The precise chemicals have not yet been identified, and could be either natural or manmade. Bacteria, viruses, ultraviolet radiation, pesticides, toxic metals, leg-munching predators or some combination of these factors are among the proposed culprits. In the case of one Sierra Nevada pond, parasites called trematodes were causing multiple limbs to form. This parasitic flatworm creates cysts in a frog's tissue, forming a physical barrier to limb growth. Developing limbs will split in two around the cysts so that two legs form where one should be.
Plastic beads embedded at the site of a developing limb bud can have the same effect.
However, parasites and other infections have been ruled out as the cause of deformities in Minnesota. Experts think of frogs as sentinel species, and are concerned about what the abnormalities might mean for humans. It is logical that frogs would be among the first species to feel the effects of chemicals in the water. From eggs floating on the surface of a pond, to wriggling tadpoles, to adults, frogs spend their lives immersed in water. These animals easily absorb pollutants through their skin, and are highly sensitive to hormones and other chemicals that disrupt metamorphosis.