Frog legs

Missing Limb. American Bullfrog, Fairfield County, Ohio, July 16, 2001. Brandon Ballangee © 2009 greenmuseum.orgAnother mystery of science has been solved, which gives me hope that the origin of CCD is only a little research away, or perhaps just a flash of inspiration. Strangely, the only coverage of this discovery that I could find is at the BBC Legless frog mystery solved, even though mysteriously deformed frogs have been found all over the world. Since the late 1980s, frogs have been found with missing legs, additional legs, and other deformities (actually, hideously deformed frogs have been found much earlier than that, but the numbers seemed to increase in the late 1980s). Various causes were suggested, but most people were convinced that the cause was chemical in nature.

Amphibians, due to their moist porous skin, are particularly good at absorbing chemicals like pesticides from the environment, so the theory made sense even though there wasn’t any specific proof. Of course, environmentalists rallied around the deformed frogs as an excuse to demonize agriculture. In 1990, the chemical hypothesis was at least partially disproven. An ingenious yet simple experiment by Stan Sessions showed that the presence of small foreign bodies (whether resin beads or cysts caused by a parasite) could cause the growth of extra limbs. The most likely scenario is that over-fertilization of agricultural fields in the late 1980s led to fertilizer running into streams and wetlands. The extra nutrients contributed to larger than typical amounts of algae, on which grazed larger than typical numbers of snails. Those snails are the primary host of trematodes, some species of which carry out part of their life cycle in frogs, and those trematodes caused the deformities. In 2008, Jason Rohr (with Sessions and others) showed that four common pesticides play a role as well. Each seemed to have a negative effect on trematode larvae (which should benefit frogs) but also caused tadpoles to be more susceptible to infection, even though the pesticides (tested at maximum expected exposure levels) did not affect mortality directly.

So, the environmentalists were partially right – agriculture and pest control are partially to blame, although not in the way that was initially assumed. Happily, over-fertilization is less common today (at least in the US) – fertilizer application rates now more closely match what the plants actually need (partially because fertilizer costs have been increasing), and some pesticides are being replaced with genetically engineered crops (hopefully this will be improved upon in the future). Studies on the relationships between human activities and ecosystems strengthen the need for more research into alternatives in agriculture and pest control.

Still, the problem of the missing limbs was not resolved, and Sessions continued his research on amphibians. In the meantime, “ecological artist” Brandon Ballengee was observing amphibians in the wild for his art. Ballangee collaborates with scientists, working with them in field research to create art that engages the public in environmental issues. He became interested in the deformed limbs and wondered if they might be due to predators.

Ballangee and Sessions worked together to test this hypothesis by placing various predators in cups with tadpoles. Only dragonfly nymphs took the bait, biting off the hind legs of developing frogs (the developing front limbs are protected by the tadpoles’s gill chamber). The cause of frog deformities therefore, is natural, though man-made chemicals aren’t helping. I expect that the plight of the bees will be similar – natural causes, exacerbated by things like miticides in the hives.

ResearchBlogging.orgSessions, S., & Ruth, S. (1990). Explanation for naturally occurring supernumerary limbs in amphibians Journal of Experimental Zoology, 254 (1), 38-47 DOI: 10.1002/jez.1402540107

Rohr, J., Raffel, T., Sessions, S., & Hudson, P. (2008). Understanding the net effects of pesticides on amphibian trematode infections Ecological Applications, 18 (7), 1743-1753 DOI: 10.1890/07-1429.1

Written by Anastasia Bodnar

Anastasia Bodnar is a science communicator and science policy expert with a PhD in plant genetics and sustainable agriculture from Iowa State University. Anastasia has had various risk analysis roles in US government and military service. She serves as BFI's Director of Policy and as Co-Executive Editor of the Biofortified Blog.


  1. I am working with nitrifying bacteria as commensal organisms that live on the surface of eukaryotes and set the basal NO/NOx level from ammonia diffusing out through the surface.

    These bacteria are pretty strongly inhibited by the triazine herbicides. I think there is a threshold effect, where if the inhibition reaches a certain level, then the biofilm on the surface depopulates (NO is a quorum sensing compound, and low NO triggers the transition from sessile to planktonic). Because NO is the normal regulatory molecule for steroid synthesis, a change in the basal NO level from the loss of the biofilm will cause endocrine disruption (not might, will).

    The NO/NOx on the surface also prevents surface infections, including bacterial and fungal.

