Paradoxical Frog: How Does a Giant Tadpole Turn Into a Little Frog?

Growth is a natural process in any living organism, including animals. A complex variety of factors determines animal growth, but these can be reduced to the supply of nutrients, the environment, and the animal's gene pool. In the animal kingdom, there are creatures with bizarre characteristics, such as a species of amphibian known as the paradoxical frog, which shows a different growth pattern in its life cycle.

What is a Paradoxical Frog?

Pseudis paradoxa, or paradoxical frog, refers to a species of hylid frog from South America. These animals inhabit lakes, lagoons, ponds, and similar water from the Amazon and the Guianas to Venezuela and Trinidad.

As tadpoles, paradoxical frogs feed on algae and are equipped with teeth to help them scrape this off of rocks. Upon reaching adulthood, they become carnivorous and feed exclusively on invertebrates.

Also known as shrinking frogs, these animals are highly aquatic, with eyes and nostrils set at the front of the head to breathe and see when under the water.

This species is unique in its ability to "shrink" during metamorphosis into an ordinary-sized frog, which is only a quarter or third of its former size. Its tadpoles grow up to 11 inches (27 centimeters) long, considered one of the longest in the world. Meanwhile, the adult shrinking frog measures up to 3 inches (8 centimeters) long.

There are noticeable local variations in the final sizes of the tadpoles. It was observed that those in large temporary waters with enough food and few aquatic predators grow larger than those in smaller waters with less food or more aquatic predators.

Other members of the genus Pseudis also have large tadpoles and ordinary-sized adults, but the record holder was a tadpole in Amapá, which belongs to the Paradoxa species.

Female paradoxical frogs lay normal-sized eggs, which hatch into tadpoles that grow continuously for months. As they develop into froglets, the giant tadpoles lose their tails, comprising most of their overgrown bodies.

Why Do Giant Tadpoles Shrink Upon Maturity?

When the paradoxical frog was first discovered, it was believed that the frogs and tadpoles were two entirely different species, so they were given another scientific name. Only when these animals were raised in captivity did experts confirm that they are the same species.

While the exact shrinkage mechanism in Pseudis paradoxa is unclear, the change mostly occurs during metamorphosis. In a study published in 2009, it was noted that the growth rate of its tadpoles is similar to that of other tropical species, but they continue to grow long after most other frogs have undergone metamorphosis.

It was found that several organs of this 'baby adult' microscopically resemble adult tissues, an indication that these tadpoles likely gain some ecological advantage by performing some of their adult development in the juvenile stage. This might be due to increased access to food and a decreased chance of getting eaten by predators, although more studies are needed to verify them.