Frog legs

posted in: Science | 14

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

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Anastasia is Policy Director of Biology Fortified, Inc. and the Co-Executive Editor of the Biofortified Blog. She has a PhD in genetics with a minor in sustainable agriculture from Iowa State University. Her favorite produce is artichokes!