Women After All: Sex, Evolution, and the End of Male Supremacy (8 page)

Probably the most complex animal with sexual cannibalism is the octopus. One female lingered outside her den near a Pacific coral reef as a small male mated with her thirteen times in four hours. Octopus males can have an intimate encounter or they can mate at a safe distance, depositing sperm with one of their arms; this male kept his distance throughout. Often he jumped back a few feet, sensing danger. Five times, “she showed only a subtle head bob and a faint darkening of her body pattern. On five occasions, the male blanched white briefly,” a sign of fear, yet cautiously came on again.

Thirteen was not his lucky number. Shortly after the last mating attempt—it’s not clear whether he actually deposited sperm—she sidled up and forcefully swatted him off a ledge of coral. He released abundant ink in defense. “She quickly pounced and engulfed him in her arms” and swam away with him, taking him into a cavity in the coral, where she finished him off. She spent twenty-four hours slowly eating him. The next morning another small male—discreetly, from behind a rock—extended an arm into her den and mated with her for three hours.

Maybe the unlucky first male never injected sperm in all those tries, and his beloved finally thought,
Since you’re not good for anything else, I might as well get a few meals out of you.
But it’s also possible that he deposited sperm and she collected more from the next guy, whose destiny may also have been her culinary enjoyment. Of course, she didn’t think through these strategies; she didn’t have the brain for it, and few if any animals other than us do. But evolution gave her the means to act in her own best reproductive interests, almost as if she had thought it through—and maybe better, since our conscious minds don’t always serve us well.

All this gives us a lesson in how the evolution of sex and gender work. The male widow, mantis, or octopus has only one function in life: seed the female. Having done this, he may as well find a function
in death—feeding her so she can better nourish the offspring that will carry forward their combined genes. Think of the jobless, depressed human dad who commits suicide thinking that his widow and orphans will collect his pension or life insurance. They don’t eat him, but, depending on local laws, they may live for a time on the funds generated by his death. In humans, a male might morosely weigh these grisly gains against his future value as a father. But in creatures with nothing remotely resembling fatherhood, giving your body for the cause may be the best move you can make—especially from your mate’s point of view.

Still, a male (regardless of species) has his own interests, which might lead him to want to mate another day, perhaps with another female. Now the accounting changes, and we learn a critical lesson: male and female, joined together to make an offspring, have different interests from each other and, in the end, even from their individual young. This is true not just of creatures with sexual cannibalism but of every sexual species in the great spectrum of life. And this is where females lose control of the males they invented, to the process of divergent or even antagonistic evolution.

Consider the syrupy primordial slime of three billion years ago. The early earth’s crust has mostly cooled, and in the process simple molecules have been cooked up into more intricate organic ones. These finally build a string of genetic material and an enzyme or two that can help the string copy itself. By definition, the string prevails by making more like itself, and the inevitable slings and arrows of fortune—cosmic rays, volcanic spray, toxins, and so on—will not break down all of them. If the genetic string—RNA or DNA—can elongate enough to make other large molecules, like proteins, that offer added protection, we will have new complexity and, over hundreds of millions of years, a very simple cell, which will give rise to many different kinds of one-celled creatures.

By this point, living things have begun to deal with the challenges of nutrition, predation, and parasites that might make variety
advantageous. It is not yet time for the “masterpiece of nature” to appear full-blown. But some mutants manage to trade genes, and by doing this each increases the variety of her offspring. “Her” is still right, because both partners in this soupy exchange can produce their own copies, out of their own bodies.

Some
non
sexual species have mating types—strains within the species that differ enough genetically to avoid cloning by exchanging genes. This means that although you and your mate (the partner you trade a few genes with) can both produce offspring, you choose her from the type that you are not. Opposites attract, even within a basically all-female species. A likely example is
Trichomonas vaginalis,
known as “trich”—pronounced
trick,
appropriately from the human viewpoint—the protozoan that gives vaginas and penises the wrong kind of itch. Ironically, trich is asexual, except for those gene swaps; yet it may have had a sexual ancestor. The way we know this holds a clue to the origin of sex.