  2. Hm, this is very interesting and very out of my field, so let me try to rephrase and please tell me if I am understanding:

    The bacteria that live on the outside of eukaryotes form biofilms that regulate the eukaryote’s hormones by controlling the rate of off-gassing of ammonia from the skin. When these bacteria are exposed to triazine herbicides, the biofilms breakup, causing endocrine disruption in the eukaryote.

    This sounds like just another reason to stop using atrazine, and so far it seems that RoundUp Ready crops are helping to do that. Now, we need to change the formulation of RoundUp to include less toxic surfactants. We also need to come up with other types of herbicides that can be paired with a resistance gene that can be rotated with other herbicides so weed resistance will be less likely to develop.

  3. Anastasia,

    When you hear the phrase, “endocrine disruption”, it’s time to worry. The scientist who first invented the notion that disruptors were exhaled from plastic was found guilty of fraud and barred from performing government research.

    That has not prevented others from trying to make “endocrine disruptor” claims, most of them non-scientists, or scientists safely esconced behind the barricades of this or that activist group.

    Atrazine has repeatedly been absolved of any role in the appearance of malformed frogs, but that does not deter the opponents of herbicides. The latest scandal involved the claim that Atrazine in *high* concentrations did not harm frogs, but had “significant” effects in *low* concentrations. That is, of course, an appeal to the homeopathic principle, according to which a substance becomes *more* powerful, the *more* it becomes diluted.

    Which has long ago been debunked, as well, but debunkings are no deterrent to the determined.

    The actual culprit for frog deformities is chytrid fungus. See,

    The trouble with all the other theories is that the geographic distribution deformed frogs bears little relation to agriculture, much less, the prevalence of the use of Atrazine, while the causal agent has conclusively been identified.

    Finally, beware of the tendency to add “balance” to reporting on agricultural issues. The lure of saying bad things, to balance out the saying of good things, can result in lending credibility to the incredible — or worse, painting a false picture.


  4. Andy, be careful, there can be endocrine disruption via pathways that are unknown and so are not considered. I agree that endocrine disruption from xenobiotic steroid mimetics is unlikely because endogenous substrates have orders of magnitude higher binding and orders of magnitude higher concentrations.

    What is the mechanism by which giving antibiotics to farm animals causes endocrine disruption such that they grow bigger, mature sooner and convert feed to biomass more efficiently? All of those myriad developmental pathways are under the control of the endocrine system(s). To perturb them substantially, there must be “endocrine disruption”.

    My research suggests that antibiotics in animal feed perturb the NO/NOx status of the organism, and that perturbs steroid synthesis by the cytochrome P450 pathways in the same direction that stress does. There is no “threshold” for this type of endocrine disruption because the NO/NOx status is already regulating the steroid synthesis pathways.

    These bacterial biofilms are also important in suppressing surface infections. Their disruption by atrazine would be expected to cause surface infections too. That disruption can occur at levels that are not toxic. I suspect that atrazine is one of the reasons behind what is called lobster shell disease.

  5. Daedalus,

    I’ll answer your points in order.

    The effects via unknown pathways are not detectable by any method known to science, because unknowns cannot be tested. The vast majority of unknown pathways will remain unknown forever, because the vast majority of them simply do not exist.

    The mechanism by which giving antibiotics to farm animals causes them to grow larger, faster, is that the antibiotics select in favor of the digestive flora in the gut which convert feed more efficiently. More efficient flora = better feed conversion = faster weight gain.

    Disruption of bacterial biofilms is crucial to the health of higher organisms. Lower populations of unicellular organisms can be beneficial, such as in the digestion of food. At higher populations, at the point where the unicellular organisms form a biofilm on, or in, the body or plant, the cells in the biofilm actually begin cooperating to ensure the biofilm’s survival. By up-or downregulating specific gene sequences, they engage in specialized functions within the biofilm–making the infection far more devastating. Since bacteria, even at this level of organization, are fundamentally stupid, this cooperation–which means competing with the host organism, or even digesting its tissues, can very quickly lead to the death of the host organism.

    Basically, a plant or animal with a biofilm in or on it is either dead, or dying.

    There are beneficial biofilms, such as in industrial processes involving bioreactors, or in the environment, where they perform what are loosely called “ecosystem services”.

  6. Sorry Andy, you really don’t know what you are talking about.

    Unknown mechanisms can be tested. After they are tested they are not unknown. Every pathway that is now known was at one time unknown.

    The mechanism(s) by which antibiotics increase growth rate, accelerate sexual maturity and increase feed efficiency remain unknown.

