The incredible shrinking frog
In the lakes, ponds, and slow-moving rivers of the Amazon Basin lives a frog that breaks a fundamental rule of growth. The paradoxical frog, Pseudis paradoxa, starts life as a tadpole of enormous size, only to shrink as it reaches adulthood. The tadpole can reach a length of up to 27 centimeters (10.6 inches), holding the record for the world's longest tadpole. The adult frog, however, measures a mere 3.4 to 7.6 centimeters long.
This "reverse metamorphosis" is what gives the frog its name. Early naturalists were so baffled by the size difference that they initially believed the frog must metamorphose into the giant tadpole, which then became a fish, unable to accept that shrinkage was possible. The reason for this biological puzzle lies in the tadpole's anatomy. The vast majority of its length comes from a large, muscular tail. Its actual body, or snout-vent length, is comparable to that of the final adult form even before metamorphosis begins. The process of absorbing the massive tail accounts for the dramatic reduction in overall length. Unlike other frogs that grow substantially after becoming froglets, the paradoxical frog does little to no growing after it loses its tail.
A life of extremes
The tadpole's extended growth period is an important part of its life strategy. Studies show its growth rate is similar to other frogs; it just keeps growing for much longer. By the time metamorphosis occurs, the frog is essentially mature. Males are already producing sperm and females have eggs in development, a stage most frogs only reach well after their juvenile phase. This extended larval stage may be an adaptation to its environment. Tadpoles in large, temporary pools with abundant food and fewer predators tend to grow the largest. Their immense size likely offers protection from predators that would typically eat smaller tadpoles.
The frog's diet also shifts dramatically with its form. As a large tadpole, it is an herbivore, feeding mainly on algae. As a much smaller adult, it becomes a carnivore. It spends its time in or near the water, preying on insects like beetles and flies, as well as other small invertebrates. This aquatic lifestyle is unusual, as most frogs do not spend their entire lives in water. If its watery home dries up during the summer, the frog will bury itself in the mud and enter a state of hibernation until the rains return.