A bizarre discovery shows some frogs and toads can grow their bladders around objects lodged deep within their bodies, ultimately peeing them out.

Zoologists from Australia's Charles Darwin University discovered the oddity after implanting frogs with tiny radio transmitters, which inexplicably migrated to the bladder. They describe their findings in an upcoming issue of Biology Letters.

"This is an extraordinary evolutionary trick," said Rick Shine, an evolutionary ecologist at the University of Sydney who wasn't involved in the study. "It wouldn't surprise me if we continue to find this ability in other animals. Natural selection has had a few hundred million years to solve tough problems organisms encounter."

Christopher Tracy and his team came across the amphibian ability while studying heat regulation in Australian green tree frogs. The team implanted tiny radio transmitters deep in the amphibians' peritoneal cavity, which lies just outside the peritoneum – a membrane containing the major organs of most animals.

After a few weeks of living in the wild, however, about 75 percent of the frogs' transmitters were found in their bladders. Tracy and his colleagues also found some on the ground without any signs of frog death, including being eaten by a predator, disease or other explanations.

"We thought there was a more mundane explanation until we started retrieving transmitters in frogs, and they were in the bladder," Tracy said. "At that point we connected the dots."

Back in the lab, Tracy’s team put their hypothesis to the test. They enlisted five green tree frogs and five cane toads, implanting small inert beads in each the same way they implanted the radio transmitters. Each tree frog expelled its bead within 23 days. One cane toad also gave its bead the boot, and the beads in the other four toads had migrated to their bladders.

To unravel the secrets of the process, the zoologists implanted beads in 31 more cane toads, toxic amphibians native to South America but introduced to northeastern Australia in 1935 to control beetle infestations. (Since then, Shine says, the toads have become invasive and poisoned populations of large predators such as pythons. As a result, ecologists now closely track their numbers and behavior.)

Toads dissected on sequential days revealed that the bladder grew a thin offshoot of cells to surround the bead, which later developed into mature, bladder-like tissue and merged with the organ's main cavity. From there, they "floated freely in the urine" and were peed out if near the bladder's opening.

"It could be a fluke of nature, but we've thought of many stories where this makes some sort of adaptive sense," Tracy said. "Frogs and toads have pretty thin skins and are clumsy when they hop, so it's not hard to imagine them landing on something that pierces the skin. They also don't chew on insects they eat, so the hard parts could poke through their digestive systems."

The intestines of humans and other animals, including sharks, reef fish, crocodiles and snakes, are already known to enshroud and expel foreign debris. But the handiwork of amphibious bladders was previously unheard of.

"As far as we can tell, this is a completely different mechanism than what’s seen in other animals. We're still in early stages of figuring out what's going on," Tracy said. "It may be going on in other amphibians as well."

When a snake eats, its digestive system occupies most of the creature's body cavity, Shines says, so it makes sense that the reptiles would use that organ to remove strange objects. Likewise, bladders take up most frogs' and toads' internal real estate.

"Frogs have a terrible problem with water loss, so one of their insurance policies is to have very large bladders, used as a kind of water reserve," Shine said. "It's a very plausible place you could slip an object to get rid of it."

Implanted radio transmitters have been a popular method to follow these creatures' populations, but Shine and Tracy don't think the latest wrinkle in the device's efficacy should put a hold on their use.

"It's not a terrible problem. We will have to be more cautious in interpreting telemetry data," he said. "But there may be modified ways to put in a transmitter so that it won't be removed from the body."

Carl G. Meyer, a marine biologist at the Hawaii Institute of Marine Biology, agrees.

"We have long-term data from large numbers of sharks and large reef fishes, indicating the majority of animals retain their implants despite a natural ability to expel foreign objects from the body cavity," Meyer wrote in an e-mail to Wired.com.

Smaller animals such as reef fish, however, may be better at removing debris, and thus more likely to skew population data.

"We need to run additional controlled experiments to determine whether this is in fact the case," he wrote.

Images: 1*)** A cane toad, also known as Rhinella marina. Credit: Flickr/Sam Fraser-Smith** 2) **An Australian green tree frog, also known as Litoria caerulea. *Credit: Chris Tracy

See Also: