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The peaks in estrogen and progestin that precede and accompany ovulation in female rodents have a series of profound effects upon her social behavior that increase the probability of successful fertilization by the male. Since muroid rodents are small, relatively defenseless animals with respect to their many predators, the female is otherwise shy and retiring, depending upon protective coloration, nocturnal habits, arboreal or subterranean nests, etc.for protection. But during estrus, she becomes much more conspicuous and attractive to the male. The most far-reaching effects are upon her motivational mechanisms. The exploration/marking motivational mechanism is facilitated (site 2) which leads to increased locomotion, approach to other animals, and various types of scent-marking that serve to attract males. The female sex motivational mechanism is facilitated (site 3) and the offense motivational mechanism is suppressed (site 1) which ensures that when approached by a male she will permit, indeed encourage, mounting, intromission, and ejaculation by the male rather than attacking or ignoring him. These effects are not indiscriminate, but are shown particularly in response to conspecific males, due to activation of an olfactory filter that is tuned to androgen-dependent pheromones and that facilitates exploration/marking (site 9). There are also important peripheral effects of estrogen. the quality and quantity of' her pheromones may be increased by estrogen, making them more attractive to males (see section I-H). the receptive field for the lordosis reflex is sensitized and enlarged (site 12), and her olfactory sensitivity is increased (see I-F). The female's pheromones serve as attractants for males, operating upon sensory filters that not only increase the male's exploration/marking, but also inhibit offensive attack by those males (brain site 8).
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