Wild Tracks

On behalf of the world's wild species

The Mystery of the Legless Frogs

Deformed frogs

Rana sylvatica metamorphs: two are "normal" and four have limb deformities, including missing limbs and limb duplications. (Joseph Kiesecker, Penn State)

Starting in the late 1980’s, reports began surfacing of wild frogs or toads being born with extra limbs, or limbs that were partly formed or missing altogether. The cause of these deformities became a hotly contested issue in scientific circles.

Some felt the problems might be caused by predators or parasites. Others feared chemical pollution or increased radiation caused by the thinning ozone layer.

The mystery of abnormal frog development remained unsolved until Stanley Sessions, an amphibian specialist and professor of biology at Hartwick College, in Oneonta, New York teamed up with colleague Brandon Ballengee of the University of Plymouth, UK.

It turns out the frogs with missing limbs are actually victims of the predatory habits of dragonfly nymphs, which eat the legs of tadpoles. Once they grab the tadpole, they search for the tender hind limb buds, and then snip them off with their mandibles. The nymphs rarely eat the tadpoles whole, and many are able to swim away and develop into the deformed adults.

So why do the dragonflies eat only the hind legs?

The scientists surmise that as toad tadpoles mature, they develop the poison glands in their skin much earlier than those in their hind legs, which could make the hind legs a more palatable meal. The front legs of tadpoles develop within the gill chamber, where they are protected.

The dragonfly nymphs don’t account for the amphibians with extra limbs, but the researchers also found a natural cause for these deformities.

The study established that many amphibians with extra limbs were infected by small parasitic flatworms, which burrow into the hindquarters of tadpoles and physically rearrange the cells that build the body.

I remember my horror when reports and photos of deformed frogs started appearing in the media. Like many people, I was sure it was caused by man, and due to the careless spread of chemical pollution. Amphibians breathe through their skin, and ingesting chemicals this way seemed to me the most logical answer.

If it wasn’t mankind’s overuse of chemicals, then surely it must have been increased radiation, also a problem to lay at our own door.

I should have given some thought to the delicate, interwoven web of life which has sustained the world for eons. Once again I am humbled by nature.


One response to “The Mystery of the Legless Frogs

  1. Mike August 15, 2009 at 9:31 am

    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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