Part 4

A second breeding season ensues soon after the young of the first litter leave the pouch, and these young probably soon learn to shift for themselves. Second litters are usually born in early June. On June 14, 1952, a female was taken with young only a few days old in her pouch. On July 5, 1952, two females last taken on May 19 and May 20, with their pouches recently vacated by first litters, were found to have young the size of half-grown mice, evidently two to three weeks old. In the months of October, November, December and January, a total of 11 young, thought to represent second litters, were taken. Dates of capture, weights in grams and sexes were as follows:

Oct. 3, 1950 400 grams male Oct. 6, 1950 510 grams female Oct. 8, 1950 260 grams female Oct. 8, 1950 350 grams female Oct. 18, 1950 350 grams female Dec. 5, 1951 630 grams female Dec. 30, 1950 710 grams female Jan. 1, 1951 660 grams female Jan. 1, 1950 700 grams male Jan. 9, 1950 550 grams male Jan. 11, 1950 550 grams male

estimated

The hind foot measured 48 mm. and 51 mm., respectively, in the young weighing 630 grams and 660 grams. These young, born in early summer have grown, by October, to a size comparable with that attained in July by young of the early spring litters. The variation in size is also similar but with a little wider range. The summer breeding season may be somewhat more protracted than the breeding season in early spring.

Too few females were caught in summer to compare the summer breeding season with the early spring breeding season, with respect to size of litters, percentage of non-breeders, and other factors which might affect the size of the crop of young produced. It is not clear why, among opossums trapped in winter, the young born in early spring outnumber those born in early summer by about four to one. Some females are eliminated after rearing the first litter, and others, exhausted by rearing large first litters may fail to participate in the second breeding season. 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.

NUMBERS OF YOUNG

Hartman (1952) has summarized his own findings and those of other authors regarding the embryology, birth, and early development of the opossum, and has corrected numerous popular misconceptions. He states that an average litter consists of about 21 eggs, but mentions much larger litters of up to as many as 56. However, many of these may fail to develop. The female normally has 13 functional nipples in her pouch and each one accommodates a single young. Excess young beyond this number are doomed, and soon perish from starvation if they reach the pouch after all the nipples are occupied. None of the females examined in the present study had a full complement of 13 young. Under unfavorable conditions, most or all of the young may fail to make the trip from the vaginal orifice to the pouch. Also, the pouch young are subject to heavy mortality, but observations concerning the time and cause of mortality are lacking.

Lay (loc. cit.) found an average of 6.8 pouch young in 65 litters examined in eastern Texas; Reynolds found an average of 8.9 (5 to 13) in 42 litters from Boone County, central Missouri; Wiseman and Hendrickson found an average of 9 (6 to 12) in southeastern Iowa. In the present study, 28 of the female opossums examined were carrying litters in their pouches, and all these females were caught in the months of March, April, May, June and July. The number of young varied from one to 12. Seven females each had seven young, six each had eight, three had six, three had five, and there were two each with nine, 10, and 12 young, and one each with one, four and 11 young. The average was 7.4 per litter. On several occasions females captured with young in their pouches and recaptured one or more times within a few weeks, were found to have lost some or all of the young. Some of the females examined probably had already lost parts of their litters. For instance, the female recorded with just one small young on March 1, probably had lost most of her litter and when recaptured a month later she did not have any young.

Nineteen yearling opossums were taken in the fall-winter-spring season of 1951-52; 42 per cent of the total, and 67 per cent of the females were individuals marked as pouch young the preceding spring. In the course of live-trapping, that spring, some first litters may have been missed. No second litters were marked because trapping was not continued into June and July when second litters are being carried by females. These figures suggest that the breeding population of females on an area consists chiefly of those born there the preceding spring.

COMPOSITION OF THE POPULATION

Sex ratio of opossums trapped was approximately 1:1; 59 males to 58 females. Age groups for opossums caught in the three seasons are shown in the following tabular fashion. For a few individuals age status was doubtful.

1949-1950 1950-1951 1951-1952 Total Old adults 11(25%) 9(26.4%) 11(39.2%) 31(29.2%) Yearlings: Born in late winter 29(66%) 18(53.0%) 13(46.5%) 60(56.6%) Born in late spring 4(9.1%) 7(20.6%) 4(14.3%) 15(14.2%) Total 44 34 28 106

In the 1950-51 season, small young of the summer brood seemed unusually numerous. In the 1951-52 period, young of both age classes were relatively scarce and old adults made up an unusually high proportion of the population. Excluding the 14 marked pouch young that were later recaptured, there were only four of the total of 106 that were trapped in each of two seasons. One young less than a quarter grown, that was accidentally caught in a live-trap set for woodrats, was recaptured as a breeding adult the following winter. An adult male and two adult females each caught in the 1949-50 season were each recaptured repeatedly in the 1950-51 season. Ninety-five per cent replacement of the breeding population by the following breeding season is indicated by our figures. Only 3 (or 5 per cent) of the individuals of the population trapped and marked in the season of 1949-50, were recaptured among the 62 opossums recorded in the two subsequent seasons. Various mortality factors including predation, disease, and accidents account for some 70 per cent. These are replaced by first-year young which make up the greater part of the breeding population. The remaining 25 per cent presumably shift their ranges sufficiently in the course of a year to have moved beyond the limits of an area of the size encompassed by the present study.

