Scientists have discovered that frogs can absorb foreign objects from their body cavities into their bladders and excrete them through urination, thus revealing the secret disposal system of these amphibians. “It strikes me as being a pretty incredible mechanism for getting stuff out from the body cavity,” lead researcher Christopher Tracy of Charles Darwin University in Darwin, Australia, told Nature News. In 2003, Tracy and his colleagues began a project to find out how frogs regulate their body temperature. They surgically implanted temperature-sensitive radio transmitters in the abdominal cavities of tree frogs of three species living around the city of Darwin. After several months, the authors set out to recapture their frogs to log the data and replace the transmitters’ batteries. But out in the field, they found three of the transmitters lying on the ground. “In telemetry studies of small animals, it’s not uncommon to find they’ve been eaten by something,” Tracy said. “But there’s usually some evidence that happened: scratches on the ground or a pile of predator faeces,” he added. In this case, the transmitters were pristine. The strangest discovery was that when the researchers opened up dozens of animals, in many cases, they pulled transmitters not from the body cavity, but from the urinary bladder. “That’s when we started thinking about trying to pin down exactly what was going on,” Tracy said. In 2008, Tracy and his colleagues decided to look into the phenomenon. They kept tree frogs and cane toads in the lab and surgically implanted beads in their body cavities. Within 2—3 weeks, the beads appeared on the floor of the frog cage. Only one cane toad out of five excreted a bead, but Tracy opened some other toads after the surgery and caught them in the act of enveloping the beads into their bladders. In just two days, the bead was surrounded by a transparent tissue devoid of blood vessels, which subsequently became vascularized and muscular. The finding will be of interest to field researchers, who often implant tiny radio transmitters into frogs to track them.
It also helps to explain how these little creatures survive a life leaping around in thorny forests and consuming spiny insects whole. “It makes sense for an animal to get an object out of the body cavity,” said Rick Shine, a herpetologist at the University of Sydney, Australia. “The remarkable thing is that they are able to do it,” he added.