Pond dipping in South America is surprising, mainly because it isn’t that different from pond dipping in a British ditch. There are familiar faces in the water: beetles, pond skaters, water boatmen, damselfly and dragonfly nymphs, even the odd tadpole. That said, sometimes the scale of things can take you by surprise – for example, when I encountered my first Pseudis paradoxa tadpole, known as the paradoxical frog.
Usually, tadpoles don’t get much bigger than a fingernail but this brute of a thing made my net bulge. When extracted, it filled my palm, leaving the considerable length of its ridged, muscular tail to flop over the edge. This gargantuan tadpole measured more than 21cm long from nose to tip – though it still fell short of the record length of 27cm attained for this species.
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Pushing the veils of aquatic weed aside with my pond net revealed a bumping mass of these animals fleeing my gaze. I assumed, logically, that this large tadpole would become a large frog, maybe a bullfrog or marine toad. I didn’t realise that the parents of these behemoth water babies were, in fact, all around me, the regular green and brown froggy-looking frogs that sat in the surface weed, creaking away. They were unremarkable in size, too – barely reaching 7cm. And that, right there, is the paradox.
What's special about the paradoxical frog?
This frog doesn’t progress through life in a predictable linear fashion, getting bigger as it grows and continuing this growth trajectory through metamorphosis into a froglet and then a mature adult frog. Instead, at the point of becoming a frog, it appears to take a hop backwards, at least in length and weight, giving it and the other members of its genus the alternative name of shrinking frog. It’s no exaggeration: an adult can be as small as a quarter of the size of its tadpole. While much of the bulk of any tadpole is the tail and associated musculature, this is still extreme shrinkage.
How big are paradoxical frogs?
The paradoxical frog's tadpole grows up to 22cm (9 inches), whereas the adult frog measures approximately 8cm (3 inches) long.
Why does the paradoxical frog shrink in later life?
What is the advantage of this bizarre life strategy? Nobody really knows, but the explanation could be connected to the places it lives. These frogs breed in and around a variety of still-water habitats throughout tropical South America, and there is quite a variation in sizes: not all the tadpoles get to be as big as others. The biggest seem to be produced from temporary lagoons, which dry up periodically. These lagoons contain water that is usually a living soup of small plants, algae and tiny invertebrates that can’t support large predators such as fish.
In such situations, it is advantageous for Pseudis to remain at the tadpole stage. After all, if life is good, why change anything? And while these lagoon tadpoles may appear not to progress to adulthood, if we could take a peek at their internal workings, we would see that they are, in fact, mature in every way. Reproductive organs are already fully developed inside each of these behemoth youngsters, something scientists describe as delayed metamorphosis.
How do paradoxical frogs develop?
Normal tadpole-to-frog development involves a vulnerable froglet stage where juveniles leave the water to embark on a terrestrial growing phase, sometimes lasting several years. But when proto-paradoxical frogs get to this stage, the building blocks of becoming a frog are already in place and the eventual change happens rapidly, within 10 days or so, and they can be up and ready to breed very soon after they’ve absorbed their tails and grown their legs. Indeed, those tadpoles living in habitats where predators are rife will need to get out of the water quickly, unlike their larger, lagoon-dwelling cousins.
Whatever reasons evolution has come up with for this frog paradox, it obviously works, as these small frogs with the leviathan tadpoles are common and widespread throughout their range.
How can paradoxical frogs help humans?
Scientists have recently discovered a compound called Pseudin-2 in the skin of paradoxical frogs, which is supposed to protect the frogs from infection. In humans, this compound has been found to stimulate insulin production and could be used as a treatment for type 2 diabetes.
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Top image: paradoxical frog. Credit: Getty
