Men in relationships always complain about their girlfriends PMSing. And yes, there comes a time of the month when we are a little whinier, needier, and bitchier than usual. But what guys don’t know is that our PMSing is offset by a short window of time exactly between our periods, when we’re more receptive to sexual advances and more likely to initiate them ourselves.

The word “estrus” refers to a state of heightened sexual arousal and activity during the ovulation period of the menstrual cycle in certain nonhuman mammals. During this period, female animals are especially attractive to males and receptive to mating. They will often exhibit the “lordosis reflex”—where they spontaneously elevate their hindquarters in the presence of a male to signal their sexual availability. In some species, mating only occurs during the estrus period. For example, in a study conducted by S. Matteo and E.F. Rissman (1984), it was found that “captive troops of rhesus monkeys… copulate nearly exclusively during the females’ ovulatory phases of the menstrual cycle.”

Up until recently, it was believed that only nonhuman mammals experience an estrus period. However, recent studies documenting the fluctuations in the sexual interest of women throughout their menstrual cycles have suggested that human females exhibit similar (albeit less pronounced) estrus symptoms. It is difficult to get exact information due to the complications that arise from couples using contraception (women on the pill typically don’t ovulate, and thus exhibit little variation in sexual behavior throughout their cycles), and due to the influence of the male sexual partner (since sex can be initiated by either partner, recording the mere number of sexual encounters is misleading).

These problems are rectified by gathering data from the women who occupy much of our fantasies: strippers and lesbians. In Matteo and Rissman’s study, lesbian couples recorded the frequency and nature of their sexual interactions throughout a complete menstrual cycle and found “significant peaks in sexual encounters and orgasms during the midcycle portion.” Because their research was based on lesbian couples, they could examine sexual behavior “without the confounds caused by contraceptive method and male influence on female sexual interest and initiative.” And although we don’t go around sticking our asses out and batting our eyelashes at anyone who crosses our path, this study suggests that human females do indeed show midcycle increases in sexual behavior, like most other female mammals.

What makes women more prone to masturbate, fantasize, and initiate sexual encounters in the middle of their menstrual cycle? To stimulate ovulation, a woman’s pituitary gland releases an increased amount of LH (luteinizing hormone) and FSH (follicle-stimulating hormone). These hormones influence estrogen synthesis and release, and some studies suggest that this increase causes increased sexual activity in ovulating women. Another study, conducted by Van Goozen, Wiegant, et al (1997) suggests that there is a significant peak in the levels of testosterone in women who are ovulating and that this is what is responsible for women’s increased horniness.

Whatever the cause of the increase in sexuality in the middle of women’s cycles, the implications of a “human estrus period” are abundant. Professors Geoffrey Miller and Brent Jordan of the University of New Mexico managed to quantify the midcycle increase in a woman’s sexuality by researching strippers. They gathered data via an anonymous web site where strippers provided information about their earnings, productivity, and menstrual cycles, and were able to get information on 296 work shifts—or 5,300 lap dances. According to this study, strippers who were ovulating made an average of $30 per hour more than women who were menstruating and $15 per hour more than women who were not in the middle of their cycles. They also found that strippers who were on the pill (and who thus don’t ovulate), made significantly less than naturally cycling women overall, mainly because they didn’t have the “estrus earning peak” that the other strippers had.

Since estrus impacts earnings, it could be useful for women in the workplace to schedule important meetings or big job interviews during certain weeks of the month. And men, if you’re having trouble getting your partner to agree to your kinkier suggestions, it might help to keep track of their cycles and proposition them when they’re ovulating.