|LEGLESS FROGS MYSTERY SOLVED|
Matt Walker, Earth News
Scientists think they have resolved one of the most controversial environmental issues of
the past decade: the curious case of the missing frog legs. Around the world, frogs are
found with missing or misshaped limbs, a striking deformity that many researchers believe
is caused by chemical pollution. However, tests on frogs and toads have revealed a more
natural, benign cause. The deformed frogs are actually victims of the predatory habits of
dragonfly nymphs, which eat the legs of tadpoles.
In the late 1980s and early 1990s, researchers started getting reports of numerous wild
frogs or toads being found with extra legs or arms, or with limbs that were partly formed
or missing completely. The cause of these deformities soon became a hotly contested
issue. Some researchers believed they might be caused naturally by predators or parasites.
Others thought that was highly unlikely, fearing that chemical pollution, or UV-B radiation
caused by the thinning of the ozone layer, was triggering the deformations.
Deformed frogs became one of the most contentious environmental issues of all time, with
the parasite researchers on one side, and the 'chemical company' as I call them, on the
other," says Sessions, an amphibian specialist and professor of biology at Hartwick
College, in Oneonta, New York. "There was a veritable media firestorm, with millions of
dollars of grant money at stake."
After a long period of research, Sessions and other researchers established that many
amphibians with extra limbs were actually infected by small parasitic flatworms called
Riberoria trematodes. These creatures burrow into the hindquarters of tadpoles where
they physically rearrange the limb bud cells and thereby interfere with limb development.
"But that was not end of the story," says Sessions. "Frogs with extra limbs may have been
the most dramatic-looking deformities, but they are by far the least common deformities
found," he explains. "The most commonly found deformities are frogs or toads found with
missing or truncated limbs, and although parasites occasionally cause limblessness in a
frog, these deformities are almost never associated with the trematode species known to
cause extra limbs."
The mystery of what causes frogs to have missing or deformed limbs remained unsolved
until Sessions teamed up with colleague Brandon Ballengee of the University of Plymouth,
UK. They report their findings in the Journal of Experimental Zoology Part B: Molecular and
Developmental Evolution. For a decade, Ballengee and Sessions have collaborated on a
series of art and science projects that image amphibians' bodies to show the detail within,
the most recent of which is funded by the Arts Catalyst organization, based in London. 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
Remarkably, many tadpoles survive this ordeal. "Often the tadpole is released and is able
to swim away to live for another day," says Sessions. "If it survives it metamorphoses into
a toad with missing or deformed hind limbs, depending on the developmental stage of the
If tadpoles are attacked when they are very young, they can often regenerate their leg
completely, but this ability diminishes, as they grow older. The researchers confirmed this
by surgically removing the hind limbs of some tadpoles and watching them grow. These
tadpoles developed in an identical way to those whose limbs had been removed by
dragonflies, confirming that losing a limb at a certain stage of a tadpole's development
can lead to missing or deformed limbs in adulthood. Adult amphibians with one hind limb
appear able to live for quite a long time, Sessions says, explaining why so many deformed
frogs and toads are discovered.
Why do the dragonflies like to eat the hind legs only? As toad tadpoles mature, they
develop poison glands in their skin much earlier than those in their hind legs, which could
make the hind legs a far more palatable meal. The front legs of tadpoles also develop
within the gill chamber, where they are protected.
Sessions is careful to say that he doesn't completely rule out chemicals as the cause of
some missing limbs. But 'selective predation' by dragonfly nymphs is now by far the
leading explanation, he says. "Are parasites sufficient to cause extra limbs?" he asks. "Yes.
Is selective predation by dragonfly nymphs sufficient to cause loss or reduction of limbs.
Yes. Are chemical pollutants necessary to understand either of these phenomena? No."