When Times Get Tough, These Toads Make Hybrid Babies

12:07 minutes

an illustration of four frogs of different colors facing out to the for corners of the image. their hind legs and rear ends overlap in the center. the background is abstract pebbles
Illustration by Emma Gometz

Scientists have long thought that when two animals from two different species mate, it’s a colossal error and the end of the road for the mismatched couple. It’s called interspecies breeding, and many hybrid offspring often end up sterile, such as zonkeys —a cross between a zebra and donkey. Or they can develop serious health problems, like ligers and tigons. 

One biologist even went as far to call interspecies breeding “the grossest blunder in sexual preference.” But is breeding across species lines always a dead end? One critter —the plains spadefoot toad—shows us that maybe it isn’t. In fact, it can give them a leg up in survival.

Katherine Wu, staff writer for The Atlantic, talks with Ira about the complicated sex lives of the female plains spadefoot toads, the trade-offs females make when choosing a mate, and why hybridizing critters may not be such a biological abomination after all.

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Segment Guests

Katherine J. Wu

Katherine Wu is a staff writer at The Atlantic based in Boston, Massachusetts.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. If two people are given the same set of facts, why do they make different decisions? Well, later in the hour, we’re going to dig into some of the biggest flaws in our judgment with Nobel Prize-winning psychologist Daniel Kahneman. But first, a story from the animal kingdom. When two animals from different species mate, it’s thought to be a big mistake, the end of the road for those critters.

It’s called interspecies breeding. These hybrid offspring often end up sterile, like zonkeys– a cross between a zebra and a donkey– or with serious health problems, like ligers and tigons. But is breeding between species lines always a dead end? One critter, the plains spadefoot toad, shows us maybe not. Here to tell us more about the strange sex lives of those toads is my guest, Katherine Wu, staff writer for The Atlantic, based in New Haven, Connecticut. Katherine, welcome back to Science Friday.

KATHERINE WU: Hello. It’s good to be here, and very excited to talk frog sex.


I guess we are. Before we get to that sex part, how do plains spadefoot toads usually reproduce?

KATHERINE WU: Yeah, so it is pretty standard froggy stuff. The male plains spadefoot toads will sit in some ponds and sing some songs. These ones actually sound like little ducks. They make these short quacking noises.


The females mosey over. And they say, wow, you sound super sexy. They couple up. The females lay eggs, and voila. They hatch into tadpoles that then grow up into more plains spadefoot toads.

IRA FLATOW: Yeah, that’s basic eighth grade stuff.

KATHERINE WU: Right, so the huge asterisk that I have to put on this process that I just described is that’s what happens when the ponds they’re in are pretty deep. So these toads are actually living in the desert, especially in parts of the Southwestern US, maybe part of Mexico, maybe part of Canada. It’s really dry there. And sometimes the ponds that they’re mating and laying their eggs into, they dry up super fast. And that actually becomes a problem. If those ponds dry up before the tadpoles become adults, they will turn into something that is very grotesquely called tadpole brittle. You’ll just see this gross, crunchy, peanut brittlely stuff.

IRA FLATOW: Hate it when that happens.


Yeah. When you open that tin, and no, this is not peanut brittle. It’s tadpole brittle. It’s not something any person or any mother frog wants to see. So that’s bad. Basically it is a race between the tadpole developing and the pond drying up. So when things get harsh, these female plains spadefoot toads will actually find a different species to mate with, knowing that the hybrid offspring can actually develop faster and maybe beat the drying up of those ponds.

IRA FLATOW: So do they start listening for a different mating call?

KATHERINE WU: Yeah, that’s exactly what they do. So it’s a pretty closely related species. It’s the Mexican spadefoot toads– one-word difference in the species name. But they do sound pretty different. Instead of making that quacking I described earlier, these males make this kind of baritone trill. But it sounds kind of more croaky.


And so the females really can tell the difference, and not only that. They’ll actually pick out the Mexican spadefoot toads that will likely give them the best, fastest-developing hybrid offspring. So they know which calls to listen for specifically.

IRA FLATOW: That’s amazing. So the female plains spadefoots will listen out for the Mexican spadefoots. How do they choose who to mate with?

KATHERINE WU: So they will actually sound a little bit different. So every call a frog makes, I guess you can picture it like a word. And some frogs will call faster. So it’s like they’re speaking really, really, really fast. Or others are speaking really slow. Others will kind of up the cadence of the trills. It’s a little hard to describe because we don’t have a human equivalent for it. But frogs can distinguish all these different characteristics and figure out, oh, maybe if you’re trilling faster or slower, that tells me something about your underlying genetics and how great our babies are going to be.

IRA FLATOW: My apologies to the toads for calling them frogs.

KATHERINE WU: Actually, so I hate to break it to you, but “toad” is kind of a social construct. All toads are frogs, but not all frogs are toads. And even though there are technically toads out there, not all the things that we call toads are actually true toads. So it’s really confusing and kind of a mess.

IRA FLATOW: Poof! You just blew my brain on that one. That’s terrific. And it sounds like these females are really weighing the consequences when they pick a mate, right? Walk me through that decision-making.

KATHERINE WU: Right. I mean, think about how high stakes this is. They pick the wrong species, and maybe all their eggs and their tadpoles end up dead before they reach maturity. That’s a horrible outcome for any mother to be. Or they pick someone of the other species, but it’s someone whose hybrid babies are going to be too slow developing– the same risky outcome. So they really, really, really have to be choosy. You will see these frogs, these female frogs swimming around these ponds, sort of assessing, how deep is the water? How risky is it going to be if I lay eggs?

IRA FLATOW: No, really?

