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You're listening to Shortwave from NPR. Hey, shortwavers, producer Burley McCoy and the host chair

today with a biology mystery that I learned about from Catherine Wu. She's a staff writer for the Atlantic

covering science. And a couple years ago, she was talking to a scientist as part of her recent

kick, reporting on frogs. As one does. And she was talking to me about hybrids. Hybrids, the result of members of two different species mating.

In this case, two different species of frogs...

It just struck me as so bizarre that there would be a situation in which one frog was

seeking out a mate of another species. It made zero sense. It had made zero sense to the

researcher at the time she discovered it. I basically had to know more. In her reporting, Katie learned

that female plains spadefoot toads that live in the North American desert actively choose to mate

with males outside of their species when the pools they're in are at higher risk of drying up.

They do this because it turns out the tadpoles from that unlikely union mature just a little bit

faster, giving them a better opportunity to hop away as adults before the pools dry up,

which would be a mushy death if you're a not quite ripe tadpole. But there's a catch.

The offspring are less fertile, so the males are totally sterile and the females don't produce as

many eggs. Because the fitness of hybrid animals across nature is often subpar, like these

froggy fertility issues or other health problems.

Biologists have long thought of interspecies mating as a disaster most of the time.

Think of the sterile hinnies and mules that come out of horse donkey unions.

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