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Reproductive performance is the currency of evolution. All things being equal, an organism should reproduce as often as possible. The puzzling questions in evolutionary biology, therefore, are not how and why an organism does reproduce, but rather how and why an organism does not reproduce. It is difficult to understand why any individual, particularly a female, might forestall reproduction when one of the biggest limitations for female mammalian reproduction is time (that is, reproductive lifespan). The answer, now widely cited throughout behavioral ecology is quite simple: Reproductive suppression can be an adaptive strategy.
Beehner et al. (Sun,) studied this question.