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.  To avoid competition with other tadpoles, the Paradoxical Frog tadpoles grow very large very rapidly to take advantage of another food source.  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.  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.