Isolation accelerates reproductive senescence and alters its predictors in female rats

The process of reproductive senescence in female rats (Rattus norvegicus, Sprague-Dawley strain) was altered by their social environment during adulthood. The incidence of constant estrus (CE), which marks the end of estrous cyclicity, was nearly twice as high in females living in isolation as it was in females living in groups. Isolated females also entered CE at three times the rate of group-housed females. In addition, the characteristics of a rat's estrous cycle when she was young predicted whether or not she would enter CE during reproductive senescence. However, the characteristics of the cycle that predicted if a rat would enter CE were different for isolated females than for females living in groups. In isolated rats, entry into CE was predicted by a pattern of regular cycles followed by irregular cycles, a lordosis reflex of consistently high intensity during irregular cycles, and an absence of spontaneous pseudopregnancies. Furthermore, a long duration of CE was predicted by early cessation of estrous cycles. In rats living in groups, increased estrogenization of the vaginal smears during irregular cycles was the only predictor of entry into CE. In both environments, the onset of an acyclic lordosis reflex predicted the timing of CE. The potential role of ovarian steroids as mediators of these effects is discussed.