Plant thorns, spiny insects and even radio transmitters don't stick around for long inside tree frogs. Researchers have discovered that these amphibians can absorb foreign objects from their body cavities into their bladders and excrete them through urination.
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 strikes me as being a pretty incredible mechanism for getting stuff out from the body cavity," says lead researcher Christopher Tracy of Charles Darwin University in Darwin, Australia. By contrast, humans and other mammals typically develop peritonitis, a potentially deadly infection of the body-cavity membrane, if the membrane is punctured or damaged by sharp objects.
As if by magic
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 (Litoria caerulea, Litoria dahlii and Cyclorana australis) 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 says. "But there's usually some evidence that happened: scratches on the ground or a pile of predator feces."
In this case, the transmitters were pristine. And the strangest discovery was yet to come: back in the lab, the researchers opened up dozens of animals and in many cases 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 says.
Better out than in
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 researchers describe their findings in a poster presentation today at a meeting of the Society for Integrative and Comparative Biology in Seattle, Washington.
Although the study is the first to show an animal using its bladder to expel foreign objects, researchers have observed similar phenomena in other vertebrates. Several species of fish and snake absorb objects into their intestines from the body cavity and expel them by defecation, for instance.
Rick Shine, a herpetologist at the University of Sydney, Australia, who has seen snakes expel implanted radio transmitters, says that the research has broad implications for understanding vertebrate physiology. For reptiles and amphibians, at least, moving objects from one part of their innards to another may pose no problem. "It makes sense for an animal to get an object out of the body cavity," he says. "The remarkable thing is that they are able to do it."