BREEDING HABITS OF THE OPOSSUM — HARTMAN. 353 



REPRODUCTIVE PERIOD AND THE SPAN OF LIFE. 



As has just been stated, the opossum breeds the first year— "year- 

 lings" have furnished the writer with some of his best material. 

 The weights of pregnant females range from 651 grams to 2,200 or 

 even 2,600 and 2,800 grams, averaging 1,337 grams; in other words, 

 they continue to grow very considerably after becoming sexually 

 mature. On this basis, from a rough calculation based on a hundred 

 records of size and weight, it would seem that the opossum lives at 

 least seven years. What the reproductive period is, that is, the num- 

 ber of years that the opossum may bear young, it is almost futile to 

 estimate. However, many very old females with greatly dilated 

 pouches are anually brought to the laboratory. Some of these ani- 

 mals have doubtless passed the menopause because after months of 

 feeding they fail to show any sexual activity and on killing them their 

 ovaries and uteri are found to be of infantile size. It is quite pos- 

 sible, therefore, that even in nature some individuals actually die of 

 old age. 



NUMBER OF LITTERS PER YEAR. 



The number of litters a year is stated by Audubon and Bachman 

 (1850) to be three, and this statement is currently copied in various 

 descriptive accounts. Certain considerations have led the writer to 

 question the correctness of this statement. First, most writers under- 

 estimate the length of time necessary to rear a litter of young. The 

 attached stage of the pouch young is given as one month, which is 

 less than half the actual time. For Doctor Meigs observed one litter 

 attached for 72 to 74 days ; and the writer can state definitely that 

 the period is certainly not less than 65 days. Thus, if one add to this 

 the period of gestation of 11 days and an additional month in which 

 the young move freely about but still suckle, it is seen that a iitter 

 can not be reared in less time than three and one-half months. One 

 must, furthermore, allow several weeks for the greatly enlarged teats 

 to retrogress before the birth of a new litter ; for it is manifestly im- 

 possible for newly-born young to attach themselves to any but very 

 small nipples (cf. fig. 6, a, pi. 2, and fig. 45, pi. 10). Finally, during 

 November and December all females that were ever brought into our 

 laboratory were in the resting stage; that is, in ancestrus. The cal- 

 endar year of 12 months does not, therefore, allow enough time to 

 rear more than two litters of young with the possible exception of an 

 unusually vigorous female that begins breeding at the end of De- 

 cember and continues uninterruptedly until the following November. 



MATING. 



As has been stated above, the act of copulation in the opossum has 

 twice been described in the literature. On this point it is not neces- 



