Amphibians
Paradox Frog
Pseudis paradoxa · also called shrinking frog, jackie frog
The paradox frog earns its name from one strange fact: its tadpole is a giant, up to 25 cm long, while the adult frog it turns into is a modest 4–7 cm. The animal appears to shrink as it grows up.
Overview
Pseudis paradoxa is a small, almost entirely aquatic frog found across northern and central South America, from the Orinoco and Amazon basins south to Argentina and out to Trinidad. It has smooth green skin with dark markings, powerful webbed hind feet, and upward-facing eyes for floating at the surface among water plants.
The shrinking tadpole
The tadpole grows for months and becomes one of the largest tadpoles known. Much of that length is tail and a long gut suited to a plant diet. At metamorphosis the tail is reabsorbed, the gut is rebuilt for eating insects and other prey, and the body takes its compact adult shape — so a 25 cm larva becomes a 6 cm frog. It is the ordinary frog life cycle taken to an extreme.
A historical puzzle
The life cycle was painted in Suriname around 1705 by Maria Sibylla Merian, and Linnaeus later gave the frog the name paradoxa, “contrary to expectation.” For a while some naturalists wondered whether the frog was turning into a fish.
Status
The paradox frog is Least Concern with a stable population. It uses almost any still or slow fresh water, including ditches and farm ponds, which helps keep it common across its enormous range.
Paradox Frog: common questions
How can the paradox frog's tadpole be bigger than the adult?
The tadpole simply grows for a long time and gets very large, reaching around 25 cm, and then loses a lot of length during metamorphosis. Much of a big tadpole's body is tail and a long coiled gut suited to a plant diet. When it changes into a frog the tail is reabsorbed, the gut is rebuilt for eating animal prey, and the trunk and limbs take their adult proportions, which are compact. The result is a frog of 4 to 7 cm. Nothing is broken by this; it is an extreme version of the normal frog life cycle, where tadpole and adult are almost different animals sharing one body.
Is the paradox frog the only frog whose tadpole shrinks?
All frogs lose their tail at metamorphosis, so all of them get shorter in that sense. What makes the paradox frog and its close relatives in the genus Pseudis unusual is the scale of it. Their tadpoles are exceptionally large to begin with, so the drop from larva to adult is dramatic and obvious rather than slight. A few other frogs have big tadpoles, but the Pseudis frogs are the classic example, and Pseudis paradoxa is the one that carries the name.
Where does the paradox frog live and is it rare?
It is common and found across a huge area of South America, from the Orinoco and Amazon basins and the Guianas south into Argentina, as well as on Trinidad. It lives in still and slow-moving fresh water — ponds, lagoons, flooded grassland, marshes, ditches, and the quiet edges of rivers — where it floats at the surface among water plants. Because it uses a wide range of wetland habitats, including man-made ones, it is listed as Least Concern with a stable population.
Can the paradox frog be kept in an aquarium?
It is occasionally kept by specialist keepers as a genuinely aquatic frog. It needs a large tank with plenty of surface plants to rest among, clean warm water, and live foods such as insects and small crustaceans. The giant tadpole stage is the real novelty and the reason people seek the species out. It is not a beginner animal and, like all amphibians, should be captive-bred rather than taken from the wild, though wild populations are currently healthy.