KATHERINE WU: And then they’ll listen and say, OK, so am I picking this species or that species? And if it’s this species, what kind of call am I looking for? I mean, I don’t want to overanthropomorphize here. But it’s incredibly complicated. Like, I don’t think this complex of a thought when I’m picking a mate. I don’t know about you, but my calculus is not that complicated.

IRA FLATOW: No. It’s their way of saying, do you come here often, sort of thing.

KATHERINE WU: Right. But also, please answer this long list of survey questions to make sure that we’re compatible.

IRA FLATOW: Oh, that’s good because I was going to ask you, what are the trade-offs of a female plains spadefoot mating with a Mexican spadefoot? I imagine they don’t create a super toad, right?

KATHERINE WU: Right. Yeah. So I mean, based on everything I’ve told you, you could ask, well, why not just only mate with the Mexican spadefoot toads? It sounds great. But as you were saying at the top of the show, there are often some pretty big trade-offs when you go between species. You’ve had some time to diverge. Not all your genes are the same. Your eggs and sperm aren’t going to be perfectly compatible. And so these hybrid babies, they develop fast. But their fertility kind of pays the price. All of the male hybrids are actually sterile. They can’t have babies of their own. And the females, they lay fewer eggs than nonhybrid females. So there is a price to pay, but the logic is better to be a little bit less fertile than entirely dead.

IRA FLATOW: Absolutely, I can see that on a bumper sticker.


Oh, I’ve got that bumper sticker. No, I wish.

IRA FLATOW: Now, I imagine with climate change happening, we’re going to be seeing more and more ponds drying up. What’s going to happen to the breeding here?

KATHERINE WU: It’s such an interesting question. And you know, my rule especially of the past 2 and 1/2 years is never predict the future. And I won’t do that here, but it’s such an interesting question. Right? What is going to happen to these frogs when things get dry? Hybrid babies are the way to go. Are we going to see an increase in this behavior? What is that going to mean for these species?

Because even though the hybrid males are sterile, the hybrid females aren’t. And these researchers have actually seen these hybrid females mating back into both parent populations. And so you see this kind of melting pot of genes, and so it’s kind of cool. You can actually see that the hybrids are really nice, funny blends of both parents. Like, it’ll have the pointy chin of Mom and the bumpy head of Dad.

And even their calls kind of sound intermediate between the two, like a trilly quack. It’s kind of weird sounding. But then when the hybrids go back and mate with a purebred toad of one species or another, you can see everything just blending together again. It’s really fascinating. And so it’s kind of interesting to think about, how is this melding of different genetic material going to shape the evolution going forward in a world that’s getting drier and hotter and just a lot more difficult for animals to navigate? It’s a pretty cool strategy. When times get tough, make hybrid babies, I guess.

IRA FLATOW: I know you quoted a biologist who said that interspecies breeding is, quote, “the grossest blunder in sexual preference.” But these toads are really impressive. I mean, are scientists rethinking what they know about interspecies breeding?

KATHERINE WU: I think they definitely are. And you know, what’s interesting is the grossest blunder idea was really the prevailing narrative for so long. And it’s definitely not entirely wrong. I mean, think of the mules that everyone has heard about, the examples you gave at the top of the show. Even the costs that these hybrid frogs are paying with their fertility. There are costs to mating outside of your species. And it is a kind of weird thing for a lot of animals to do. And certainly, if their environments were an extreme, I don’t think these female frogs would be doing it terribly often. But when push comes to shove, when the world is changing, I think it very clearly is a way that animals go.

There are other animals that have been shown doing this too– fish and rabbits that are acquiring genes to handle pollution from other species or changing their coat colors so they can better blend in with landscapes that are less snowy now that climate change is happening. This may be a really important way for animals to continue just trying to keep up with how fast the world around them is changing.

IRA FLATOW: Do we have any idea how fast that’s happening, what percentage of the species might be doing this?

KATHERINE WU: Yeah, so this is an area of pretty active research. It’s thought that at least 10% of animal species regularly make hybrid babies with another animal species. And honestly, the number is probably a lot bigger now that we have this big genetic revolution where we can go inside the genome of different animals and say, wow, that kind of doesn’t belong here. You must have gotten that from this other species that you hybridized with somewhere back in your ancestry. I think that number is going to go up in the coming years.

IRA FLATOW: This story makes me think about interspecies breeding in a different way. What was your big takeaway in researching and reporting this story?

KATHERINE WU: I think one thing that is really worth thinking about is we as human beings have really put boxes around the other animals and plants and other life forms in our environment. Species really is kind of a human construction, and it is useful to think about. But one of the questions that is actually really challenging to answer is, what is a species?

Do you define it by what it can and can’t mate with? Do you define it by what its genome looks like and how different that is from something else’s? Do you define it by where it lives and the fact that it can’t interact with other things very often? It’s a really complicated answer. And maybe you plop an animal into an environment where it can mate with another animal, and they produce functional hybrids. Are they the same species, or are they just two different species that can produce functional hybrids? Really just shows us the limitations of how we sometimes oversimplify the world around us.

IRA FLATOW: Absolutely.

KATHERINE WU: I mean, even we humans have the genes of other species in our genomes. And it has helped us survive till today.

IRA FLATOW: The Neanderthal in me thanks you very much, Katherine, for that interspecies chat. And I will never look at a tadpole the same way again.

KATHERINE WU: Yeah, hopefully you only have nonbrittlelized tadpoles in your future.

IRA FLATOW: Katherine Wu, staff writer at The Atlantic, based in New Haven, Connecticut, thank you for joining us today.

KATHERINE WU: Thanks so much for having me.

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