Sometimes in the life of the female, it is advantageous to postpone reproduction. On the one hand, her genetic code is biased towards reproduction since it is an inheritance from ancestors who did reproduce. On the other hand, reproduction can be a great risk. During her pregnancy and lactation she must increase her metabolic intake enormously. And it is a relatively long-term commitment; after all, gestation and lactation may require about 50 days which may be at least 10% of her lifespan. Furthermore, during this time she is a special and vulnerable target for predators; her location is fixed, she and her offspring must use communications systems which predators may be able to intercept, and they are a particularly rich potential source of food for such predators. And finally, she and her offspring are subject to the attacks of conspecifics who can potentially gain in the struggle for differential reproductive advantage if they can kill her offspring.
There are two situations in which it is particularly useful to postpone reproduction: during periods of climatic adversity and during periods of overcrowding and dwindling food supply. In these situations it is less likely that her offspring will live to reproduce, in which case the risk is not worth the gain of reproduction
The state of reproductive postponement is mediated by a hypothetical "anti-gonad system" that includes secretion of CRF, ACTH, and adrenal corticosteroids. The effects of these hormones are profound and far-reaching in the life of the female. On the one hand, her reproductive physiology, her sexual attractiveness, and her sexual receptivity are turned off, as her secretion of gonadotropins and gonadal hormones is suppressed (site 13). On the other hand, under conditions of adversity, these functions are at best irrelevant, and perhaps they may be quite disadvantageous; her attractant pheromones could serve as attractants for predators as well as conspecific males. Her exploratory/marking and sexual behavior is reduced, due to lack of estrogen facilitation at brain sites 2, 3 and 9, and she does not wander from home or engage in scent-marking which would spread her attractant pheromones. Her tendency to become involved in fighting is similarly suppressed; in this case, as well, there is little to be gained, and much to be lost if the fighting serves as an attractant for predators. The tendency towards fighting is suppressed at two points: at site 1 her offense motivational mechanism is suppressed by ACTH; and at site 6 her consociate modulator is facilitated by corticosteroids, instead of fighting back against another attacking conspecific with behaviors such as lunge-and-bite, she responds with submissive behaviors such as freezing, upright posture, and ultrasound that inhibit the offense of the opponent. She yields dominance to the opponent, but is not forced to fight or flee.
What is the advantage of such a state, the state of reproductive postponement? There is no sexual behavior, no fighting, not fighting back against an attacking conspecific. Instead there is only submission and olfactory neutrality. Only against a predator is her defense behavior unmodified and fully effective. In terms of evolution, it can best be understood if we assume that in such a quiet, self-demeaning way, this female can wait out the winter or the period of overcrowding and live to reproduce another day. One may assume that she will survive and enter once more a reproductive state, thus making her contribution to a future gene pool that will include the genetic basis for a reproductive postponement state.