It was just another day in southern Minnesota in the summer of 1995 when a group of middle school students went to collect frogs around a small man-made pond. As they gathered the frogs, people noticed with growing concern that a large proportion of the frogs in this pond were malformed. There were frogs with extra legs, some with too few. Others had legs in irregular places, while others had unusual numbers of eyes. News of this spread quickly. As people began looking broadly for similar trends, it became clear that this phenomenon was not limited to Minnesota, but also across the United States and into Canada. The proportion of malformed frogs had increased compared to frog data from the early sixties. As Minnesota was the first place citizens observed the malformed frogs, it became the central focus for research into what was going on.
Frogs are often considered “sentinel species”; because their porous skin absorbs whatever is in their environment they are especially sensitive to changes in their environment. Thus, frogs can inform people of potential hazards before they affect the broader ecosystem and have garnered much attention as a source of research. Researchers from a number of colleges and agencies worked on determining the causes of these mutations with much of the research focused on the northern leopard frog, the most common frog species in Minnesota.
Thanks to this research, there are now a number plausible theories to explain the malformed frogs. The first is that the parasite Ribeiroia ondatrae, a flatworm that hatches out of snails and then infects tadpoles, can cause malformed hind legs in frogs. However, some ponds without this parasite still have malformed frogs, so it cannot be the whole picture. Many different chemical pollutants have also been implicated in the deformation of the frogs, fertilizer run-off and pesticides are two such pollutants. The Earth is being exposed to increasing amounts of ultraviolet light, and this has also been found to cause malformations. Most likely, it is a combination of all of these factors, along with more of which we are not yet aware, causing the increase in malformed frogs. In 2001, funding for research on the explanation behind mutant frogs was stopped, so little new information is available. Nonetheless, people have found that as lakes and ponds are cleaned, the number of malformed frogs decreases. So, even without a concrete understanding of the causes, there is much that can be done to help protect the frog’s environment and well-being. In one such well-protected place, Kettle Hole Marsh in the Arboretum, you can now hear the calls of the wood frog and the chorus frog, hopefully present with only their four legs.