You may recall that we commented in Linda Bower's leeches video that the tadpoles only had hind legs. I always assumed that the front legs would start as little buds, growing larger slowly like the hind legs do. It turns out that they do, but hidden inside the gills. Notice above the bump below and behind the eye. That is the front leg awaiting its debut. So how does a tadpole become a frog?
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| "Look Ma, no hands!" |
"As a tadpole matures, it most
commonly metamorphosizes by gradually growing limbs (usually the back
legs first, followed by the front legs) and then (most commonly in the
case of frogs) outwardly absorbing its tail by apoptosis. Lungs develop
around the time of leg development, and tadpoles late in development
will often be found near the surface of the water, where they breathe
air. During the final stages of external metamorphosis, the tadpole's
mouth changes from a small, enclosed mouth at the front of the head to a
large mouth the same width as the head. The intestines shorten to
accommodate the new diet. Most tadpoles are herbivorous, subsisting on
algae and plants."
Wikipedia
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| Front leg peeking out of gill opening |
The front legs develop internally inside the gill pocket of the tadpole’s body. They don't emerge until the tadpole is almost ready to make its frog/toad debut and leave the water. The
seemingly obvious reason used to be that the delay in emergence was because the front legs would obviously slow the tadpole's swimming speed, making it more vulnerable to predators. Recent research published in
Functional Ecology has proven the common explanation wrong.
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| Three legs and a leg lump in the gills |
This article in the
Telegraph summarizes the findings.
As seen in Linda's video, front legs rarely erupted simultaneously. Generally one leg appeared five or six hours before
the other. So for a short spell the tadpoles were swimming around with
three legs which common sense would say might prevent the tadpole from swimming straight. Wrong, they swam as straight as those with two or four legs.
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| Right elbow out but the "fingers" still in the gills. |
But of course the little legs dangling down would surely slow them down. Wrong! They tested tadpoles having two, three or four
legs to see the effect on how fast tadpoles could swim. They measured
the escape or “burst speed” - how fast a tadpole set off when startled by a jet of
air from a pipette. "Tadpoles swam faster with four legs than with two. They even swam faster with three legs than with two."
Now watch in
Linda's video and pay special attention starting at 2:24 and you will see the front leg on the right emerge and start functioning.
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| "Free at last. Time to suck up my tail and eat meat!" * |
* Stay tuned as tadpoles complete their transition to frogdom.
