Decoding Love - Andrew Trees [17]
Once you look at sexual activity primarily as a way to strengthen the pair-bond, though, you can begin to make sense of a variety of oddities about human beings. For instance, women have a tipped vagina, which promotes more intimate face-to-face copulation, and large breasts, which are on permanent display and act as a constant advertisement of sexual receptivity totally disconnected from ovulation. In contrast, most female mammals only develop enlarged breasts when they are pregnant. Ethologist Desmond Morris argues that various other features—fleshy earlobes, protruding noses, and everted lips—are also designed to promote face-to-face copulation. Even the loss of body hair was possibly a means of promoting the pair-bond.
Perhaps most important, women developed concealed ovulation, which makes it impossible for men to tell when it’s the ideal time to mate. This makes her distinct among primate females, all of which have visible displays of their fertility (think of certain primates in which the females buttocks turn bright red during estrus). Further confounding male efforts to determine the time of peak fertility, women do not limit their sexual activity to the time they are ovulating. These developments likely played a crucial role in cementing her bond with a man. Instead of guarding a woman jealously for a few days during ovulation, the man had to develop a long-term relationship to try to ensure that her offspring would also be his. Anthropologist Helen Fisher has called this the “sex contract” that evolved to secure women the help they needed to raise their children.
CHEAP SPERM AND PRECIOUS EGGS
So, with all of these traits to promote pair-bonds, everything should be great when it comes to the relationship between a man and a woman, right? Unfortunately, no. To understand why, we need to explore the crucial role of two largely unmentioned participants in all of this pair-bonding, the sperm and the egg. It is at their fruitful conjunction that everything happens. But what they do to get there and how their carriers (i.e., men and women) feel about that journey makes all the difference.
It took a while for scientists to realize the significance of this. Although they were busy studying and refining Darwin’s arguments, sexual selection didn’t receive a lot of attention, particularly when it came to one particular segment of the animal kingdom—human beings. While happy to study the mating rituals of everything from slugs to lemurs, scientists proved reluctant to put humans under the microscope, albeit with a few high-profile exceptions such as Alfred Kinsey. That all began to change in 1972 when Robert Trivers published an essay entitled “Parental investment and sexual selection.” Despite the rather pedestrian title, that essay is possibly the single-most influential piece of evolutionary theory to come along since Darwin’s original concept of sexual selection. What Trivers discovered was no less than the key to sexual selection, the engine, as it were, that made the whole thing run. That engine was parental investment.
Trivers’s revolutionary insight was simple. The investment a parent makes in his or her offspring has a huge influence on how that parent will approach mating. The more investment a parent makes, the more selective that parent will be in choosing a mate. The less investment a parent makes, the more sexual competition there will be to attract a mate. Think of the many men who often crowd around an attractive woman at a bar, and you have a pretty good picture of this dynamic at work. Which brings us back to the sperm and the egg. You