Trich is capable of meiosis (
my-oh-sis
). This bane of high school biology comes down to something simple. Cell division, or mitosis (
my-toe-sis
), yields two identical daughter cells. It goes on in many parts of our bodies all the time. Meiosis differs in that it randomly allots half the genes or chromosomes of the dividing cell to each of the two daughters. This happens when we make eggs or sperm, so that when they join they can create something new. Gene swaps without meiosis involve a risk of mismatch between the parts of the genomes traded and their future genetic hosts. Meiosis elegantly divides each partner’s genome in half so that the halves can combine with their complements in the mate; this reduces the mistakes made when mates mix their genes. It’s the same principle as egg and sperm, except that the combining halves are not so different. But the advantages of meiosis may help explain why sex evolved and stuck in so many species—it’s a better way to mix and match genes and get the gold ring of adaptive variation.

The sexual version of this unfolded two billion years ago among
some one-celled organisms, and most plants and animals have stuck with it. But some, like the vaginal trich and the virginal whiptail, somehow became asexual again. It’s difficult to know why, but in one rather startling animal a reversal has been followed in real time, and it gives us clues to how sex evolved in the first place.

The creature is the asexual New Zealand mud snail. At least it
used
to be asexual. In a wonderful case of evolutionary change seen in a human lifetime—there are a lot of these, despite what creationists claim—these snails re-evolved sex. They became infected with a worm, called
Microphallus
because it sterilizes the snails—male or female, sexual or asexual. This, needless to say, is bad for the guy or gal and the species.

In the lab, when snails are deliberately infected with this worm, both snails and worms evolve faster—the Red Queen running in place again. But the real power of this research is that it shows that in nature, in New Zealand lakes, a small minority of snails that became sexual outevolved their far more common asexual counterparts.
Microphallus
quickly wiped out the most common asexual clones, and the more resistant ones were outbred by those snails that (re)invented males. So the Red Queen favored the sexual snails, crowding out the offspring of what we might call virgin queens.

Nevertheless, as we’ve seen, some all-female species do persist in nature. In fact, as Olivia Judson and Benjamin Normark point out in “Ancient Asexual Scandals,” some of those may never have been sexual. Other species consist of hermaphrodites—individuals that are both male and female, making both eggs and sperm—and these can teach us a lot about our own battle of the sexes. Ordinary garden snails are an example. In a pinch, they can fertilize themselves (“selfing,” for short), but instead they mostly seek an equally versatile partner. Now it gets interesting. Each two-sex snail tries to inject sperm into the other. They mate for hours, so presumably it’s fun. But they are competing, even if both succeed.

In one of the strangest mating rituals in nature, they shoot love darts—that’s the scientific term—a few millimeters long into each other. The dart looks like a spear point, and when snails mate, one or both will have a dart lodged in its body, like an arrow sticking out of an enemy. The dart usually does no great harm, but it does seem to hurt and can leave lasting damage. So why inject it? Because it’s coated with hormone-containing mucus that aids the sperm of the snail that shot it and, at the same time, ups the number of eggs in the mate that takes the hit.

Each snail tries to launch these missiles during sex, having stored one or more darts near the penis, which is attached to each owner’s
female
organs. Snail biologist Ronald Chase has said, “Love is coming down to war in a way. Sexual conflict plays out,” even between hermaphrodites. Apparently all’s fair in the snails’ love-war, and it does seem a pretty macho way to have sex. Yet I would argue that we can fairly call both snails female, because each goes on to create new life, laying around eighty eggs in the soil that, with luck, will grow up in a year or two. Also, although it’s not ideal, each as a last resort can reproduce alone.

Chase thinks that the myth of Cupid and his love darts came from the ancient Greeks’ knowledge of snails. The Greeks were good naturalists and would have noticed this display. Cupid’s darts, according to the story, make you fall in love—you are smitten, we often say—and while this can happen to both partners, it is not always equal, and sometimes the one more deeply smitten gets put at a serious disadvantage. This is an asymmetry we have in common with garden snails.

