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These Creatures Are Probably the Closest Thing Nature Has to Real Werewolves
Under the right conditions, the spadefoot tadpole will transform into a voracious predator of its own species.
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October 30, 2023
These Creatures Are Probably the Closest Thing Nature Has to Real Werewolves
Under the right conditions, the spadefoot tadpole will transform into a voracious predator of its own species.
By Brian Gutierrez edited by Jeffery DelViscio
[A smaller tadpole is actively eaten by a larger one on the pebble-strewn bottom of a pond]
Oxford Scientific Films/Getty Images
[Illustration of a Bohr atom model spinning around the words Science Quickly with various science and medicine related icons around the text]
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Brian Gutierrez: Werewolves aren’t real. Everyone knows that. But stay with me for a moment while I tell you about the spadefoot frogs—specifically, their tadpoles, which just might be the closest things to werewolves in nature.
I want to tell you about the bizarre and true story of cannibal tadpoles.
I’m Brian Gutierrez, and this is Scientific American’s Science, Quickly.
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[CLIP: Science, Quickly show music]
Gutierrez: Spadefoot tadpoles are born as peaceful bottom-feeders that eat little bits of algae and poo floating in the water.
But under the right conditions, each tadpole has a chance to transform into a hulking, agile predator. And the first things on the menu are other tadpoles.
David Pfennig: And so, and I have some right here.
Gutierrez: I’m in the office of David Pfennig on the third floor of the biology department at the University of North Carolina at Chapel Hill.
Gutierrez (tape): Oh, okay. Could you describe what ...
Pfennig: Yeah, so, so what you’re seeing here is: I’m showing you a little vial filled with ethanol, and it’s got some tadpoles in it. And some of these tadpoles you see are small; some of the tadpoles you see are large.
Gutierrez (tape): Oh, okay, so this is one of the huge ones. Yeah.
Pfennig: And so that huge one is what we call the carnivore morph.
Gutierrez: Learning about the carnivore morph—what I think of as a tadpole werewolf—is why I came to see David.
The tadpoles in this particular vial are from the deserts of Arizona. The desert is a pretty tough environment for any organism to survive, but that’s especially true for frogs, which need to stay moist to breathe and they usually lay their eggs in water.
An amphibian surviving in the desert is like an ice cube surviving in a dutch oven. Making it work requires some pretty extreme adaptations.
One of those is hibernation.
Pfennig: They’ll go into their permanent burrow, and they’ll dig with their hind feet. That’s why they’re called spadefoots, because they’ve got this little keratinized spade on the back of their feet. And they basically sort of do this little wiggle dance, you know, to sort of dig into the ground. So they’re digging backwards, if you want to think about it that way.
Gutierrez (tape): How deep do they go underground?
Pfennig: Well, it depends upon the time of the year. So they have been recorded as digging almost a meter deep—so 90 centimeters deep.
Gutierrez: That’s almost three feet into the earth.
In arid environments, spadefoot frogs sleep patiently underground for months, a year or even two years if they have to—waiting for the perfect moment to emerge. 
When he goes out looking for them, David waits for the trigger that will bring the frogs out of their deep sleep: rain.
[CLIP: Rain sound slowly starts and gets more intense]
Pfennig: We don’t really know how they know it’s raining, but somehow they know it’s raining.
Gutierrez: The frogs come up from under the ground and start looking for newly formed pools that dot the landscape in all shapes and sizes.
Pfennig: Some of them could be as small as your bathtub. Some of them could be as, as big as your bedroom. Some of these ponds can be as big as your whole house and maybe your backyard, you know, and so it just depends on the location. It depends on how much rain you get. But one thing that unites all the places where these toads breed is that they’re all temporary.
The males typically will arrive first, and then they’ll start to call. They have very loud calls, like a lot of frogs, but these guys in particular have really loud calls.
[CLIP: Sound of spadefoot frogs]
Pfennig: It’s, like, really raucous. You can’t even hear yourself talking, you know, this can be so loud. You have to yell for somebody else to hear you. So it’s very, very noisy. The water is just full of frogs calling. And that will attract the females to the site as well. And then all the breeding will take place in one night.
Gutierrez: After months of barely moving, the frogs get to work incredibly quickly.  That night, each female will lay between 800  and 1,500 eggs. Those eggs will hatch as soon as the next day in these fresh rainwater pools.
Pfennig: It’s just, it’s clean rainwater just falling on dirt, right? And so there’s nothing, there’s no algae or anything like that growing in them initially. Presumably, if  you wanted to, you could probably drink out of them, I guess.
Gutierrez:  It’s the perfect nursery for baby tadpoles. But it doesn’t stay that way for long.  After that first night, the pristine water that’s clean enough to drink slowly turns into sludge. Algae starts to bloom across the surface, and large animals such as cattle come by to drink and do their business around the pool.
Pfennig: And so then they’ll start getting a little smelly, you know, and so then you wouldn’t want to drink the water out of them, of course. And so basically, over time, it just starts, you know, it starts getting nastier and nastier, basically.
Gutierrez: At first, this is all food for the new tadpoles. But as the water starts to evaporate, that nastiness gets more and more concentrated, making it harder and harder for them to breathe through their gills. If the pool dries up too quickly, there might not be any water left at all.
Pfennig: You’ll go in there, and there’ll be thousands, tens of thousands of tadpoles all dying because the, the water has disappeared, and they’re just basically desiccating in the sun.
Gutierrez: David showed me a picture. It’s really tragic. As the pool of sludgy water gets smaller, the tadpoles gather closer and closer together until there’s no water left, just a mound of tadpoles on top of the drying mud.
On top of that, parasites and predators start to arrive. The tadpoles aren’t poisonous, so they’re an easy snack for snakes, birds and even insects such as wasps and beetles.
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