If your garden has a pond, or you just like to keep your eyes open while out for a walk, you will be familiar with tadpoles.
So familiar, in fact, that a really weird aspect of tadpole development will probably seem completely normal. That is, frogs have four legs, but tadpoles have only two — the back ones.
The front legs of tadpoles develop along with the back ones, but they do so internally, erupting only at the very end of development, just before the tadpole officially becomes a frog and leaves the water. This is not a peculiarity of our common frog — all frogs and toads are the same.
Why? The widely accepted explanation is that it's got something to do with swimming.
Specifically, that tadpoles can swim faster without their front legs, and are therefore better at escaping from predators. The trouble is that, like many widely believed explanations for all kinds of things, this one had never been tested.
Rightly offended by this unsatisfactory state of affairs, a team of biologists from the Universities of Glasgow and Edinburgh set out to investigate, and recently published their findings in the journal Functional Ecology. And to make sure they did a proper job, they used common frogs, common toads and four tropical frogs and toads as well.
Right away they noticed another strange thing about all their study species: the front legs rarely erupted simultaneously. The species differed a bit, but generally one leg appeared five or six hours before the other. So for a short spell the tadpoles were swimming around with three legs.
Surely, the scientists thought, this must be a bad idea — like trying to steer a rowing boat with only one oar.
So they conducted some experiments to see if having two, three or four legs affected how fast tadpoles could swim. What they actually measured was "burst speed": how fast a tadpole set off when startled by a jet of air from a pipette.
And second, did having just one forelimb affect the angle they set off at? Essentially, did tadpoles with only one emerged forelimb show any tendency to swim in circles?
The results were quite unexpected. Tadpoles swam faster with four legs than with two. They even swam faster with three legs than with two.
If escaping from predators is a priority (which it probably is; tadpoles are a favourite food of a whole zoo of aquatic predators), they would apparently be better off if their front legs emerged earlier. Not only that, but having just one front leg didn't seem to affect the direction they swam in.
Weirdly, startled tadpoles of all the species had a tendency to set off at an angle to the right. They had this bias when they only had two legs, and showed exactly the same bias after one front leg had emerged, irrespective of whether that was the right one or the left.
So what's going on? Why do tadpoles' front legs develop internally, emerging only at the last minute? There's a long list of possible explanations, but most seem pretty unlikely to me, and of course all are basically untested. Frankly, no one has any real idea.
Science isn't about knowing all the answers; it's about asking the interesting questions. Full marks, by the way, to the Glasgow Natural History Society for helping to fund this study.
Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written five gardening books, including Compost and No Nettles Required. His most recent book is Where Do Camels Belong? The Story and Science of Invasive Species.
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