Evolutionary scientists have never had difficulty explaining the male orgasm, closely tied as it is to reproduction.
But the Darwinian logic behind the female orgasm has remained elusive. Women can have sexual intercourse and even become pregnant - doing their part for the perpetuation of the species - without experiencing orgasm. So what is its evolutionary purpose?
Among the theories that Dr. Lloyd addresses in her book is one proposed in 1993, by Dr. R. Robin Baker and Dr. Mark A. Bellis, at Manchester University in England. In two papers published in the journal Animal Behaviour, they argued that female orgasm was a way of manipulating the retention of sperm by creating suction in the uterus. When a woman has an orgasm from one minute before the man ejaculates to 45 minutes after, she retains more sperm, they said.
Another hypothesis, proposed in 1995 by Dr. Randy Thornhill, a professor of biology at the University of New Mexico and two colleagues, held that women were more likely to have orgasms during intercourse with men with symmetrical physical features. On the basis of earlier studies of physical attraction, Dr. Thornhill argued that symmetry might be an indicator of genetic fitness.
In the 1981 book "The Woman That Never Evolved" and in her other work, Dr. Hrdy argues that orgasm evolved in nonhuman primates as a way for the female to protect her offspring from the depredation of males.
She points out that langur monkeys have a high infant mortality rate, with 30 percent of deaths a result of babies’ being killed by males who are not the fathers. Male langurs, she says, will not kill the babies of females they have mated with.
In macaques and chimpanzees, she said, females are conditioned by the pleasurable sensations of clitoral stimulation to keep copulating with multiple partners until they have an orgasm. Thus, males do not know which infants are theirs and which are not and do not attack them.