The Romans considered that female mammals were driven mad by sexual desire. The term "estrus" describing female sexual heat is derived from the word "oestren" or gadfly. Cows race madly around their pastures to escape being bitten by the gadfly. A recent scholarly book argues that women also experience estrus.
This is something of a revelation given that the received wisdom of sex researchers was that women had lost estrus entirely, or had "concealed" it so as to permit continuous sexual receptivity. Being sexually receptive all of the time kept men in the dark as to when ovulation was actually taking place. This meant that if a man was to impregnate a woman, his chances were improved by staying around rather than just showing up during the few days of the month when a woman is most likely to conceive.
In making the case that women have estrus sexuality (1), Randy Thornhill and Steven Gangestad of the University of New Mexico, had to hack away at what they see as a false stereotype of sexual heat in females. Contrary to the view that females in heat are indiscriminate, Thornhill and his longtime collaborator argue that the opposite is true. They conclude that although female sexual interest is heightened at estrus, they become highly discriminating in their choice of a mate.
Female chimpanzees mate with all the adult males in their group when their sexual swellings appear signaling impending estrus. When they are likely to conceive, however, they disappear with a single high-quality consort male who will father their offspring.
Changing views of female sexuality
Prior to Darwin, females were considered to have inconsistent sexual interests, if they had any at all. Then Darwin proposed that female choice is an important mechanism in evolution, resulting in such phenomena as the peacock's tail but others were skeptical. A century passed before Darwin's ideas on sexual selection were taken very seriously. Researchers began to amass evidence of male adaptations designed to impress choosy females, such as the gaudy plumage of male birds.
In addition to hooking up with men having good genes, women also turned out to be picky about the social prospects of potential mates. They preferred men who were socially successful, had valuable skills, or enjoyed high social rank, or wealth. A willingness to invest in children was also valued.
Thornhill and Gangestad believe that women have essentialy two different kinds of sexual responsiveness. That occurring around ovulation is referred to as estrus sexuality. Sexual responsiveness outside the fertile phase of the monthly cycle is referred to as extended sexuality. They believe that estrus sexuality is designed to obtain good genes for offspring whereas extended sexuality involves those other benefits, e.g., food, that women may extract from men by being sexually receptive when they are not fertile.
Given that women are sexually receptive for most of their monthly cycle, estrus cannot be defined by receptivity alone. This means that the greatest burden for Thornhill and Gangestad in establishing estrus sexuality for women is placed on the argument that women are more discriminating in their choice of a mate during this phase of their cycle.
They found a remarkable amount of evidence in support of the view that women are far choosier around the time of ovulation. In particular, during estrus women are more attracted to men with highly masculine faces and bodies. They are also more swayed by bodily symmetry, which is a reliable index of genetic quality. They are even more attracted by the scent of symmetrical men. In estrus, women find highly masculine voices more attractive as well.
Like other mammals, it seems that women are choosier about what sort of man they will mate with when they are most fertile. This implies that their sexual psychology is designed to obtain good genes, rather than simply to secure any sperm.
Just when we feel that we are finally sorting out the difficult topic of human female sexuality, we discover that it is still more complex. If Thornhill and Gangestad are correct, then when women become more sexually liberated in some societies, this would be due to the benefits of sex outside estrus.
Yet, women in sexually liberated societies are both more interested in male physical attractiveness (2, suggesting estrus) and more interested in having various partners (suggesting extended sexuality). There is not a watertight distinction between getting good genes and mating for non genetic reasons after all.
1. Thornhill, R., & Gangestad, S. W. (2008). The evolutionary biology of human female sexuality. New York: Oxford University Press.
2. Swami, V., & Tovee, M. J. (2005). Male physical attractiveness in Britain and Malaysia: A cross-cultural study. Body Image, 2, 383-393.