WHEN radio transmitters surgically inserted into wild tree frogs started appearing on the ground without explanation, scientists had a puzzle on their hands. How could a transmitter - albeit tiny - dislodge from the frog?

''We had occasionally found a transmitter in a pile of predator poo belonging to something which had eaten our frog,'' said zoologist Chris Tracy from Charles Darwin University in the Northern Territory. ''But finding transmitters out on the ground with no sign whatsoever that the frog had been eaten was a bit of a mystery to us.''

So a study that started out trying to establish how frogs regulate their body temperature turned into a quest to solve the mystery. The results, published online in the Royal Society journal Biology Letters this week, were unexpected.

It turns out that Australian frogs have a unique way of removing foreign objects from their body. After swapping transmitters for beads, which were surgically implanted in the frogs' bodies, researchers found the beads made their way to the bladder from where they were expelled in 10 to 23 days.

The research team used beads because they were small enough to pass through the pelvic girdle of green tree frogs and cane toads. The frogs were closely monitored in the lab, so there was little chance the bead's return would be missed.

''Sure enough, we came in one morning and there was a green tree frog sitting next to the bead, and over the next two weeks that batch of five frogs all peed them out,'' Dr Tracy said.

Of the cane toads inserted with beads, just one of the five expelled the bead but the other toads had moved the bead from the left side of the body to the bladder.

Dr Tracy said while scientists knew vertebrates such as humans and crocodiles expelled foreign objects through the skin or intestine - the bladder being an exit point for foreign objects was new.

''It seems to be a really different mechanism from what all the other species do,'' he said. ''As near as we can tell, it looks like what most species do is a pretty standard response, [but] we've never seen anything like this before.''

With most frogs and toads, the bladder is a large organ that does more than hold urine before it is expelled from the body. The bladder, which takes up a significant proportion of the body cavity, is often used ''as a canteen'' to store water in dry periods.

''In some of the desert frogs, when the bladder is full, the mass of that can be greater than the mass of the rest of the frog,'' he said. ''But now we know that it's also the exit route for foreign objects.''