Paradox Frog

AmphibianSouth America
Written By Alexander Julius Jensen

Galaxias gollumoides

A paradox frog tadpole can grow to be 27 centimetres (11 in) long — the largest tadpole of any frog. It eventually metamorphoses into a shrunken adult frog, only some 7 centimetres (2.7 in) long.

Tadpoles

Most frogs don't hatch looking like frogs. They pop out of their squishy, translucent eggs as teardrop-shaped larvae — just bulbous heads and tapering tails, with no limbs at all. They live in water, in wetlands and ponds, swimming by flicking their tails from side to side. They breathe with a pair of external gills, not unlike those of an axolotl, and many have keratinised "beaks" with which they scrape up algae. As they mature, their gills are covered by a layer of skin and their limbs grow in — first the hind legs, then the front — until they look like little frogs with long tails, before those tails are gradually absorbed into their bodies. They have completed their metamorphoses, from tiny tadpoles to fully-formed frogs.

Throughout the entire process, they are growing, becoming larger — and they'll continue to grow as frogs until they've fully matured, albeit at a slower rate. American toads begin life as 22-millimetre (~1 in) long tadpoles and, over 50 to 65 days, turn into 75-millimetre (3 in) adult toads. Goliath frogs hatch from tiny, 3.5 mm eggs. Over the next 75 to 90 days, the tiny tadpoles grow to about 5 centimetres (2 in), and by 10 to 12 months old, they've matured into 30-centimetre-long (12 in) frogs that can weigh up to 3.3 kilograms (7.2 lb). The growth from newly-hatched tadpole to adult frog is mostly linear — they continually grow bigger, losing some length as they absorb their tails, but gaining more mass than they ever had as tadpoles. There is, however, one paradoxical exception.¹

The Paradoxical Frog

The subject of this paradox is the paradox frog, Pseudis paradoxa, a South American amphibian ranging from northern Colombia to the island of Trinidad and down to northern Argentina. The paradox frog begins its journey like many other frogs: as a single, tiny egg within a frothy mass of hundreds of its siblings, laid among aquatic vegetation by its mother, and fertilised by its father. It hatches as a tiny green tadpole and undergoes the same stages of metamorphosis as your average frog. But, unlike your average frog, the paradox tadpole balloons to a behemoth size of up to 27 centimetres (11 in), the largest tadpole of any frog — almost as long as an adult goliath frog. You might then expect, quite reasonably, that the adults must be massive, larger even than the goliath. This is where the "paradox" comes in; an adult paradox frog is only some 7 centimetres (2.7 in) long. The paradox tadpoles are often four times larger than the adult frogs.²

Solving a Paradox

Paradox frogs at different stages of metamorphosis.

"How?" and "why?" are the questions.

How does a paradox tadpole grow so large? A tadpole's entire purpose is to gorge and grow. The paradox tadpole isn't on some special diet of steroids and growth hormones — like other tadpoles, it mostly grazes algae with its hardened mouth parts. It also seems to grow at a similar rate as other tropical frogs, it just doesn't stop until it becomes a titan of a tadpole. And then it shrinks.

A hatchling paradox tadpole features a striped pattern along its body that helps it hide; a pattern which it loses as it grows larger, clearly no longer worried about predators, many of which it has outgrown. Most of the paradox tadpole's size is in its long and wide tail, with its actual body and head comparable in size to its final adult form. So, unlike other frogs, which absorb their tails and continue to grow, the paradox frog loses its giant tail during metamorphosis, but its body doesn't grow any larger, leaving it as a "shrunken" adult.

Why would it give up its size advantage? There are a few hypotheses.

An adult paradox frog spends much of its time outside the water, chasing insect prey, perhaps restricting its ability to grow — meaning that all its growth has to happen in its tadpole stage. Another hypothesis has to do with the paradox frog's maturation rate. Most frog species will only reach sexual maturity some time after they've metamorphosed into their adult forms. While the paradox frog's growth rate isn't exceptional, by the time the paradox frog has grown its limbs and lost its tail, males have begun producing sperm and females have eggs in stock — they are already sexually mature. Perhaps this immediate maturation is enabled by the paradox frog's exceptional size as a tadpole. The hormone prolactin (PRL) — which promotes growth in tadpoles and can potentially delay metamorphosis — has been suggested as the cause, or the mechanism, behind this frog's weird growth patterns.

Perhaps a paradox tadpole benefits from being larger and less vulnerable to predation, while an adult paradox frog benefits from being smaller — being harder to spot when croaking (with its croaks sounding like a pig's oinks) and more agile for catching insects, but, as of now, we can only guess at the exact answers to this paradox.

¹ There are actually seven exceptions in the genus Pseudis, including the paradox frog. All exhibit the same developmental patterns, although not to the same extreme degree as P. paradoxa.

² While it’s called paradoxical, this frog's development isn't an illogical or impossible situation — not a true paradox. It’s simply a counterintuitive and very odd biological pattern.

Where Does It Live?

⛰️ Shallow waters in ponds, lakes, and lagoons.

📍 South America; from Columbia, east to Trinidad, and down to northern Argentina.

‘Least Concern’ as of 13 April, 2021.

IUCN

  • Size // Small

    Length // Up to 27 cm (11 in) tadpole, 7 cm (2.7 in) adult

    Weight // N/A

  • Activity: Nocturnal 🌙

    Lifestyle: Solitary 👤

    Lifespan: Up to 11 years in captivity

    Diet: Omnivore

    Favorite Food: Algae as a tadpole and arthropods as an adult

  • Class: Amphibia

    Order: Anura

    Family: Hylidae

    Genus: Pseudis

    Species: P. paradoxa

  • The paradox frog, also known as the shrinking frog, hails from South America. You might hear its croaks — akin to pig-like oinks — coming from ponds, lakes, and lagoons.

    How does the paradox tadpole grow so large?

    Like other tadpoles, the paradox mostly grazes on algae with its hardened mouth parts, and it grows at a similar rate; it just doesn't stop until it becomes a titan of a tadpole.

    Does the paradox frog undergo metamorphosis?

    A paradox frog does undergo metamorphoses like your average anuran. It hatches from a tiny egg, swims and feeds as a teardrop-shaped larva, grows larger and larger, develops its hind and front limbs, and, finally, absorbs its tail.

    How does it shrink as an adult?

    By the time a paradox tadpole reaches its peak size — up to 27 centimetres (11 inches) long — most of its length is in its tail, with its actual body and head comparable in size to its final adult form. During its final stage of metamorphosis, it absorbs this tail and becomes a "shrunken" adult — only about 7 centimetres (2.7 in) long.

    What is the purpose of this "paradoxical" growth pattern?

    While most frogs take a while to become sexually mature, the paradox frog can reproduce as soon as it metamorphoses into its adult form, perhaps imparting some reproductive advantage. The tadpole may also benefit from its large size — keeping it safe from most predators as it grazes on algae — while the adult may benefit from its small size — keeping it hidden as it croaks for mates and allowing it to agilely pursue insects.

Alexander Julius Jensen
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