Another set of' positive feedback loops has been postulated to operate in the case of the parental motivational system, the secretion of prolactin, and the nursing of infants. Infant nursing is at once facilitated (indeed, made possible at all) by the parental motivational mechanism and by lactation that depends upon prolactin, and at the same time, the stimuli from the nursing facilitate both the parental motivational mechanism and the secretion of prolactin. The more the parental motivational system is activated, the more the motor patterning mechanisms of assuming the nursing posture and secreting prolactin (and consequent lactation) are activated. And the more these contribute to nursing, the more the nursing contributes to the activation of the parental motivational mechanism and the secretion of prolactin. other things being equal, the mother in the presence of her pups should maintain herself in a state of ever-increasing, mutually-facilitating activation of prolactin and the motivational system of parental behavior. This state may be called a "reproductive fulfillment state" to distinguish it from the reproductive readiness state described above.
There are limits and cyclic fluctuations within the states of reproductive readiness and fulfillment of the female. Rather than being continuously receptive, the female is periodically receptive, either in four to seven day cycles of spontaneous ovulation (estrus) which is the case in most muroid rodent species or else during ovulation that occurs in response to coitus which is the case in microtine species (Dewsbury, 1977). Following ovulation" the female enters a period of relative unreceptivity associated with pregnancy or pseudo-pregnancy; this occurs reflexively in response to coitus in the case of spontaneous ovulators and spontaneously following ovulation in the microtine rodents (Dewsbury, 1977). The hormonal basis for these checks and limits upon the female reproductive readiness state has not yet been determined; although progestin has long been thought to play a critical role (Arlmura , 1977), other hormones may be lnvolved as well (Baum et a1, 1979).
The operation of the anti-gonadal. system, postulated in Section II, may be cons1dered to create a th1rd reproduct1ve state that suppresses and replaces the states of reproductive readiness and reproductive fulfillment. Its hormones, that include the adrenocorticotrophic hormone (ACTH), inhibit the secretion of the gonadotropic hormones, thus suppressing the reproductive readiness state. Furthermore, it has been suggested that ACTH may 1nhibit the parental motivationaJ. mechanism, the ultimate effect of which would be to disrupt. the reproductive fulfillment state. This "anti-gonadal" state may be initiated by seasonal stimuli or by :prolonged activation of the submission motivational mechanism by social defeat or prolonged activation of the defense motivational mechanism by stimuli associated with predators or fear of predators. As discussed below, it may be called a "reproductive postponement state."
There may be still another reproductive state in muroid rodents associated with emigration. as will be discussed below. Its hormonal basis is not known.