times within a few days, weight was usually found to fluctuate sometimes more than 200 grams, or more than 10 per cent of the animal's body weight. Opossums recaptured soon after their original capture and toe-clipping were generally found to have lost weight, reflecting the deleterious effect of marking by this method. The temporary laming of the animals prevented them from traveling as far or as fast as they normally would have; consequently they probably obtained correspondingly less food. They were also handicapped in digging, grasping and climbing. Nineteen such animals taken within a month of the original capture and marking, averaged 94 per cent of their original weights. The minimum was 82 per cent. Only 2 of the 19 had gained.

The stumps of amputated toes did not heal rapidly in opossums--contrary to experiences with many other kinds of mammals, reptiles, and amphibians also marked by toe-clipping. For many weeks the toes remained unhealed, sore and swollen. In several instances after periods of months the clipped toe stumps were unhealed. This was observed even in some of the opossums that were marked as pouch young and recaptured when grown to nearly adult size.

Some adult opossums trapped were heavier than the 2000-gram capacity of the spring scale usually used in the field, and no definite weights were recorded for most of these animals. Some of them that were caught near the laboratory were brought there for weighing.

Even within the same age- and sex-group at any one time, opossums varied widely in general condition and in weight. Some were emaciated and sickly in appearance with sparse, ragged pelage, while others were in excellent condition, fat and with thick, glossy pelage. Seasonal trends are partly obscured by these differences in individuals, by the tendency to lose weight in those recently marked, and by the irregular fluctuations that occur in each animal.

The few opossums caught in summer were thin and appeared to be suffering from infestations of ectoparasites, especially chiggers (Eutrombicula alfreddugesi) and ticks (Dermacentor variabilis). Those trapped in October and November were mostly fat and in good condition. For individuals caught at different seasons, maximum weights were generally recorded in these two months. The maximum weight record of the study was one of an adult male weighing 5000 grams on December 23, 1950. The weight records of this individual were more complete than most and are recorded below to illustrate seasonal trends for adults. May 10, 1950, 1925 grams; May 14, 1830 grams; May 17, 1940 grams; November 5, 4540 grams; November 28, 4540 grams; December 23, 5000 grams; February 18, 1951, 3300 grams; March 6, 3080 grams; March 28, 3080 grams; May 28, 3080 grams; June 18, 2620 grams.

Of opossums that were trapped alive, the weight ranged from the maximum of 5000 grams to a minimum of 126 grams. The maximum in males was higher than in females. In fall, three rather poorly defined age-size groups were discernible in each sex: adults more than a year old and including all the largest individuals; large young born late the preceding winter and approaching small adult size; smaller young born in early summer and still less than half-grown. 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. The smaller young probably are subject to drastic reduction in numbers as a result, directly or indirectly, of severe winter weather. Many of these smaller young, weighing considerably less than 1000 grams, did not survive overnight when caught in live-traps in cool autumn weather, whereas adults and well-grown young generally survived exposure even for several successive nights in various extremes of weather conditions.

BREEDING SEASON

Hartman (1928:154) stated that there were at least two litters of young per year in the southern states with a small percentage of unusually fecund females producing a third litter. Lay, in eastern Texas, concluded (1942:155) that "The present investigation substantiates Hartman's deduction of two litters being normal, but fails to disclose any evidence of a third litter." He found females carrying young in the pouch only within the seven-months period January to July with definite peaks in February and June, and stated that second litters appear in the pouch from early April to as late as May 20 to 23. Reynolds (1945:362) found that the breeding season in central Missouri in 1941 and 1942 began about the first of February, with known or calculated birth dates of 42 litters rather evenly distributed throughout the periods February 12 to April 2, and May 16 to June 4. Eight of these females had given birth to young between March 16 and April 2, approximately six to nine weeks after the beginning of the breeding season. Reynolds assumed that these were individuals that had failed to find mates during the first oestrus of the season and that after completing the regular dioestrus of about 28 days they had then mated and borne young. Wiseman and Hendrickson (1950:333) in southeastern Iowa recorded a female with a litter no more than two days old on February 23, and several other females with young were estimated to have borne litters at approximately this same date, while still others bore litters as late as early March. Two lots of small young found in early June may have been second litters.

For the region represented by the present study, the data indicate a breeding season with later onset and sharply circumscribed limits as compared with an earlier onset and less circumscribed limits in Texas, central Missouri, and even southeastern Iowa, which is a little farther north. The available data indicate that there are two distinct and well-defined breeding seasons in the course of the annual cycle on the University of Kansas Natural History area. The whole population, including young of the preceding year, some still far below average adult size, breeds from about the middle of February into early March, and first litters are born mainly in early March. Individual females may vary as much as two to three weeks in the time of breeding, and varying weather conditions from year to year may hasten or delay onset of the breeding season. Data are recorded below for all females caught in March that were carrying litters.

Weight of Date female Number of in grams young Development of young

March 1, 1952 2000 9 Newborn March 2, 1952 1450 6 Newborn March 2, 1952 1230 7 Newborn March 5, 1950 1200 10 About 16 mm. snout to vent March 5, 1950 1300 1 About 14 mm. snout to vent March 6, 1951 1110 4 Newborn March 18, 1952 1930 8 Not present when female was trapped on March 1 March 18, 1952 1520 6 March 18, 1952 1230 12 About 40 mm. snout to vent March 19, 1951 1000 8 Estimated 1 week old March 22, 1950 1040 9 About 34 mm. snout to vent March 24, 1950 1280 10 74 mm. snout to vent March 24, 1950 1480 8 March 27, 1950 965 8 Total length 26 mm., weight .8 g. March 28, 1951 820 7 20 mm. crown to rump; born since previous capture of female on March 7 March 30, 1950 1325 9 Total length 33 mm. March 31, 1952 1930 8 March 31, 1952 1630 5 Total length 73 mm.

None of the females trapped in February was carrying young in the pouch, but probably some early litters are born in the last week of February or even earlier. 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.

Of 13 females examined in April, 12 were carrying young, and the remaining one was known to have been carrying a single young on March 1, but had lost it. Eleven females were examined in May, four of which were the same ones examined in April. Eight of the eleven females were carrying young; of the remaining three, one had lost the litter of young that it had been carrying when trapped in April. Two had empty pouches on May 19 and 20, but probably had successfully reared the litters of young which they had been carrying when trapped in April. The young of all those females trapped on different dates in April and May were in stages of growth indicative of birth about the first week in March. The latest date on which a female was recorded with first-litter young in the pouch was May 22, 1951, and these were the largest pouch young observed. Their eyes were recently opened, they were estimated to weigh 60 grams each with hind feet 20 mm. long. Young continue to grow rapidly after leaving the female's pouch. A young female caught on June 16, 1949, weighed 126 grams. For seven young caught on July 5 and 6, 1952, weights and hind-foot measurements were, for males: 660 grams, 52 mm.; 560 grams, 46 mm.; 550 grams, 48 mm.; 450 grams, 44 mm.; 370 grams, 44 mm.; 330 grams, 37 mm.; and for the one female: 430 grams, 46 mm.

The wide variation in size in this small group of young of nearly the same age is noteworthy. Size and condition of the females carrying them, number of competing litter mates, and early success or handicap in independent life causes so much divergence in size that at the age of four months some young are twice as large as others.

By late fall the young grow to small-adult size. For example, the female that weighed 126 grams when first caught on June 16, 1949, was recaptured on November 29, 1949, and on that date weighed 1710 grams.