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Some aspects of reproduction in female Microtus townsendii MacFarlane, Joan Damon

Abstract

In 1966 and 1967, data obtained from a breeding colony of Microtias townsendii townsendii, supplemented with field data, have been utilized to examine the following aspects of reproduction: onset of pubescence in female; nature of the estrous cycle; ovulatory pattern; pregnancy and events following pregnancy. Onset of pubescence: Vaginal perforation occurred with the greatest frequency between days 30 and 45 of age (Range 19-82) among isolated litter females. There was no predictable relationship between the initial occurrence of perforation and first estrus in animals studied (Range 1-36 days). Field females became perforate with the greatest frequency between 15 and 20 days (utilizing laboratory growth chart). Six of seven females in this range demonstrated both vaginal perforation and estrus by smear. Inasmuch as field statistics of age of sexual maturity vary greatly in Microtus on several ecological bases (Krebs and Myers, 1974), these data are of minimal significance in the field. Nature of the estrous cycle: On the basis of vaginal smear studies and correlated histological examination of ovaries and reproductive tracts, M. townsendii appears to be an induced ovulator. Of 466 smear days of isolated females: 57 percent of the smears were estrus; 35 percent dies-trus; 8 percent proestrus and metestrus. Cyclic periodicity of estrus was from 2-7.5 days. No ovulation, corpora lutea or corpora albicantia were observed in any unmated female. Analysis of variance demonstrated significant difference between estrus and the other two stages, but no significant difference between diestrus and anestrus except for the larger diestrus mean maximum ovarian diameter. Interstitial tissue appearance and number of medium sized follicles are posed as explanation for this difference. Two metestrus females appeared most similar to observations of estrus. The inference is made that stages reflect low to high estrogen levels in the order: anestrus, diestrus, metestrus, estrus. Anestrous females given P.M.S. became perforate and 7 of 9 were in estrus 51-53 hours later. This supports the existence of a prior extrinsically based P.S.H. block. Coitus-induced ovulation: No unmated females examined had ovulated; three females ovulated after sperm in the vaginal smear were observed. Evidence, though not conclusive, is strongly presumptive in support of coitus-induced ovulation. Pregnancy and events following pregnancy: Three dated pregnancies were obtained with gestation periods of 21, 23, and 24 days. Litter size was 8.48 of laboratory litters (n = 13); 6 of pregnant field female embryo counts (n = 20); 5.41 of field females which had litters in the laboratory (n = 12). All pregnant females demonstrated diestrus vaginal smears; two instances of post partum estrus were documented.

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