Induction of maternal behavior in rats: effects of pseudopregnancy termination and placenta-smeared pups.

The onset of maternal behavior in Long-Evans rats was examined after pseudopregnancy (PsP) termination, both with and without exogenous estrogen administration, and in response to either clean or placenta-smeared stimulus pups. Natural (spontaneous) PsP termination was as effective in hastening the onset of maternal behavior as ovariectomy plus estrogen injection. If clean foster pups were presented as soon as pseudopregnancy terminated (first proestrus or cornified smear), maternal behavior was exhibited within 2 days; placenta-smeared foster pups presented at the same time elicited maternal behavior within 2 hr. The combination of initiating maternal-behavior testing immediately after the natural termination of pseudopregnancy and proffering placenta-smeared pups apparently simulates the hormonal milieu as well as the environmental cues present at parturition, noninvasively, producing optimal conditions for the rapid induction of maternal behavior.