The Mystery of Legless Frogs Solved

It was one of the most contentious debates in the history of environmental issues: what was causing wild frogs to develop with missing limbs?

During the late 1980's and early 90's, researchers received reports of wild frogs being found with missing and or extra limbs.  Some felt predatory insects were to blame, and others thought it was caused by environmental degradation such as thinning of the ozone layer.

Biology professor Stanley Sessions and other researchers were able to determine that a parasitic flatworm disrupted the developing structure of a tadpole, leading to extra limbs.  However, what was causing limb loss was unknown until recently.

Sessions and colleague Brandon Ballengee of the University of Plymouth, U.K., found the apparent answer during one of their recent collaborations.





As part of this work, Ballengee and Richard Sunter, the official Recorder of Reptiles and Amphibians in Yorkshire, spent time during the summers of 2006 to 2008 surveying the occurrence of deformities in wild amphibians at three ponds in the county.

In all, they found that between 1.2% and 9.8% of tadpoles or metamorphosed toads at each location had hind limb deformities. Three had missing eyes.

"We were very surprised when we found so many metamorphic toads with abnormal limbs, as it was thought to be a North American phenomenon," says Ballengee.

While surveying, Ballengee also discovered a range of natural predators he suspected could be to blame, including stickleback fish, newts, diving beetles, water scorpions and predatory dragonfly nymphs.

So Ballengee and Sessions decide to test how each predator preyed upon the tadpoles, by placing them together in fish tanks in the lab.

None did, except three species of dragonfly nymph.

Crucially though, the nymphs rarely ate the tadpoles whole. More often than not, they would grab the tadpole and chew at a hind limb, often removing it altogether.

"Once they grab the tadpole, they use their front legs to turn it around, searching for the tender bits, in this case the hind limb buds, which they then snip off with their mandibles," says Sessions.

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The study by Ballengee and Sessions puts forward an interesting potential explanation for some types of frog deformities. Unfortunately, their research fails to live up to the media hype; it does not solve the mystery of deformed frogs. In the laboratory, they found that some dragonfly larvae will remove limbs of tadpoles. However, Ballengee and Sessions did not actually test the predictions of the dragonfly hypothesis with rigorous data from the field. For example, a clear prediction of their hypothesis is that as the frequency of dragonfly larvae in wetlands increases, the frequency of missing-limb deformities in those wetlands is also expected to increase. Ballengee and Sessions did not test this prediction. Testing such predictions is a fundamental component of science. Until there are well-designed studies that examine the relationship between dragonfly density and frogs with missing limbs in nature, the relative importance of the role of predation in amphibian deformities will remain unknown.
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So, all those environmentalists who said that the loss of limbs *was* caused by pollution were lying to us the entire time!!!eleventy!!11!!
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This is not the only solution. It is also known that limb buds in tadpoles are attacked by trematode larvae which cause a deformation as the limb grows. Additionally, certain pesticides in the water can weaken and slow the tadpoles so that they can be more easily eaten or damaged by predators.
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