POPULATION DENSITY

No precise measurement of the population density on the study area was obtained. It was not practical to capture every individual present there, and rapid population turnover, due to mortality and wandering, obscured the trends. The information obtained concerning movements of opossums suggest that one may habitually forage as much as 900 feet from its home base. Assuming that 900 feet is the typical cruising radius, the areas drawn upon by the trap lines in the three different seasons were approximately as follows: 1949-50--400 acres; 1950-51--350 acres; 1951-52--220 acres. In these same three seasons the numbers of opossums caught were, respectively, 46, 37, and 30. If these figures represent the numbers actually present, densities of one to 8.7 acres, one to 9.5 acres, and one to 7.3 acres are indicated. However, some opossums using the area probably were missed; and on the other hand, not all those caught in the course of a season were present there simultaneously. Many of those present early in the season would have moved away a few months later, and others would have moved in, replacing them. The number present at any one time could scarcely have been more than half the number caught in the entire season.

CENSUS WITH HALF-MONTHLY SAMPLING PERIODS

Number of Number of Number of Computed individuals individuals recaptures population Sampling period taken taken in in for in following following sampling period period period period

Early November 1949 3 7 1 21 Late November 1949 7 8 3 18.7 Early December 1949 8 11 3 29.3 Late December 1949 11 7 4 19.2 Early January 1950 7 3 1 21 Early March 1950 5 8 2 20 Late March 1950 8 6 3 16 Early April 1950 6 3 1 18 Late April 1950 3 6 2 9 Early May 1950 6 3 2 9 Early November 1950 1 3 1 3 Late December 1950 3 6 1 18 Early February 1951 4 13 3 17.3 Late February 1951 13 6 3 26 Early March 1951 6 4 3 8 Late March 1951 4 5 2 10 Early April 1951 5 1 1 5 Late April 1951 1 5 1 5 Early May 1951 5 3 2 7.5 Early February 1952 9 4 2 18 Late February 1952 4 9 1 36 Early March 1952 9 6 2 27 Late March 1952 6 5 2 15

CENSUS WITH MONTHLY SAMPLING PERIODS

Number of Number of Number of Computed individuals individuals recaptures population Sampling period taken taken in in for in following following sampling period period period period

November 1949 9 16 7 21 December 1949 16 9 3 48 March 1950 11 9 3 33 April 1950 9 7 2 32 October 1950 9 3 3 9 November 1950 3 3 1 9 December 1950 3 7 3 7 January 1951 7 14 3 33 February 1951 14 7 4 25 March 1951 7 5 3 12 April 1951 5 6 3 10 November 1951 3 6 1 18 December 1951 6 5 1 30 January 1952 5 11 3 18 February 1952 11 13 4 36 March 1952 13 9 5 23 April 1952 9 3 1 27

Crude census-figures were obtained by utilizing the Lincoln Index and computing the total on the basis of the ratio of marked (and recognizable) individuals to others caught in a sampling period. A large number of census figures were obtained over the three-year period of the study. Each separate census, however, was based on an inadequate sample as the number of marked individuals taken at each sampling, as recaptures from the previous sampling period, varied from one to five. While little confidence can be placed in any one census computation, the trends of figures from series of such computations reveal the approximate number of opossums on the area if due allowance is made for certain distorting factors. Presumably the differences in figures obtained at different samplings result chiefly from the margin of error in the data, although it is true that there is rapid change in the actual number of opossums.

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. These young do not become independent until late May or early June, and during the intervening months there is a further reduction of the adults and yearlings, so that the active population reaches its annual low point in late spring. At that time of year most opossums are in poor physical condition.

The area represented by the opossums trapped totaled more than 500 acres, but not more than 400 acres were within the area drawn upon by the trap line at any one time. Usually the area represented at any one time by the trap line was less--100 to 350 acres, with from 25 to 45 traps. Traps were moved from time to time depending on the distribution of opossum sign and food sources, the weather, and the time available for this study. As a result, successive samples are not strictly comparable and a major source of error is introduced into the census computations. Lack of exact correspondence in the area represented by successive samples would result in a disproportionally small number of recaptures, and an erroneously high census computation. While adequate adjustment cannot be made, examination of the data suggests that census figures are too high, by as much as 50 per cent in many instances as a result of this factor, while in some other instances when there was little or no alteration of a trap line from one period to another, the census figure was not affected. 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 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.

MORTALITY FACTORS

Many of the opossums trapped were suffering from injury, disease, or parasite infestation, and some were in critical conditions. A large adult male trapped on April 2, 1952, seemed to be dying from disease. It was much emaciated and the pelage was sparse and ragged, as if the animal had been sick for a long time. The skin had numerous light-colored pustules 1 to 2 mm. in diameter, and these were especially prominent on the ears, lips, and penis. When released, the opossum was too weak to move away. It was excited by movements of the trapper, and stood erect with violent involuntary rocking movements. After a few seconds it gradually slumped to the ground and subsided into quiescence. On the next day no trace of it could be found.

Most of the opossums caught in summer and early fall had eye infections, and all of them were infested with ticks (Dermacentor variabilis). Sometimes ticks were attached in dense clusters of several dozen on the animal's ears and scattered over other parts of the body.

In March and April, 1950, seven adult opossums were found dead in the traps. None of these showed any evidence of disease or injury and they were normal in appearance except that they were thin. It was concluded that death had resulted from exposure and starvation in the traps in these animals already in critical condition as a result of winter food scarcity and frequent fasting. Up to this time the procedure had been to check the trap line only on alternate days and no mortality had resulted, even in the coldest part of the winter. The implication is that by spring, opossums are in a condition so critical that they are unable to withstand exposure or fasting and die whenever weather conditions are unusually severe.