It was not too long ago that sex, the newfangled approach to multiplying organisms, burst onto the evolutionary scene. Before that, no partners were involved; organisms simply split in two and produced carbon copies of themselves. A colony of asexual offspring was the ultimate in harmony, since every individual had the same genetic makeup and, therefore, the same genetic interests at heart. Peace aside, cloning didn’t win particularly high scores for innovation. Everything was fine as long as asexual organisms remained in a stable environment. If the environment suddenly changed, however, cloning didn’t allow genetic mutations to happen often enough for the species to adapt.
It took sex, the combining of genes from two different individuals, to produce the novel combinations that allowed animals to change quickly. When sperm met egg, the random shuffling of genes was much more likely to produce a mutation
—a new twist on an old theme. If this mutation proved beneficial to survival, it
was adaptive. The lucky individual would survive and pass on the adaptive trait, and if it were truly a breakthrough, the trait would spread quickly through the whole population. In the same way, a bad mutation would be edited out of the population, since the tainted individuals would usually not survive to reproduce.
This ability to adapt quickly allowed early organisms to emerge from the sea and colonize the land and the skies. Today there are millions of sexually reproducing species changing and evolving in response to their environments.
But sex, for all its advantages, also has some drawbacks. Primary among them is the fact that members of a sexual species can’t produce offspring by themselves. There must be a union of egg and sperm, small cells that reside inside two unrelated individuals. To get the two together, an animal must find and attract a mate of the opposite sex, confirm that they are of the same species, reduce the mate’s aggression, arouse its sexual interest, and then synchronize its partner’s sexual cycle with its own. It’s a lot to orchestrate, especially when you factor in the mates’ conflicts of interest in both the genetic and the gender departments.
STRANGE BEDFELLOWS As we have seen, animals are more willing to cooperate with animals that share their genes. Since mates don’t have any genes in common (a “rule” that prevents inbreeding), there’s very little to temper their natural distrust for one another. As they get closer and closer to mating, the animals are held prisoner by three competing forces: sexual attraction, fear, and aggression. Sometimes the fear is on top, and the animal’s retreating movements flash the signal “I am about to flee.” Counteracting these are forward-approaching movements that reveal a strong sexual desire. As the animal approaches, it may suddenly express aggression, brought on by the mere proximity of a stranger. The three driving forces war with one another, like elastic bands pushing and pulling the animal in different directions. Though it may try to escape or launch an attack, its desire to stay pins it to the spot and holds its aggression in check. You can actually see the ambivalence as the animal bobs its head forward and back as if to say, “I’m staying, I’m leaving.”
This visible ambivalence has been ritualized through eons of evolution until, eventually, the back-and-forth, flee-or-approach movements have become elaborate stereotyped displays. To the potential mate, these displays prove that
the performer is in turmoil and that its motivation for staying is probably sexual rather than aggressive. Once fear and aggression are neutralized, the flames of arousal can win out in the war of emotions, allowing the pair to get close enough to copulate.
OPPOSING GAME PLANS To further complicate sex relations, the male and female strategies for reproducing themselves are worlds apart. To understand the reason for this division, we have to travel deep inside the animals’ bodies and take a look at the cells that do the work of reproduction—the female egg and the male sperm. Talk about differences! The human egg is 85,000 times larger than the sperm, and that outsizing is echoed in all species, from newts to blue whales.
Naturally, it takes a lot more energy and resources to produce an egg than it does to produce a sperm. In the case of humans, a female will produce, at most, 400 eggs in her lifetime. The male, on the other hand, produces up to 300 million sperm with each ejaculation! Each sperm is so tiny, however, that the energy invested in it is minuscule.
Once the egg is fertilized, the female pours even more energy into the embryo, and, in many species, she continues to invest in the young even after birth. To protect her investment, it pays for the female to find only the fittest father, perhaps one that will help defend or feed her offspring. For the male, whose initial investment is minimal, promiscuity pays. His strategy is “quantity not quality,” and he seeks to impregnate as many females as he can, taking his chances that some of his sperm will find their mark and develop into adults that carry half his genes. The female, by contrast, puts her bets on only a few good mates.
These differences are most obvious when the female of a species is capable of raising the young successfully on her own. The male, assured that his genes are in good hands, dedicates his energies to mating with as many females as possible. This is especially true with mammals. Since the females have the exclusive patent on milk-producing breasts, many males need never look back on the raising of their offspring. These males belong to the 95% of all mammal species that are nonmonogamous.
In a scant 5% of the mammal species, however, it takes two individuals to raise the young successfully. In these species, males change their strategy and act
more like females—staying close to home and lavishing attention on a few offspring. Male lions are needed, for instance, to protect the cubs from enemies, and male foxes are needed to bring the nursing vixen enough food. If these males were to leave the female, the chances of their offspring surviving would be negligible. Choosing quality over quantity, therefore, they opt to stay. Ninety percent of all birds are monogamous for the same reason; the female can’t incubate eggs, brood the chicks, and catch food for the family all by herself.
In our Western culture, which values monogamy, people seem to approve of animals that form long-term pair bonds. It’s important to remember, however, before we jump to biased conclusions, that these animals are not displaying “true love,” but simply following the dictates of their genes. They are survival machines, and their mission is to multiply their own genes in the gene pool. If a male felt that his partner could raise young without him, he’d be off in a flash.
But this would not be abandonment in our terms, and we need not feel sorry for the female. Both are pursuing their own game plan that will lead to the best positioning of their genes—a pursuit that is adaptive and, therefore, beautiful.
COURTSHIP DISPLAYS Now that you know how dicey the sex game really is, you can see why courtship displays are so important. Miraculously, these displays encourage the merger of sperm and egg, even between mates that have opposite agendas and a nagging reluctance to be together. As you watch these mating preludes at the zoo, try to see how well they perform their functions.