In biology, a malformation is an irregular or abnormal structural feature of a living thing. Most often, these structural anomalies compromise function and reduce the chances of survival. In general, they represent errors that occur very early on in development. Although the causes of about half of all malformations remain unknown, some may be traced to genetic abnormalities. Still others are caused by exposure to environmental factors.

Indicator species are certain living things that provide scientists with early warning signs that a change is affecting the health of the environment. Frogs, like those featured in this video, are extremely sensitive to environmental variability. Because frogs live both on land and in water, their highly permeable skin and soft eggs make them more susceptible than most other animals to their physical surroundings. For example, a change in the environment early in a frog’s life cycle—when it’s still a tadpole—can alter its genes, causing malformations made visible after the tadpole has metamorphosed into a frog. Malformations in an indicator species signal ecosystem disruption that may extend to other kinds of organisms that share the same ecosystem, including humans.

The range of malformations exhibited in frogs includes missing limbs, extra limbs, partial limbs, twisted limbs, webbed skin, malformed jaws, and missing eyes. Some of these malformations are symmetrical, meaning they appear on both sides of an affected frog; others are asymmetrical, appearing on just one side. Identifying what is abnormal about a particular frog can provide clues as to when, during development, its malformation commenced.

Scientists and public health officials in many countries have conducted extensive research into malformations. Three environmental factors considered responsible for malformations are ultraviolet (UV) light from the Sun, chemical contaminants, and parasites. Research supports the notion that UV light may be sufficient to cause major developmental changes in amphibians. Certain chemical pollutants found in rainwater, lakes, and other water sources can interfere with natural hormones that control development, growth, and behavior of organisms and lead to malformations. Many frog malformations appear to be caused by a type of flatworm that develops inside aquatic snails before burrowing into tadpoles. Although parasite infections have been definitively linked in laboratory studies to some types of malformations, such as extra limbs, recent studies have shown that parasites do not produce other types, such as missing limbs.

While periodic field counts of an animal population can clearly demonstrate a change in the occurrence of malformations, pinpointing the cause or causes responsible for them is a more challenging task. Complicating matters are the results of previous studies that show that the frequency of frog malformations may peak at different times of the year at different study sites, and that the types of malformations found often differ from site to site. This suggests that malformations are caused by more than one factor and that those factors may vary from one region to the next.

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