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Reproductive biology of the eastern and least chipmunk Venhuizen, Bradley Austin


Eastern and least chipmunks from northern Minnesota were studied in regard to fecundity, age at maturity, timing of reproduction, and changes in the reproductive organs with age and season. A computer program for discriminate analysis was used to determine relative age of the individual animals. Traditional age indicators were used as variables. The result is a measure of the dispersion of the individual animal from a selected control group using all variables at once. Subjective decisions are almost eliminated by this technique. The histology of the reproductive and accessory organs is similar to that of other sciurids as described by other authors. An apparent difference is the presence of two very distinctive areas in the epididymides. The epithelium of one region has central nuclei and is about 31.4 mu. in height. The epithelium of the other area has basal nuclei and is about 16 mu. in height. Sperm is absent from the lumina of the first tubule type, but abundant in the lumina of the second tubule type. The chipmunks in both Oregon and Minnesota had only one breeding season per year which occurred shortly after emergence from hibernation. Hence juveniles do not breed until the spring following their birth. A significant change in ovulation rate of the eastern chipmunk was detected between 1967 and 1968, and for the least chipmunk in Oregon between 1968 and 1969. The rates changed from 7.09 in 1967 to 4.35 in 1968 for the eastern chipmunk, and from 4.78 in 1968 to 6.31 in 1969 for the least chipmunk. Preimplantation loss and resorption of embryos increased in the years of lower ovulation rate.

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