Hormonal induction of maternal behavior in the ovariectomized nulliparous rat

A nulliparous female rat, if kept continuously in the presence of young, will begin to behave maternally. However, she will take, on the average, some six or seven days to display the behavior. In contrast, the puerperal female responds maternally as soon as the young emerge from the birth canal. This difference in latency was attributed to the characteristic action near term of a triad of hormones, consisting of estrogen, progesterone and prolactin. The attempt was made to impose this triad on ovariectomized nulliparous females. The experimental animals were administered subcutaneously estradiol benzoate, progesterone and prolactin. The estradiol was injected from Day 1 through Day 11; the progesterone from Day 6 through Day 9; and the prolactin on the Evening of Day 9 and the Morning of Day 10. The Control females, assigned respectively to four groups, were administered either only two of the three inductor hormones—the vehicle in each case having been substituted for the hormone omitted—or simply all three vehicles. On the Afternoon of Day 10, six normally-delivered foster young, 6–20 hours of age, were proffered each female. Of the 10 experimental animals, each, without exception, showed full maternal behavior at between 35 and 40 hr. Not only does this represent a significant reduction in latency from the average of 6–7 days characteristic of untreated nulliparae, but represents as well a uniformity in time of onset closely approaching that exhibited by the puerperal female. In contrast, each of the control groups showed marked variability in onset and a significantly higher median latency. Just how estrogen, progesterone and prolactin acted to induce maternal behavior was discussed. Also discussed was the possibility of reducing even further the obtained latency of 35–40 hr.