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Looking at both sides of the equation

by Rowan Hooper

Someone asked me the other day if I wouldn’t like to be a woman, just to see what it was like. Sure, I’d love to try it, I said, for a day or two. Imagine seeing the world from the other side, seeing how men assess you and wielding power over them with a glance. Or if you’re a woman, imagine being a man, and knowing first hand that we truly are apes, subtler and more complex, to be sure, but unmistakably apes.

Then I read a paper published this week and imagined being both at once.

It might sound weird, but biologists do this sort of thing all the time. For one thing, it cuts to the important questions about sex: whether it is better, in evolutionary terms, to be a female or to be a male. For another thing, biologists just like thinking about sex.

In humans about one in 250,000 births is hermaphroditic, though the preferred term is intersexual. Another medical term used to describe the condition is “ambiguous genitalia.”

You get the idea. A duplication or deletion of one of the sex chromosomes can cause male or female sexual organs to grow when they shouldn’t. Intersexual individuals can also develop from an imbalance of hormones — too much of the adrenal hormones in a genetic female, for example, can cause her to develop masculine traits.

But millions of species are both at once, hermaphrodites that produce both eggs and sperm (or in the case of plants, eggs and pollen). Hermaphrodite animals are, curiously, often soft-bodied slimy things, like snails and worms, though many fish turn from male and female at different stages of their lives.

Some species, however, have the luxury of being both male and female at the same time, and it was an animal like this that I read about this week.

Tropical sea slugs have their own special sexual problems. They don’t self-fertilize, so they still need to go through the mating and courtship game. The difference is that they want a partner who will give them sperm for their eggs and receive their sperm at the same time.

If male and female animals were perfectly cooperative, they would each contribute the same amount of resources to sex and reproduction. But of course they are not cooperative — they are in conflict. It’s the reason there are two sexes in the first place.

Nevertheless, it would save much time and energy if the two sexes could cooperate. So 20 years ago, some fair-minded biologists thought that in hermaphroditic animals there should be a rule whereby both partners agree to donate and receive equal amounts of sperm. This idea was called the “sperm trading” hypothesis.

However, in practice, when biologists looked, they found that many hermaphrodites let their male side dictate their behavior. In other words, they donate sperm and then flee. One slug is left holding the babies, while the other is back on the dance floor looking for more.

In these animals the male half of the hermaphrodite seems to be winning the evolutionary game of sexual conflict. If you are a male, you want, in evolutionary terms, to fertilize the eggs of as many females as possible. If you are a female, you want to snare as much of a male’s resources as possible. If you are both male and female at the same time — the bizarre situation sea slugs find themselves in — then logically you want to give but not receive sperm.

Now Nils Anthes and colleagues at the University of Tybingen, Germany, have demonstrated for the first time that sperm trading does occur. A sea slug swims up to a partner (see photo) and each animal inserts its penis into the other. One donates a small “packet” of sperm, and the other reciprocates. It’s the sexual equivalent of “You scratch my back, I’ll scratch yours.”

Anthes generated individual hermaphrodites only capable of performing a “dry” copulation without sperm transfer, by cauterizing the sperm duct that leads from the testes to the penis. These animals could happily insert the penises into their partners, but not transfer sperm.

Surprisingly, the researchers found that sea slugs mating with animals that did not transfer sperm ended sexual intercourse significantly sooner than did normal pairs.

“If an animal realizes that the partner is not donating sperm, then it terminates the copulation,” Anthes told The Japan Times. “How exactly they determine whether or not the partner has inseminated is not known yet. One could imagine that they are able to ‘feel’ the loading of their sperm receiving organ.”

In 20 years, this is the first time that anyone has found hermaphrodites behaving like this. The tragedy of the commons is that individuals will usually act in their own best interests. Often this means “donating” sperm as much as possible — as much as they can, to as many females as they can.

The message from sea slugs is that it doesn’t have to be that way. Cooperation can sometimes be the best thing to do. That’s certainly the case if you have both male and female genitalia, but perhaps that could apply to the rest of us, too.