One further case of two-sexes-in-one: the red-tipped flatworm, named for its gorgeous coloring, which includes a red-tipped white stripe down the back of an almost iridescent blue body. It is about two inches long and speeds around coastal bays, reefs, and lagoons in the oceans from Myanmar to Australia. In addition to female reproductive organs, each worm has two penises, which it uses to
fence—again, the scientists’ word—in the ritual leading up to mating. The two would-be lovers rear up with their back ends on the ocean floor and fence it out in a way that looks hypermasculine, each striking and parrying as best he—she?—can.

The contest—romance?—takes up to an hour; it is described in detail in a paper called “Sex and Violence in Hermaphrodites.” The worms are not using weapons that some might claim symbolize penises. They are using their actual penises, two each, slapping them against their partner’s—opponent’s?—penises and trying to jab at least one of their own into the other’s flesh, to stick without getting stuck. They don’t have to aim for any special spot or cavity, just pierce the skin. They can inject anywhere, and the stream of sperm will find its way to the other’s ovaries, making pale streaks that look like lightning, visible through the worms’ translucent bodies.

Thus the war between the sexes—without sexes. Each is at once aggressive and coy, intrusive and choosy. But as Leslie Newman, who coauthored the paper with her colleague Nicolaas Michiels, said, “It is better to stab than to be stabbed.” Each dueler is trying to choose by resisting fertilization. This limits mating to only the most skillful rivals and ensures that the victor’s offspring will have the same skills. Naturalist William Eberhard called it “selective surrender.” If you are pricked, so to speak, you make offspring. On the other hand, if you stab but escape stabbing, you have no flesh wounds to heal and no burden of eggs, yet you pass on your genes.

It is not difficult to see how males might evolve in such a system, and there are other species of flatworms that are conventionally male and female. Those females avoid some males and welcome others, and it’s not the most aggressive males who get the prize. Perhaps sex originally evolved from hermaphrodites, though it could also have happened the other way around. The possibility of evolving back and forth seems clear.

Either way, these instances of intense competition during sex itself highlight a key fact: all organisms are to some extent in conflict
with all others, no matter how intimate the relationship. Yes, many species have cooperation and even altruism, but those nice behaviors always involve limits. The red-tips are engaged in the most important cooperation, the one at the heart of all evolution itself. Yet each wants the upper hand—or, rather, the upper penis; each has something to gain at the other’s expense. And that’s in species that are not even split into males and females.

Which brings us back to our old friend the unisexual whiptail lizard. The species didn’t go from “male
or
female” to “both”; it just got rid of males. It is highly evolved, with a far more complex brain than that of a snail or a flatworm. In the vast spectrum of life, the all-female whiptails are not much simpler than we are. Yet they unambiguously evolved an end of males. Of around forty-five species of whiptail lizards, in one-third males need not apply. Here we can always say “she,” because these species are made up entirely of shes; each is a sorority in which all the sisters make babies on their own.

Almost.

It turns out that these lizards—let’s call them parths—have something called “pseudosex.” However, if it were happening in people—or, for that matter, in our close ape relatives the bonobos, about which a lot more later—it would be called sex between females, and in these lizards that is obligatory. One female mounts another just as a male would a female in a sexual lizard species. Any female can at some point in time be either mounter or mountee, and each may play either role in varied encounters. There is nothing to insert—and there are no sperm or any sharing of genes.

The mounter, though, is usually either past ovulation or has undeveloped ovaries, while the mountee’s ovaries brim with ripe eggs. Once she lays them,
she
can be the mounter in the next same-sex tryst. This transition is made possible by progesterone, which (as in humans) surges in the egg producer, but in parths that same hormone induces mounting. Because the parths are descended from
whiptail species that had males, parth females respond to testosterone if you give it to them, even though they don’t normally have much. As neurobiologist David Crews, who has studied them for decades, puts it, the whole thing is a “snapshot of evolution.”