Swimming in a pool of cannibals after being abandoned by one's parents is a pretty grim situation. But a tadpole that finds itself here doesn't passively await its fate. Instead, it tries to jump onto the back of any visiting frog and hitch a ride to safety. Even if the frog has no interest in a rescue, the tadpole is ready to rescue itself. Not all
variabilis parents abandon their young. These Peruvian poison dart frogs lay two to six eggs at a time in water, and the fathers usually return after the eggs have hatched to separate the young. That's because the tadpoles are vicious cannibals. The fathers put their babies on their backs and ferry them, one by one, to separate pools of water.
But sometimes the father doesn't come back. For unknown reasons, he abandons the young to fend for themselves. When this happens, the tadpoles attack each other until "only one individual survives," write Lisa Schulte and Michael Mayer of Trier University in Germany. The researchers wanted to know whether abandoned tadpoles actively seek help when they've been left behind like this. They gathered 15 fresh clutches of tadpoles and put them into pools in a lab. They they dropped adult frogs at the edge of the pool. Some were male
variabilis frogs, like the tadpoles' missing fathers. Others were females—the scientists didn't think tadpoles would react to the females, since mothers aren't usually the ones carrying them to safety. Adult male frogs of two other, related species visited the pools also.
To the scientists' surprise, the tadpoles swam toward every adult. They didn't care whether the frog was the right sex, or even the right species. Any frog was a potential lifeboat. In two cases, tadpoles even managed to climb onto the adult frog's back. The scientists note that several other tadpoles seemed to be aiming for the same thing, but didn't succeed. While frog fathers may scoop their backs or use their legs to help their own babies get onboard, the adult frogs in the experiments didn't help at all. In fact, they seemed to be ignoring the tadpoles. But that didn't deter the young swimmers. "It seemed like the tadpole rather jumped than climbed onto the frog’s back," the authors say—a move that "nearly resembled an attack." When the scientists put 3D-printed frog models at the edges of the pools instead of real frogs, the tadpoles weren't fooled. The models may not have been convincing enough, they say. But it's also possible that tadpoles use some chemical cue to help them find adult frogs. In the real world, a frog visiting the tadpoles' pool would mostly likely be their dad. But a frog father might rescue only one of his tadpoles before he runs out of places to put them. So the tadpole that can climb onto its dad's back first has an advantage—it might be the only one to escape the deadly nursery. This leads to an unusual dynamic between parent and offspring. "When in acute danger," the scientists write, tadpoles "do not passively wait for their parents to pick them up."
Image: John Clare (via Flickr)