In addition to the toxic effects of pesticides and other environmental contaminants, some pollutants act like hormone mimics, causing disruption of the endocrine systems of fish and wildlife, and potentially, humans who might be exposed. These endocrine effects occur because the “foreign” chemicals bind to receptor molecules in hormone sensitive “target” cells (cells which respond to hormones, and produce effects related to the hormone action), and result in an abnormal response. Hormone disrupters are most injurious to developing embryos and to animals exposed during sensitive stages of the life cycle, such as during the breeding season.

Several examples of endocrine disruption in wildlife have been documented, including abnormal thyroid function in fish and birds exposed to PCBs and dioxins in the Great Lakes, testicular feminization of gulls exposed to DDT in their eggs, feminization of male fishes living downstream from municipal waste water discharge facilities, and male alligators with abnormally small penises living in a Florida lake contaminated with a variety of agricultural chemicals. A recent review by Colborn, et al. (1993) summarizes the information well.

Several pesticides, some industrial chemicals, and some natural plant compounds have estrogenic properties, that is they mimic the action of female steroid sex hormones. One chemical form of DDT, the “orthopara” isomer (designated o,p′-DDT) acts like an estrogen when it bioaccumulates in bird eggs, and causes male embryos to develop testes with partly ovarian characters, and to retain oviducts as in female birds. In southern California, the gull colonies exposed to DDT suffered population losses and an excess of breeding females, because feminized males did not reproduce, and did not even return to the coloy.

Fish in some British rivers have been exposed to chemical effluents from municipal waste water treatment plants. Estrogenic alkyl-phenols (a group of industrial surfactants common in many household products) in the treated water are absorbed by the male fish, and the contaminants stimulate the male's livers to synthesize yolk proteins as if the male fish were going to lay eggs. This is a very recently discovered effect (White et al. 1994), and it is not yet known how common these pollutants may be. The effects, however, have prompted regulatory agencies to quickly begin to develop methods to screen for hormone disruptors, and to begin reassessing the regulations for testing chemicals and regulating waste discharge.