The body doesn’t like foreign objects. For example, our skin is often able to “work out” small splinters, and we can reject many implants with nasty allergic responses to them. Some species (such as catfish), when you give them an implant, will work it into the intestine, and poop it out (there’s even a report of this happening in a human with a leftover surgical sponge)! Of course, pooping something out is one thing, you’ve got a little room to spare. Peeing it out? That is entirely another, and brings up feeling akin to the horror of passing a kidney stone. But it doesn’t stop some species, and in this case, it left some scientists scratching their heads.
Tracey et al. “Removing the rubbish: frogs eliminate foreign objects from the body cavity through the bladder” Biology Letters, 2013.
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When you do studies of animals in the wild, you often use radio transmitters to tell where they are going and what they are doing. You capture some animals, apply the transmitter, and release, tracking them for a time. You can then try to recapture to remove the transmitter, which can allow you to recover more data, or you can use estimates of how often you recapture to determine the size of a population.
The radio transmitters vary in how they are applied. Ones for sea turtles, for example, get stuck on the back of the shell. Ones for mammals or birds are often collars or rings attached to a foot. For bats they come in glue on. But what about for amphibians? They don’t have necks, or ankles really. You can’t put something glued on to the skin because it will interfere with their ability to take in oxygen (also, it’s slimy). So…implants!
There are subcutaneous radiotransmitters, but again, animals have an amazing ability to get them out. So in this case, to study frogs the authors implanted inert radio transmitters into the body cavity of the frog. It’s easy to do, and the frog will not presumably react to the presence of the transmitter.
…except that, in this study, the transmitters kept disappearing. In a few cases, the scientists found the transmitters on the ground, but with no sign that the frog previously attached to it had been eaten (usually you will see something around to indicate predation). But in 75{9f43b4361d9a125bc126dd2a2d1949be02545ec69880430bc4fed2272fd72da3} of the cases, the transmitters were still IN the frogs, they just weren’t where the authors expected to find them. Instead, they were in the bladder.
And a transmitter in the bladder indicates that it might be possible for the frogs to…pee them out. It seems like something of a stretch (especially for their poor cloaca!), so the authors decided to test the hypothesis.
They took 5 native Australian tree frogs (pictured above), and 5 invasive cane toads (pictures below).
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In this case, instead of wasting the (presumably more expensive) radio transmitters, they implanted small beads into the peritoneal cavity. But they were only small to us, to a frog, these are pretty significant, about 7.8 mm long, or a third of an inche. Then they held on to the frogs and toads and got ready to catch beads.
After 19 days, ALL of the Australian tree frogs had successfully peed out their beads. The cane toads didn’t fare so well, only 1 of the five got his bead out, but the other four had them in the bladder (perhaps they were just too uncomfortable to pass).
So…how does it happen? I mean, these are large beads, it’s not like they can just slip in through the blood or something. Nope, instead, when they authors tracked the beads, they found that the beads became surrounded by bladder tissue as soon as 2 days after implantation! First there was a then tissue extension from the bladder, and then thicker layers came in to envelope the bead and move it in to the bladder. Once they were in the bladder, the beads just kind of hung out until they got near the opening between the bladder and the cloaca.
Remember, frogs and toads don’t have a urethra like we do, instead they have the cloaca, an all purpose hole for urine and poop (and, in internally fertilizing groups like birds and reptiles, for semen as well). So it’s not like this is a teeny tiny passage (though it was probably smaller than the bead in question).
So why the bladder? Why not the intestine like in other species? Well, you underestimate the bladders of frogs and toads! They can be impressive pee-ers. In some cases, the volume of urine in the bladder can be MORE than the entire body weight of the frog alone! Imagine carrying around a 135 lb bladder! This also means that the bladder takes up a LOT of the frog or toad’s body cavity space, so in many cases it may be the closest and easiest contact when a foreign object pierces the skin and settles in the body cavity (like, say the frog landing on a pointy stick). In other species, the bulk of the body cavity is intestine, so that might be a better route (though this is all hypothesizing, of course).
But what it means is that people tracking frogs and toads may need to keep an eye on their transmitters! When you see one on the forest floor, it might not be a sign of a dead frog. Instead, it might be the one that peed away.
Tracy, C., Christian, K., McArthur, L., & Gienger, C. (2010). Removing the rubbish: frogs eliminate foreign objects from the body cavity through the bladder Biology Letters, 7 (3), 465-467 DOI: 10.1098/rsbl.2010.0877
