While adult dancing frogs have caught the eye of biologists for over a century, due in no small part to the habit of the males to tap and wave their feet about in an attempt to attract a member of the opposite sex, the tadpoles of the amphibians have remained strangely elusive. For the first time, researchers have confirmed that they have discovered the tadpoles of one species of dancing frog, uncovering just why they have been so difficult to find: they burrow underground.
Found in the damp forests and fast-flowing streams of the Western Ghats in India, there are known to be around 14 species of dancing frogs. They get their name from the unusual behavior of the males, which, in order to defend prime territory and attract the attentions of females, will sit on rocks calling while at the same time extending and waving their back feet in a weird mix of dancing and semaphore. If successful, the male will then mate with a female, who proceeds to bury her eggs in the sand of the stream bed.
Yet despite the frogs living and displaying in the clear streams and pools of the Western Ghats, the water has remained mysteriously free of tadpoles. It now seems that rather than looking in the ponds, researchers should have been looking under them, as the larvae were there all along burrowing around in a subterranean world. From hatching out of the eggs to developing into little froglets, the tadpoles lead an entirely burrowing life, finding their way through the gaps between the sand and gravel of the stream beds.
The tadpoles, which were confirmed by genetics to be the larval stage of the species Micrixalus herrei, finally fill a massive gap in the frog’s life cycle, with the results being published in PLOS ONE. Described as having an “eel-like” appearance by the researchers, the tadpoles have a whole host of adaptations for their burrowing lifestyle not normally seen in amphibians. They have strong muscular tails for pushing themselves through the sand, and a well-developed rib cage for the muscles to attach to. Only a handful of species of frogs have rib cages, and it is thought that they aid in protecting the tadpoles' organs when underground.
An adult Micixalus herrei dancing frog. SD Biju
The larvae have relatively underdeveloped eyes, which are in turn covered by skin to protect them from the abrasive gravel. The researchers also found their gut to be full of fine sand, and they suspect that the tadpoles are injesting the substrate, and then extracting the nutrients from the organic matter surrounding it. Even their mouths are adapted to their fossorial lifestyle, with serrated jaw sheaths thought to act as a filter to prevent large sand particles from entering the mouth. Finally, and also unusually for frogs, the tadpoles have a lime sac, which provides the larvae with a source of calcium.
The finding finally answers the question of how the frogs develop, which has plagued scientists since their discovery 125 years ago. They also hope that it will underline the importance of the Western Ghats as a biodiversity hotspot, containing many unique and unusual species.