Frogs debug themselves by absorbing tracking devices into their bladders

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.

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To track the beads’ expulsion more carefully, Tracy captured another 31 cane toads, stuck more beads into them and dissected them on successive days. He found that a thin flap of tissue slowly grew from the bladder and soon enveloped the beads, in as few as two days for some animals. Once surrounded, the beads were covered by another thicker layer, full of blood vessels. Soon, they were pulled into the bladder itself, floating freely until they happened to be peed out.

Many animals have managed to expel transmitters and other foreign objects from their bodies, including fish, snakes and crocodiles. Even in humans, surgical sponges left in the body have sometimes wormed their way into the intestines, only to be removed through the usual route. But all of these species purge their internal rubbish through the intestine or the skin – the bladder is a new route, although perhaps not an unexpected one.

Frogs famously jump a lot, and risk getting punctured by sharp nearby objects; the insects they swallow whole can also come in spiny shells. It makes sense for them to be able to get rid of any unwanted bodily junk. The bladder presents a good route – frog bladders are very large and they take up a lot of room. Any intruding objects are more likely to come into contact with the bladder than any other organ.

It’s a cute result, but one with potential importance for scientists who track amphibians – it could lead to people drawing false conclusions about the death or disappearance of frogs that had simply managed to debug their bodies.

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