Explaining frog deformities

Causes of deformities in frogs

source: Andrew Blaustein and Peter Johnson
Scientific American vol 288 no 2 February 2003
starts p48, 6 pages long

Amphibians throughout the US have been affected by deformities, noted in over 60 species, and 46 US states since 1995. Elsewhere in the world, missing and extra legs and other deformities have been noted. There appears to have been an increase in these deformities, and one suspect is an excess of ultraviolet radiation, leading to genetic mutations and damaged immune systems. Some researchers have found a link between missing legs and ultraviolet radiation, but this fails to explain extra legs, and why animals are affected when they are nocturnal, or hide from the sun in shade and mud.

Pesticides may also be a contributory factor, but again, they do not appear to be the only cause of deformities. Trematode parasites have, however, been linked to the development of extra limbs. Amphibians can become infected with Ribeiroia ondatrae parasites, which spend part of their lives in snails, which can infect frogs. Affected tadpoles do not develop properly, and can be more easily caught by herons and other birds. The parasites mature within the bird, and are excreted, and then infect snails.

An increase in infections by parasites could, then, explain why deformities have become more common, and increased fertilizer run-off from farmed land into wetland may be the reason why infections of amphibians with this parasite have become more common. The run-off promotes the growth of algae, which can, in turn, boost the population of snails. Amphibians may also become more vulnerable to the parasites because of excess radiation, or pollution.