Christopher Tracy found the three radio transmitters lying on the forest floor. They were still intact and sending off a strong signal, but there was a big problem – all three of them were meant to be inside the body of a frog.
Several weeks before, Tracy had implanted transmitters into three species of Australian frogs to track their whereabouts. He had placed the devices into the frogs’ peritoneal cavity, a space within its belly that contains its stomach, guts and liver. But these ones were alone, with no bodies nearby or any signs of predators. The frogs hadn’t died or been eaten, but they had somehow removed the transmitters from an enclosed space within their bodies.
When Tracy located his other tagged frogs, he found an important clue: around three-quarters of the transmitters had moved to the animals’ bladders. Tracy was intrigued. He rounded up five more Australian tree frogs and five cane toads, implanted small beads into their bodies, and tracked them solidly for two to three weeks. After that time, he found that four of the toads had the beads in their bladders, and the other animals had urinated theirs out.
The invasion of land by the tetrapods – four-limbed animals that include mammals, reptiles and amphibians – was surely one of the most evocative events in animal evolution. The march onto terra firma began some 365 million years ago and was driven by a suite of innovative adaptations that allowed back-boned animals to live out of water.
Lungs were among the most crucial of these for they allowed the first land-lubbers to extract oxygen from the surrounding air. That ability is so important that it’s rare for tetrapods to lose their lungs completely. Until now, the only groups that we know have done so are two families of salamanders and a lone species of caecilian (a type of burrowing worm-like amphibian).
Now, David Bickford and colleagues form the National University of Singapore have expanded that list with the discovery that a species of frog – the Bornean flat-headed frog (Barbourula kalimantanensis)- also lacks lungs of any sort.
The small species, also known as the Kalimantan jungle toad, is one of the most primitive of all frogs. It’s almost completely aquatic and lives in fast-flowing streams on the island of Borneo. Bickford found nine specimens on a recent expedition to the island in 2007 and before then, only two specimens had ever been found in almost 30 years of searching. Through dissections carried out right there in the field, Bickford confirmed that the frog has no lungs.