In order to study sexual desire, scientists naturally sought to manipulate certain handles of sexual desire in an attempt to determine if manipulating these triggers resulted in changes in sexual desire or behavior. The search was a difficult one, however. Interrupting nerve signals to sexually stimulating parts of animals did not work. Damaging key parts of the animals’ brains did not work either. Neither did lobotomizing them, removing their eyes, or zapping parts of their brains.
Finally, they were able to successfully eliminate a female cat’s capacity for sexual desire by removing its ovaries. This stops it from engaging in any of the telltale mating behaviors typical of female cats (clawing at the ground, spraying, yowling, rubbing themselves on objects, and so on). These cats would nonetheless exhibit sexual desire once a suitable synthetic estrogen chemical could act as a functional replacement (at least on a behavioral level) for its ovaries.
Next, rather than subcutaneous estrogen injections (by which they could artificially produce mating behavior in the cats), they implanted a device that could directly distribute estrogen into the hypothalamus of the cats. While the cats showed none of the physiological signs of being in heat, they still exhibited the aforementioned mating behaviors. In fact, they became hypersexual cats, engaging in intercourse continually for almost 2 months. Thus, it appears that estrogen acts as a signal within the hypothalamus that is independent of the physiological changes associated with being in heat. Normally, however,
“a feedback system with the hypothalamus acting like a network telephone exchange “listens” for the physiological changes of estrus induced by ovarian hormones and then coordinates behavior so that a female can take action to become pregnant when the time is right”(Young, 2013).
Most rodents are like humans in that they ovulate after estrogen and progesterone production explodes during a four-day period. It is the behavioral effects of this process which scientists are able to artificially stimulate by means of estrogen in a female mammal whose ovaries have been removed.
“Estrogen exerts its powerful behavioral effects by docking at receptors concentrated in the ventromedial hypothalamus, just behind the MPOA. When these receptors bind to the regulatory regions of certain genes, the chemistry of the neurons changes. They increase neurotransmitter production and become more sensitive to other signalling molecules…when estrogen docking as increased the production of progesterone receptors in the ventromedial hypothalamus, the resulting heightened sensitivity to progesterone allows the hypothalamus to time ovulation with amazing accuracy. This estrogen-progesterone sequence leads the female rat to start her seductive hoppoing and darting precisely at the time she is most fertile, and then to assume the lrodosis position if a male takes the bait.”(Young, 2012).
During ovulation, women tend to be more flirtatious, receptive to sex, more likely to initiate sex, and more attracted to “rugged” men than at other periods of their cycle. Female behavioral patterns during ovulation are perhaps not as obvious as a rodent engaging in lordosis, but female humans clearly have our own parallel.
Parts of the brain responsible for sexual desire are closely bound up with parts of the brain responsible for stress. In particular, it is the pituitary, the hypothalamus and the adrenals that are responsible for the stress response. The brain stem, prefrontal cortex and amygdala act as input devices to this system. The hypothalamus is able to communicate with the brain stem, which is in charge of the autonomic nervous system. In females, the autonomic nervous system regulates the function of the ovaries and levels of sexual desire. The prefrontal cortex is in charge of goal-oriented behavior. The amygdala communicates with the hypothalamus, telling it how and when to produce such and such a hormone at such and such a level.
Higher levels of estrogen cause women to relax and to be less likely to be anxious. The hypothalamus is thus alerted that it is safe for the woman to reproduce. When ovaries are preparing to release an egg, estrogen tells the woman that it is time to mate. It is at this point that the female becomes less risk-averse and more open to flirting and mating. Their perception of males becomes more focused on the positive and tends to excuse the negative.
Functional MRI studies were conducted on women at various times during their cycles. That is, brain activity was surveyed both when estrogen was low and progesterone high, on the one hand, and during ovulation, when progesterone was low and estrogen high. When presented with a picture of a male and asked to entertain a scenario in which the male was willing to have sex with the woman, the medial orbitofrontal cortex exhibited notably more activity during ovulation than at other times of the month. Since this part of the brain is involved with reward-seeking, decision-making and goal-directed behavior, it is likely that, in such a scenario, the woman would be seriously considering the possibility of having sex with the man. The scientists also found that ovulation is associated with lowered inhibition in general, not only towards sex, but towards risky, non-sexual decisions.
Just as estrogen mediates sexually behavior in important ways for women, testosterone does so for men. Castrated males tend to exhibit a major decrease in sex-directed behavior, whereas these tendencies are restored when testosterone is applied to specific circuits of the brain. Male testosterone tends to naturally decrease in certain situations, such as when a male becomes married and have kids or when they become depressed or disappointed.
Sexual arousal in males is associated with an increase in testosterone levels. Most men may not realize it, but, just as women are more flirtatious during ovulation, men themselves are more attracted to women. For example, when men are given odor samples taken from ovulating women, they experience a spike in testosterone. It should perhaps be unsurprising that ovulating strippers consistently make notably more money than those at other points in their cycle. This may also have to do with differences in the quality of a woman’s voice (it becomes higher-pitched during ovulation), differences in waist-to-hip ratio, skin texture, etc. that change during ovulation. Men are able to pick up on these signs. Furthermore, the strippers may exhibit greater risk-taking behavior associated with lowered anxiety. This may lead to behavior that appears more authentically desirous, leading men to be more attracted to them. As noted before, these behaviors peak during ovulation.
Young PhD, Larry; Brian Alexander (2012-09-13). The Chemistry Between Us: Love, Sex, and the Science of Attraction (p. 44). Penguin Publishing Group. Kindle Edition.