In previous experiments, we have shown that age at vaginal introitus predicts adult patterns of reproductive, maternal, and aggressive behavior of female Mongolian gerbils. In the present series of studies, we explored the relationship in gerbils between age at vaginal patency and three other indices of rate of female sexual development: ovarian weight, occurrence of vaginal estrus, and onset of attractiveness to males. We found that female gerbils, classified on the basis of their ages at vaginal introitus as early-maturing: (a) exhibited greater ovarian weights throughout development than did their late-maturing, female sibs; (b) exhibited estrus and Diestrus I stage vaginal smears and, by inference, ovulation at an earlier age than did late-maturing females; and (c) elicited mounting by males at an earlier mean age than did late-maturing females. The data were consistent with the hypothesis that age at vaginal introitus is a reliable indicator of rate of sexual development in female Mongolian gerbils.
Age at vaginal opening is bimodally distributed in female Mongolian gerbils (Meriones unguiculutus) (Clark, Spencer, & Galef, 1986b). Some female gerbils, those we have described in previous papers as early-maturing (E-M), exhibit vaginal opening before reaching 23 days of age; (x = 15.9 & .5 days); other female gerbils, those we have called late-maturing (L-M), fail to exhibit vaginal opening until they are more than 27 days old (3 = 35.2 -+ .7 days). Fewer than 1 in 100 gerbils exhibit vaginal opening when they are between 23 and 27 days of age. (Clark et al., 1986b). Hence, female gerbils can be conveniently defined as either early-or late-maturing depending on whether they exhibit vaginal introitus before or after reaching 25 days of age (Clark et al., 1986b).