Reproduction in a Population of Yellow-Bellied Marmots (Marmota flaviventris)

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Abstract
The annual reproductive cycle within a yellow-bellied marmot population was studied in the central Sierra Nevada of California about 30 mi. N Lake Tahoe. Seventy animals were live-trapped and ear-tagged, and the reproductive organs of 47 marmots were examined histologically. the approximate date of emergence from hibernation apparently was governed by the time of spring thaw. the animals were ready to breed when they emerged, so the thaw apparently would determine the time of mating and copulation, parturition, weaning, and juvenile emergence from the natal burrow. During the 2-week mating and copulation period, almost all adult females, but few subadults, became pregnant. the average gravidum per pregnant female was 4.8 and litter size was slightly higher. the adult sex ratio was approximately equal, as judged by trapping records. However, juvenile males were seen much more often than adult males. a high population in 1966 dropped to a low level in 1967, seemingly as a result of over-winter loss. Juveniles grew most rapidly, followed by subadults, and then by adults. the average number of active days required to grow 1 centimeter in length was 4.7 days for juveniles, 11 days for subadults, and 16 days for adults.