DON’T LEAP TO CONCLUSIONS
THE CASE OF THE MALFORMED FROGS
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In 1995, students were taking a field trip to a wetland near Henderson, Minnesota. To
their surprise, they found many malformed frogs. Some frogs had missing or extra legs.
Others had malformed eyes as well as other malformed body parts. This information was
reported and not long after this discovery near Henderson, malformed frogs were turning
up in other places in the Minnesota River Valley and in less than a year, there were
reports about this phenomena all over the state and even in other states.
Scientists were very alarmed by this problem since frogs are highly sensitive to pollutants
in the environment because frogs breathe through their skin and inhabit both the land and
water. Scientists studied a number of possible causes they hypothesized might be related
to this problem. Some scientists wondered if increased UV radiation from the sun due to
CFCs and a thinning ozone could be responsible for this problem. Other scientists
thought pesticides might the problem. Methoprene, a chemical that is used to control
insects, was dissolved in water and resulted in malformed frogs compared to a control
group. Still others thought it could be caused by predation or some kind of a parasite or
virus. Use the research on frog malformation and write answers to the following
1. Write an If…then hypothesis statement for this experiment.
2. In 100-150 words, describe the experiment(s) conducted?
3. List 2-3 of the most compelling finding of this research.
4. What was the conclusion of this research? Indicate acceptance or rejection of the
5. How could changes in an environment affect the survival of a species?
6. Write a hypothesis to examine if this could be a problem related to genetically
inherited traits. Describe a simple experiment to test this hypothesis.
The widespread occurrence of "deformities" (or "malformations") in natural populations of amphibians, especially anurans, has recently been perceived as a major environmental issue (Tietge et al., 1996; also see Links to other web pages). The majority of observed abnormalities are frogs with missing limbs or parts of limbs, or with one to several partial or complete extra limbs. Similar kinds of deformities have been found in the past ( past research ), but renewed attention has been focused on them since 1996 when a group of school kids found some deformed frogs in Minnesota and broadcast their findings on the World Wide Web with the help of the Minnesota Pollution Control Agency (Helgen et al., 1998). Reports of these deformities are geographically widespread across the United States and Canada ( Narcam), and are thought to be linked with the general problem of amphibian decline. There is current concern that recent reports reflect a sudden increase in the incidence of deformities in natural populations of amphibians, possibly indicating an environmental problem of possible risk to other organisms, including humans (Ouellet et al., 1997; Gardiner and Hoppe, 1999). Leading hypotheses to explain these deformities are predation, parasitism, UV-B radiation, and chemical pollution.
One possible hypothesis to explain frog deformities is that the deformities are caused by
predators or by cannibalistic acts of the tadpoles themselves. If this happens during the
early stages of tadpole development, then it is likely that the missing limb will regenerate
and one would never know that it was missing at all. On the other hand, when a tadpole
begins morphing into a frog, there is a decline in their ability to regenerate a limb and if a
predator successfully bites off a limb after or during this decline, then the limb may not
regenerate at all and all that will remain of the missing limb after metamorphosis is a
little growth, a spike of cartilage.
Scientists have observed captive tadpoles in a fish tank with stickleback fish. They
observed the stickleback fish nipping at developing tadpoles limbs. According to the
scientist’s observations, the stickleback fish do indeed bite off the limbs of toad tadpoles
and also of Woodfrog larvae.
In order to test the predation hypothesis, Dr. Sessions and his team of science researchers
set up an experiment. First they collected tadpoles from places where frogs with missing
limbs were found. They predicted that if predation were the cause of the missing limbs,
then they would find tadpoles in different stages of development with evidence that their
limbs had been attacked.
A left hind limb showing a cartilaginous
spike The presence of the spike indicates
that the limb was lost after it had
A deformed bullfrog caught at a confirmed
location where deformed frogs were found.
A tadpole missing both hindlimbs.
A series of Bullfrog tadpoles that were
raised in the lab, showing the kind of
damage that they can do to each other. These tadpoles have sharp mouth
parts that they can use to inflect damage
on each other.
Other researchers hypothesized that frog deformities are caused by parasites. For example, trematodes are small flat worms which parasitize aquatic birds. They have a very complex life cycle that is shown below.
