Brave Old World
What Dad didn't tell you about the birds and bees.
It's time to talk about the birds and the bees.
No, I don't mean sex. You've already heard that story: Boy meets girl, sperm meets egg, a baby grows in mommy's tummy. That's the way of all flesh. Or so you were told.
Well, you know how it is with the stories we tell kids. We leave a few things out. The truth about babies is that sometimes, there's no sperm. There's just an egg, but a baby hatches anyway. Many bees don't have daddies. Neither do some birds. It's called parthenogenesis—literally, virgin birth. We're finding more and more animals that can reproduce this way, and we're learning how to engineer it in others. We're even tinkering with mommy-daddy procreation in humans. If you think sex is kinky, wait till you see the alternatives.
In bees, fertilized eggs become females. Unfertilized eggs become males. Even without a queen, some female honeybees can keep a colony going by laying eggs that fertilize themselves.
Birds do it, too. Up to 30 percent of unfertilized turkey eggs can spontaneously begin to develop. In one study, selective breeding boosted the rate above 40 percent. A 1975 U.S. government report documented more than 50 mature turkeys that were never fathered.
Lately, parthenogenesis has been verified in various snakes and lizards, extending the list to about 70 vertebrate species, including fish, frogs, and chickens. But the beasts at the top of the food chain—lions, sharks, Komodo dragons—seemed impervious.
Still, the process hadn't been proved in sharks or mammals. And there seemed to be a good reason why. An egg that fertilizes itself makes two identical sets of chromosomes, including sex chromosomes. In birds, snakes, and most lizards, two identical sex chromosomes make a male. That allows parthenogenesis to function as a DNA survival mechanism, since an isolated female—close your ears, kids—can produce a son and mate with him. But in sharks or mammals, this wouldn't work, since two identical sex chromosomes—XX—make a female.
Or so we thought. Three weeks ago, Biology Letters delivered the second surprise: "Virgin birth in a hammerhead shark." A perfectly formed baby shark had appeared in a tank in Nebraska. Tests proved she, too, was a parthenogen.
Why hadn't we found parthenogenesis in these animals before? Because we hadn't looked. Several sharks have mysteriously reproduced in captivity in recent years. Scientists now think parthenogenesis is responsible. Of the two sexually mature female Komodo dragons in Europe, both are now known to have reproduced this way. Every parthenogenic dragon is male, which may explain in part why males heavily outnumber females. In snakes, the array of known parthenogenic species continues to grow.
Will Saletan covers science, technology, and politics for Slate and says a lot of things that get him in trouble.
Photograph of lizards by Alistair J. Cullum.