Frogs are one of the most remarkable examples of metamorphosis in the animal kingdom, transferring from little wriggly (sometimes carnivorous) tadpoles into four-legged hopping adults of all shapes and sizes. Identifying which tadpoles belong to which frog species from appearances alone can therefore be difficult – but by monitoring sites where known frog species are depositing their eggs, we can fill in this gap. For a research project in Brazil studying the Paranapiacaba Treefrog, however, this method proved tricky, as it became apparent that the tadpoles were disappearing from their birth pools (so to speak) before completing their development. So, where do they go?
The study, published in the journal PLOS ONE, first established where Paranapiacaba Treefrogs mated as well as where they were depositing their eggs. After many years, they were able to conclude that this species always mated and laid their eggs in the "leaf-tanks" (pools of water suspended by leaves) of bromeliads, a family of plants. Strangely, observation of these bromeliad leaf-tanks never yielded any tadpoles beyond a certain point of development.
“We could see in the bromeliad leaf-tanks that the tadpoles were always in their initial stages of development (up to stage 26),” said study author Dr Leo R. Malagoli of Universidade Estadual Paulista in Brazil in an email to IFLScience. “As bromeliads are always located beside or above temporary or permanent streams, there was only one explanation: tadpoles must end their development in these streams, where they could find more food and space to grow. In fact, the tadpoles found in streams were larger and had more advanced stages of development than those found in bromeliads.”
The next step was confirming if the big wiggly tadpoles they were finding in the streams were definitely the same species as the slightly smaller wiggly tadpoles in the leaf-tanks – and sure enough, DNA analysis confirmed that they belonged to the Paranapiacaba Treefrog. While frogs spawning in leaf-tanks is not new to science, the journey made by these tadpoles to complete development has never been seen in anurans (frogs and toads) before.
So how does a small tadpole make this epic migration? The researchers believe heavy rains may help them on their way, either by encouraging the tadpoles to jump or by essentially washing them out. The more it rains, the more the bromeliad leaf-tanks fill up, sending the tadpoles gushing forth from their safe – but ultimately lacking – nursery pools and out into the big, wide, edible world.
The reproductive strategy is a new one for science and after 11 years of research, it’s perhaps a satisfying world-first discovery for the team, but Malagoli says they’re not finished yet. “This species of tree frog, which is endemic to the Brazilian Atlantic Forest, still has many secrets that deserve to be researched and unveiled,” he continued. “I think one of the interesting topics is to discover in more detail the process of transferring tadpoles from bromeliads to streams.”