Reproductive cycle and litter size of the woodchuck [Cycle reproductif et taille des portées chez la marmotte des bois].

Résumé

From 1955 to 1958 the reproductive organs of 382 adult female woodchucks from a 10,000 acre area in south central Pennsylvania were examined to obtain information about reproduction and the reproductive cycle. The sexually mature males emerged from the hibernation dens around February 1 in this locality, about 3 to 4 weeks before the females and the sub-adult males. The majority of the sexually mature females were inseminated during the first 3 weeks of March and most of the young were born during a period from April 1 to April 21, the mean date of parturition being approximately April 10. From 75 to 80% of the females were pregnant each year, excep in 1956 when only 20 of 36 examined were pregnant. The collective noun gravidum (pl. gravida) has been introduced to describe the total number of embryos or fetuses in a pregnancy. Gravidum is considered to be a prenatal counterpart of litter. The mean number of implantations per gravidum varied considerably from year to year, being as low as 4,00 in 1956 and as high as 4.67 in 1957. The percentage resorption of embryos and fetuses declined from 16.6 in 1955 and 26.1 in 1957 to13.2 in 1958. The approximate average litter size was highest in 1958, 3.97 compared to 3.47 in 1955 and 3.42 in 1957. The reproductive rate increased from lows of 2.59 in 1955 and 2.57 in 1957 to 3.24 in 1958.