Big Baby

At first glance, Pseudis paradoxa looks like the most normal frog in the world. It’s green, it lives in the water, it buries itself in mud, and it eats flies. Nothing particularly unusual about it. Until you meet its monstrous offspring.

That’s the P. paradoxa tadpole. And this is the adult and the tadpole next to one another:

The tadpole is almost four times larger than the adult; while adults measure about 3 inches long, their offspring can grow to 10 inches long. And there’s the mystery behind the Paradoxical Frog. There are invertebrates in the world which are larger in their larval stage, metamorphosing from chubby caterpillars to svelte butterflies. But this is the only vertebrate whose offspring is larger than its adults, and we have no idea why. And as long as science can’t yet explain it, why not indulge the urge to irresponsibly hypothesize?

For all you non-biologists out there, let’s get something straight about amphibians. The two-sided lives (amphibios) they live are less about their ability to live on both land and in water than it is about their two stages of life as a larvae and an adult. (“Amphibious vehicles” like those touristy Duck Buses aren’t truly amphibious, unless they grew from a larval “station wagon” stage.) True, the larvae usually breathe through gills, and most — but not all — amphibians need to lay their eggs in water. But some amphibians, like mudpuppies and axolotls, never leave the water, while others, like New Mexico’s spadefoot toads, are perfectly happy being high and dry their entire adult life.

A tadpole is the larval stage of a frog. As the tadpole makes the transition, several things happen: it loses its gills and develops lungs, as well as other adaptations it will need for a life on land, such as limbs, eardrums, and eyelids. Its tail disappears. And, oh yeah, it gets bigger. Unless you are Benjamin freaking Button, you don’t get smaller as you get older. You don’t grow up to be tiny.

That is, unless you’re a Paradoxical Frog. Shortly after hatching, the tadpole goes through a huge growth spurt, and then steadily shrinks until it reaches its adult form. Why waste so much energy growing only to lose that body mass? (And how do they do it? Shrinking your skeleton isn’t the same as losing weight.) In the case of caterpillars, they need that much body mass to burn to complete their metamorphosis. But no other frog needs such an energy reserve to transition. And down there in the warm jungles of Surinam and Brazil, they’re not bulking up to hibernate or estivate, either. So the reason must be that there’s both an incentive for a tadpole to be big, and another incentive for frogs to be small. Truly, the Paradoxical Frog must be living two very different lives.

Here’s my hypothesis. The Paradoxical Frog occupies the same ponds that other species of frog inhabit. [citation needed] To avoid competition with other tadpoles, the Paradoxical Frog tadpoles grow very large very rapidly to take advantage of another food source. [citation needed] However, this put them in competition with local fish, so the species is pushed to become ever larger to be able to rule the pond. [citation needed] However, large frogs are easy prey for predators. So the tadpole, once it has lived the first few weeks as a giant, uses those energy reserves to live on while it shrinks, putting it back in competition with other tadpoles. As a regular-sized tadpole, it doesn’t get as much to eat… but because of its bulk, it doesn’t need to. [does that even make sense?] Perhaps P. paradoxa even synchronizes its breeding with other species, with the tadpoles shrinking to size just as other types of tadpole are getting out of the water. [okay, now you’re grasping at straws]

If you have another hypothesis, I’d love to hear it, fellow armchair biologists. It’s one of the greatest zoological mysteries of our time, and I’m not even wearing pants.


About quantumbiologist

Christian Drake, AKA The Quantum Biologist, is a naturalist and poet formerly of Albuquerque, NM and currently living deep in the backwoods of the Connecticut Berkshires. He has worked in aquariums and planetariums, national parks and urban forests. When not birding or turning over rocks to find weird bugs, he enjoys rockabilly music, gourmet cooking, playing harmonica and writing dirty haiku. View all posts by quantumbiologist

4 responses to “Big Baby

  • Lauryl Wagoner

    When I first stumbled across the “big baby” frog, I too lost my pants. I am so fascinated. Thank you for theorizing. I am fond of the idea that they have to be big in order to compete against large fish, but, indeed, this will require much more study.

    • quantumbiologist

      Apparently Paradoxical Frogs are dang hard to study. The adults are small, nocturnal, cryptically-colored, and mainly aquatic. Researchers are more interested the potentially diabetes-curing compound called Pseudin found in their skin than they are the mystery of how big tadpole make tiny frog.

  • Ben Bormann

    Maybe the tadpoles are that large so the usual predators of tadpoles in their habitats fail to recognize them as prey. You’d think something else would step in and eat them, though.

    Also, you’d think that with a possible cure for diabetes in their skin we’d be studying them like mad to find any other possible trait to utilize. With enough eyes on the frogs for enough time, eventually scientists would get curious about the uniqueness of them shrinking into adulthood. Because, who knows, whatever may be active in that process might be medically beneficial as well.

  • Tatyana Brown

    I’m going to contradict one of your premises and say that the tadpole’s excess bulk IS an energy reserve. I think that could make sense if:

    1. The tadpole is pulling a koala maneuver, and its food source is outrageously difficult to digest, but as such is a basically cornered market. Ooooh! Highly improbable but insanely fun idea: Maybe it’s a consumer of toxin-rich flora or fauna, and the more it eats, the more reserved energy it uses up. With less reserve energy, the tadpole becomes increasingly inefficient at processing the toxin, until the buildup triggers the transformation to adulthood.

    2. The adult Paradoxical Frog is somehow incredibly structurally complex or difficult to build. In that case, the tadpole spends its excess mass on becoming the frog equivalent of a James Bond gadget: teeny, but alarmingly sophisticated. Maybe it’s even the process of growing especially antimicrobial skin (yes, I looked up Pseudin to see if it might back up my argument).

    These are both total crackpot theories, but no more unlikely than many documented evolutionary developments.

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