Estrous Cycle and Related Aspects of Reproduction in Captive Asian Chipmunks, Tamias sibiricus

Abstract

Reproductive patterns of free-living Asian chipmunks, Tamias sibiricus, have been described by Geinitz (1980), Ognev (1940), and Shubin (1964), who reported on breeding seasons, litter sizes, approximate length of gestation, and emergence ages of young. Ognev (1940) also mentioned mating calls by females. Some aspects of reproduction, however, are observed more easily in captivity than in the wild. Because female Asian chipmunks vocalize when in estrus, even in captivity (Blake and Gillett, 1984; Imaizumi, 1972), the estrous cycle is detected easily in captive females. Herein, we report information on estrous calling behavior, length of the estrous cycle, mating behavior, breeding seasons, gestation period, number of litters per year, litter size, and sex ratio for captive Asian chipmunks. Asian chipmunks were imported into Great Britain until 1971 and since have been maintained in captivity. In the course of a study of conditions for breeding captive Asian chipmunks (Blake and Gillett, 1984), we surveyed private breeders in Britain and accumulated data on reproduction. The original stock for the chipmunk colonies was reported by importers to have come from Japan and Korea, but we do not know the exact locations. Subsequent interbreeding among the descendants makes it impossible to refer the chipmunks to subspecies. Some data on time of breeding, and number and sex of young came from our own colonies. We studied our colonies for 9 years (KEG) and 3 years (BHB), respectively, for a total of 29 female-years, where a female-year is 1 year in the life of one female. Our colonies were kept indoors in large cages, 0.6 by 0.6 by 1.8 m (BHB), or 0.9 by 0.5 by 0.4 m with daily exercise in large indoor and outdoor areas as large as 7 by 5.9 by 2.8 m (KEG). BHB's colony was in London, and KEG's colony was in Somerset Co., England for 6 years and in Amman, Jordan for 3 years. BHB's colony was observed for about 0.5 h daily, 5 days a week, and KEG's colony was observed for 4 h or more, 6 or 7 days a week. Additional data came from private breeders who recorded breeding dates, birth dates, and number and sex of young. These data and our own allowed us to determine breeding and gestation times, and number, size, and sex ratio of litters. Most of this information came from five breeders who kept detailed records for periods ranging from 5 to 17 years (115 female-years). Their colonies were in outdoor cages ranging in size from small (0.6 by 0.5 by 0.5 m) to large (2.4 by 1.9 by 2.4 m), and, in one colony, also in small indoor cages (1.0 by 0.4 by 0.6 m). Breeders observed their chipmunks daily for periods from 2 min to 3 h, with an average of about 15 min/day (and longer when mating and parturition were anticipated). Eight other breeders, with chipmunks kept under similar conditions, provided numbers and sex of young at weaning. More information on breeders and their colonies is given in Blake and Gillett (1984). Litters were born in nest boxes and remained there until they emerged at weaning. Because most breeders did not disturb young in nest boxes, litter size and sex of young were determined at weaning, rather than at birth. However, few nestlings died (except when entire litters died); one breeder who inspected infants the day after birth found that only 3.3% (four of 123 young, n = 31 litters) died between birth and weaning. In comparing litter size under different conditions, pairwise tests were used to eliminate effects of cage size, which was correlated with litter size (Blake and Gillett, 1984); comparisons were made between litters within the same female-year for first and second litters of the year, or between litters born in different years to the same female for maternal age and experience. Information on estrous calling behavior came from our colonies and two breeders' colonies. We determined lengths of estrous cycles for three females in KEG's colony from the day calling began in one cycle to the day it began in the next. These females were unmated, and dates were recorded for periods ranging from 1 to 4 years for a total of 7 female-years. We also recorded beginning and ending of the calling season for a fourth unmated female, giving a total of 8 female-years for length of calling season.