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.
Figure 5. Weight changes in opossums live-trapped; lines connect successive weight records of the same individual, showing, in most, a downward trend throughout the winter and early spring, and an upward trend in late spring.
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.