In 1978, during a routine ecological assessment of several British waterways, wildlife biologists discovered an unusually high number of abnormal fish living downstream of two sewage-treatment plants. The fish were considered intersexual because their gonads contained both ovarian and testicular tissue. Nearly 2 decades later, after the development of more-sensitive analytical techniques, researchers provided an explanation. They traced the animals' reproductive problems to low concentrations of estrogens, known as the female-sex hormones, that had entered the environment in waters released by the sewage plants.
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