Sexual selection operates under the same process of differential reproductive success described by Darwin in The Origin of Species. This success, or fitness, is determined by the amount of genes contributed by an individual to the next generation. Sexual reproduction adds genetic variation and allows for distinct sets of characteristics, some of which are advantageous and allow for the perpetuation of specific traits.
Natural selection drives population change. Sexual selection is the mode by which an individual chooses a mate with specific traits and behaviors.
The goal of any individual is to produce viable offspring, so he or she will select a mate who can ensure the achievement of this goal. Among many other species, mate selection is determined by female choice based on male display. Attributes such as a lion’s mane or a peacock’s tail feathers attract attention and lure partners. However, there are other species, like the Emperor Penguin, that have comparable nurturing responsibilities. Because the male and female take turns with the task of incubating an egg, both sexes are active in the selection process. In comparison, the male seahorse is the main contributor in ensuring the survival of the fertilized eggs. In short, the sex that expends more energy to guarantee reproductive success holds the power of choice.
In humans, sexual selection is based on both biological and cultural factors. Biologically, pheromones, chemical odors produce by the body, can attract potential mates. First discovered in silkworms in 1956, pheromones produce physiological responses and affect interaction. German researchers were able to isolate and extract a chemical compound from the female moth that resulted in the male moths beating their wings in a “flutter dance” without the presence of the actual female. Furthermore, a 1971 study of women in a college dorm showed that human pheromones influence menstrual cycles by synchronizing them. This sexual chemistry is also seen in the high levels of hormones released during initial attraction, which have similar properties to amphetamines and act to heighten awareness. On the other hand, endorphins, which create a euphoric sensation, are released in the presence of long-term partners.
Cultural factors that influence courtship rituals include concepts of beauty and social status. Universally, each culture has certain attributes that are desired in a mate, the three most important being health, age, and body symmetry. However, in the concept of health, what is defined as healthy is culturally determined. For example, a female considered overweight and unattractive by North American standards may have the ideal proportions in Latin America. Cross-culturally, however, men are more likely to place higher value on physical attractiveness, whereas women focus more on personality and social status.
Attraction, whether physically or socially determined, allows individuals to pick the best possible candidate for both reproduction and childrearing. Even though many cultures permit polygamous relationships, few individuals are wealthy enough to put this custom into practice. Because of the long infant dependency of humans, both partners must expend energy to care for the young; therefore, monogamy is a more common cultural practice. However, this evolved from a more general trend of serial pair bonding, in which individuals had only one partner at a time but more than one throughout life.
For humans, though, sex is not only about reproduction. Sexual interaction is also associated with play, intimacy, and emotional attachment and comes in many different forms. Sexual practices vary cross-culturally and are directed by learned behaviors.
- Middleton, D. R. (2002). Exotics and erotics: Human cultural and sexual diversity. Prospect Heights, IL: Waveland Press.
- Ridley, M. (1993). The red queen: Sex and the evolution of human nature. New York: Penguin Books.