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  • Author: E. M. Banks x
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Department of Ecology, Ethology and Evolution, University of Illinois, Urbana, Illinois 61801, U.S.A.

(Received 10th September 1974)

The rate of sexual maturation in females has been shown to be related to population density in a number of species of small wild rodents (Microtus californicus: Greenwald, 1957; Mus musculus: Southwick, 1958; Rowe, Taylor & Chudley, 1964; Microtus ochrogaster and M. pennsylvanicus: Keller & Krebs, 1970; Lemmus trimucronatus and Dicrostonyx groenlandicus: Krebs, 1964). It has been shown in the laboratory that the presence of an adult male accelerates the rate of sexual maturation in young female mice (Vandenbergh, 1967; Castro, 1967; Fullerton & Crowley, 1971), prairie voles (Hasler & Nalbandov, 1974) and cuis, Galea musteloides (Weir, 1973).

In this study, the influence of adult male collared lemmings (Dicrostonyx groenlandicus) on the rate of sexual maturation after weaning of juvenile females was investigated. The animals were laboratory-born descendants of lemmings captured near Fort

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J. F. Hasler, A. E. Buhl and E. M. Banks


The effect of photoperiod on sexual function and growth in weanling male and female collared lemmings (Dicrostonyx groenlandicus) was studied. Males and females maintained in 6 hr light:18 hr dark after weaning were larger at the end of 70 days than their siblings kept in 20L:4D. Males in 6L: 18D were longer and overall body growth was greater. Males in 6L:18D had heavier adrenals but there were no differences in the weights of testes, preputial glands or seminal vesicles or in the mean testosterone levels of the two groups. Females in 6L:18D had heavier preputial glands than the 20L:4D females. There were no differences between the two groups in uterine, ovarian or adrenal weights or in the number or mean size of atretic or healthy Graafian follicles. Adult males kept in 6L:18D increased more in body weight than those in 20L:4D, but there was no detectable influence of the short photoperiod on reproductive function.

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In the collared lemming, Dicrostonyx groenlandicus, ovulation occurred approximately 9 hr after copulation or injection of HCG. Ovulation did not occur spontaneously during long periods of oestrus but was induced by copulation during oestrus. Induction of ovulation was independent of the time of day at which copulation occurred. Between the time of copulation and 8 hr later, preovulatory follicles increased in size. Following copulation, meiotic changes in the ova of preovulatory follicles proceeded rapidly and 1 hr after copulation, the nuclear membranes had almost disappeared. The first meiotic metaphase was evident 2 hr after copulation. In some eggs, the first mitotic division had occurred 24 hr after copulation.