    Your hypothesis that it is due to selection for more efficient gut flora is non-physiologic. Why would more efficient gut flora not result in greater bacterial growth? In any case farm animals are fed ad lib, they can always eat more to increase growth, so food is not the limiting factor in growth rate. Animals can increase food consumption rate and increase conversion to biomass (as during lactation), so digestion capacity is not limiting. How does that affect the rate of sexual maturity? Only via endocrine disruption.

    Only biofilms containing pathogens can cause disease. A biofilm that does not contain pathogens cannot cause disease. There are no reports of an “infection” being caused by ammonia oxidizing bacteria. Ammonia oxidizing bacteria do form biofilms on the surfaces of organisms such as lobsters. They form biofilms on other organisms too.

  7. Daedalus,

    Unknown pathways cannot be detected by any means comprehensible to human science. Suspected pathways can be tested, but that requires an hypothesis as a starting-point. That is, a starting-point from some existing knowlege.

    Perversely for you, but obvious to scientists, is that the only way “unknown mechanisms” can be tested is via the “unknown test”. Perhaps, some day, someone will develop a comprehensive test to determine the effects which ensue via unknown causalities.

    Personally, I view that possibility as so impossible as to render further speculation as far beyond the scope of the genre known as science fiction. To be kind, I will say, your suggestion is so far beyond any credible conception of human science that it must relegated to the domain of nonsense — until, of course, you reveal your unknown test, which reliably detects the unknown pathways which lead to certain results.

    What I say about gut flora is not an “hypothesis”. It works, and that is why the antibiotics are used in animal feed. You might envision the availability of food to farm animals as the “limiting factor”, but the fact is, what’s eaten is only relevant to the extent that the nutrition is *bioavailable*. If gut bacteria are consuming nutrition to their own advantage, thereby depriving the animal, you don’t want those critters in your gut.

    Or, in the gut of the cattle you feed in order to feed… and, so forth.

    As for your weak defense of biofilms, the best you can say is that some multicellular organisms, such as lobsters, manage to keep them at bay.

    Many thanks. I, personally, and on behalf of many other friendly organisms, are thankful that we have developed internal defenses against what humans describe as slime.


  8. From what I’ve been told the most likely cause of Colony Collapse Disorder is a virus called the Israeli Acute Paralysis Virus. It’s a virus that slowly paralyzes bees and it appears to be vectored by Varroa destructor, the mite that causes problems in honeybee colonies. The symptoms seem to fit the condition, and there’s a very high correlation between CCD and detected IAPV.

    HOWEVER…it has not been conclusively proven that IAPV is the cause of CCD or if there are multiple factors working together. We’re still in the ‘WTF is going on’ stage.

    Since you’re at ISU, I’d recommend talking to Mary Harris in the Entomology department because she studies bees. Not honeybees, but she knows her field very well and could point you in the right direction. You can contact me through my blog and I’ll give you more info.

    Not to be a shameless self-promoter, but I kind of hit on IAPV in a recent post on beekeeping. I linked to a paper with more information about it.

    Harris will bee able to give you more information.


  9. Cheshire,

    CCD is a modern artefact of mass media, but has actually been a crucial, critical, real-world problem that can be dated back to at least 1996 (in the US), if not earlier.

    Bee-keepers, and the crops and farmers who depend upon them, are so desperate for a working theory of CCD that they’re now proceeding on the basis of experimental banning.

    That is to say, ban this, ban that, see if that works. This has been praised as the “Precautionary Principle” at work, but fact is, even this approach hasn’t done any good.

    Cheshire, if you have a novel theory of CCD that deserves a real test–and the world needs it–please describe it and I will post it at GMObelus as a world challenge.


  10. Something else out there may be killing the frogs…

    “Noise from a local building site is being blamed for the death of around 270,000 American frogs at a nearby farm in south China, the China Daily reported on Tuesday.”

    “According to the farm owner, Fu Yishin, the round-the-clock construction work killed his frogs one after another as every time there was a loud noise the frogs jumped.”

  11. Thanks, Cheshire. As far as I know, CCD has multiple causes which I too have covered in other posts… I just wanted to make the connection between the previously unknown cause of frog leg problems and previously unknown cause of CCD, and make the point that the causes are not at all mysterious or scary as some would have us believe.

  12. 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.

  13. That’s a good point mike. A lot of research only holds true in certain contexts. I hope they’re working on some field work to test their hypothesis.

Comments are closed.