Furthermore, where wildlife species are known to be experiencing declines in population associated with abnormalities, experiments should be designed to study the phenomena with respect to chemical contamination. For people, the recommendation is for additional studies to document the presence or absence of associations between HAAs and human cancers. When associations are discovered, the causality is investigated in the relationship between exposure and disease, and indicators of susceptibility to disease of certain groups of people by age and sex. The story of wild leopard frogs from a variety of areas in the midwestern United States sounds something like a science-fiction horror story. In affected areas, between 10 and 92% of male frogs exhibit gonadal abnormalities, including retarded development and hermaphroditism, meaning they have both male and female reproductive organs.
Other frogs have vocal sacs with retarded growth. Since their vocal sacs are used to attract female frogs, these frogs are less likely to mate. What is apparently causing some of the changes in male frogs is exposure to atrazine, the most widely used herbicide in the United States today. The chemical is a weed killer, used primarily in agricultural areas. The region of the United States with the highest frequency (92%) of sex reversal of male frogs is in Wyoming, along the North Platte River.
Although the region is not near any large agricultural activity, and the use of atrazine there is not particularly significant, hermaphrodite frogs are common there because the North Platte River flows from areas in Colorado where atrazine is commonly used. The amount of atrazine released into the environment of the United States is estimated at approximately 7.3 million kg (16 million lbs.) per year.