24.11–24.12

Environmental contaminants can disrupt normal hormone functioning.

Potentially hazardous chemicals are found not just in dry cleaning agents but also in most printed receipts.

Figure 24-26Effects of DDT.

Figure 24-27Some consumer products contain chemicals that act as endocrine disruptors in wildlife.

Beginning in 1939 and continuing for more than three decades, the chemical DDT was used as a pesticide, with widespread agricultural use. It is extremely effective in killing a variety of insects, including mosquitoes, which also made it popular in strategies against malaria—a disease transmitted by mosquitoes. Unfortunately, after DDT killed the intended insect pests, it remained in the environment. Animals that consumed insects exposed to DDT ingested the chemical, which was stored in their body tissues. And when those animals were consumed by others, the DDT was passed on. Because larger animals ate greater numbers of animals carrying DDT, they stored more and more of the toxic chemical.

In predatory birds such as bald eagles, peregrine falcons, and pelicans, DDT disrupted the development and functioning of the reproductive tract and impaired the birds’ ability to produce normal eggshells—the shells would crack under the weight of the parent incubating the eggs (FIGURE 24-26). These problems led to serious population declines in many bird species and may have led to the extinction of some species.

The publication of Silent Spring by biologist Rachel Carson in 1962 called attention to the negative environmental effects of the widespread use of DDT. Carson’s book represented the beginning of the environmental movement in the United States. It incited such a public outcry that DDT was eventually banned in the United States—a ban that is cited as instrumental in the rebound of bald eagle populations.

DDT is a synthetic chemical called an endocrine disruptor. When endocrine disruptors are taken up by organisms, they can mimic, block, or otherwise interfere with hormones, leading to harmful effects. These effects are a consequence of the close chemical similarity between endocrine disruptors and hormones, particularly estrogen. Many endocrine disruptors can directly bind to the same receptors as estrogen.

There are several types of endocrine disruptors, in addition to DDT. These include PCBs (polychlorinated biphenyls), which are used as industrial coolants and lubricants; phthalates, commonly found in soft toys and cosmetics; and bisphenol A (BPA), found in many plastics, including the lining of food cans, plastic water bottles, and baby bottles (FIGURE 24-27).

Endocrine disruptors are found in thousands of other consumer products, too, and have been detected in numerous natural habitats, sometimes carried in runoff water from industrial manufacturing processes or as airborne pollutants. Exposure to endocrine disruptors has been implicated in a variety of adverse physiological effects, often related to the chemicals’ feminizing effects, which has led to some endocrine disruptors being referred to as “gender-bending chemicals.” Many animal groups seem to be adversely affected by endocrine disruptors.

• 1. Mammals. Populations of Baltic ringed seals have declined significantly over the past 100 years. This is due, in part, to the presence of large concentrations of pollutants, including PCBs and DDT, in their habitats, which has interfered with female reproductive functioning in many ways, leading to partial or complete sterility in 70% of the animals.
• 2. Fishes. In a variety of fish species, including carp, rainbow trout, and flounder, exposure to endocrine disruptors in sewage runoff is known to interfere with reproductive functioning. In some cases, the exposure causes males to produce egg proteins that are typically found only in females.
• 3. Invertebrates. A group of chemicals called tributyltin compounds, or TBTs, are used to reduce the growth of organisms on the hulls of ships. TBTs act as endocrine disruptors in several marine species. Oysters, for example, when exposed to TBTs, produce defective shells, and in numerous other marine invertebrates, females’ genitals become masculinized and the animals have reduced fertility. These impacts have led to a worldwide decline in populations of gastropods such as sea snails and limpets.

Large numbers of synthetic and natural chemicals have endocrine-disrupting functions in both natural and laboratory populations of animals. It remains controversial, however, whether these chemicals cause health problems in humans. Many studies are under way—so far, with conflicting or inconsistent results—and the U.S. government has taken many steps to restrict the use of the endocrine disruptors described here.