As shown in the life cycle diagram above, aquatic birds acquire the trematodes by eating infected tadpoles. Adult trematodes use the bird to lay their eggs and then the eggs of the trematode are passed from the bird and grow into an organism called a miracidium. The miracidium infects snail as that is the organism it uses in order to develop and grow into the next stage which is called cercaria. Eventually, the cercaria exit the snail and the next host in their life cycle is the tadpole where the cercaria form metacercarial cysts.
In order to test the parasite hypothesis, Dr. Johnson and his research team designed lab
experiments in which tadpoles were exposed to a level of trematode infestation similar to
what was found in areas where deformed frogs were found. They discovered that the
cercaria targeted the hind limbs of the tadpoles and the more cercaria they added to the
experimental habitat, the more severe the deformities were that appeared in the frogs.
The following figures show the types of deformities that were observed in the frog limbs.
Note how the single frog limb
has two sets of digits and
attached bones. In this
diagram and photo, the
duplication runs from digit 5
to 1 and then continues from 1
to 5. This is called a posterior
double mirror image
The photo to the left shows a
different orientation for a
single frog limb. In this
diagram and photo, the
duplication runs from digit 1
to 5 and then continues from 5
to 1. This is called double
anterior mirror image
In some frog limbs, a mirror
image triplication has been
Parasite induced deformities are characterized by large numbers of metamorphosing or newly metamorphosed froglets which show a wide range of effects, including skin fusions, extra legs and toes, and missing limbs that are all found associated with these parasites. Dr. Sessions and Ruth found large cohorts of young frogs, where 40- 70% were infected and deformed. This is very different from fully metamorphosed frogs that simply have missing limbs which are usually found individually, not in large groups, and show no evidence of trematode/cyst involvement.
It is well known that there is a problem with ozone depletion leading to increased levels of harmful ultraviolet irradiation on different parts of the globe. Some scientists feel that this irradiation may be sufficient to cause major developmental changes in amphibians (who have weak defenses against this type of attack), leading to some of the deformities seen in the wild. While scientists who support this hypothesis feel that UV-B is not responsible for the multi-legged amphibians found, they do believe that UV can be a cause for many of the other deformities that are being found, especially limbless frogs. It has been known for many years that UV irradiation (at high levels) can prevent limb development and regeneration in amphibians.
It has been shown that UV-irradiation in the lab can cause limb deformities such as missing limbs and limb parts. Field experiments show that UV can cause abnormal development and death in early embryos, but the effects of ambient UV-B on limb development in older larvae in the field have not been shown to cause the kinds of deformities that are being reported in natural populations of amphibians. Furthermore, the effects of UV exposure on developing limbs are predicted to affect both sides of the body equally, but this is not what is seen in field-caught deformed frogs.
Frogs with missing limbs have been the most difficult deformity to explain so far, partly because examination of adult specimens has revealed few clues. For example, most of the limbless frogs that we have examined do NOT show a close relationship between deformities and trematode cysts. Nevertheless, there is one important piece of evidence in adult deformed limbless frogs that all but eliminates UV (or chemical pollution) as a cause for cartilaginous spikes.
Notice a cartilaginous (regenerative) spike
on the left hind limb. The presence of the
spike indicates that the limb was lost after
it had developed, and is a normal
regenerative response for a frog. Note that
this was an otherwise very healthy looking
frog, and that the right hind limb is normal
in every respect.
Cartilaginous spikes as shown in the picture above are a normal regenerative response of
limb amputation in an older tadpole or frog with a diminished ability to regenerate
limbs. Such spikes are inhibited by both UV and retinoids which are important to
embryonic development. Excess retinoids have been shown to cause birth defects in
humans. Thus, if an adult limbless frog exhibits a spike, it tells us at least two important
1. the limb was lost after it had developed (i.e. it was amputated by something and was
not a birth defect or malformation), and
2. there was no harmful UV or chemical pollution around to prevent the regeneration of
a cartilaginous spike.
It is important to realize, however, that the opposite is not true: cartilaginous spikes only
form in a fraction of cases under normal circumstances, so the absence of a spike tells you nothing! It is only the presence of a cartilaginous spike that provides a useful
clue. Sorry, but that's just the way it is. Cartilaginous spikes are consistent with traumatic
loss of limb such as would occur with predation or cannibalism in which the limb was
damaged or cut off. We now have evidence that this is what is causing at least some of
the deformed amphibians with missing limbs.