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Introduction
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Methods and Results
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Discussion
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References
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The "reproductive postponement state" has been described most extensively in rodents, but it may also play a major role in the life histories of primates, including humans. In humans, reproductive postponement is given different labels depending upon the social context and the age at which it occurs; often it is labeled as "depression" and considered to be pathological, as a result of which the person may be removed from society as if they had a contagious disease. In rodents, the reproductive postponement state is mediated by a hormonal condition with profound effects on the animal's behavior. The hormonal condition consists of elevated levels of adreno-cortico-trophic hormone (ACTH) and corticosteroids, on the one hand, and depressed levels of gonadal hormones (androgens and estrogens), on the other hand. The behavioral effects in rodents include the depression of the motivational systems of sexual behavior, depression of offense behavior (including aggression to maintain a territory), and depression of patrol/making (including social locomotion and investigation, and scent-making). As a result, the animal ceases to be reproductively active, females cease to conceive or raise a litter, and males cease to maintain a territory (1, 2, 3).
If a reproductive postponement state occurs in the life cycle of a rodent before the onset of sexual maturity, then the onset is simply postponed (4, 5, 6).
The triggering stimuli of the reproductive postponement state are produced by social stress, and the state is maintained by the production of ACTH and corticosteroids in response to those stimuli. In rodents, the triggering stimuli may include both the pain and other tactile stimuli associated with defeat in fighting (7) and olfactory stimuli associated with crowding (8). It is not a permanent state, but may be reversed once the social stress is removed and normal social relations are resumed. In response to stimuli (especially olfactory) from other reproductively active individuals, the animal once more secretes gonadal hormones and reproduction becomes possible.
The evolutionary advantages of the reproductive postponement state are great. For the female, there is reduced metabolic demands of pregnancy and lactation and reduced risk of predation in comparison to other females who are reproductively active. For the male, there is no need to compete in fighting for a territory which reduces risks of predation and metabolic demands, and there is no danger of being attacked by other males because he no longer produces the androgen-dependent odors that stimulate such attacks (9). All this reduced danger comes at a time when reproductive activity is not likely to be successful in any case because of the social stress that makes it unlikely the young would survive. Whereas other individuals who do not enter a reproductive postponement state are more likely to die as a result of predation, fighting, or starvation, animals with reproductive postponement are able to survive the period of stress. They can return to a normal reproductive state after the period of stress is over (10) and reproduce offspring who retain the genetic mechanisms that make possible the entire process in the future.
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