In this video segment adapted from FRONTLINE: “Fooling with Nature,” learn how Theo Colborn, then a scientist with the World Wildlife Fund, noticed a disturbing trend among animal populations in the Great Lakes. Colborn and other researchers, including Jim Ludwig, identified problems, from population decline to birth defects, which are caused by disruptions to the animals' endocrine systems. Certain chemicals, called "endocrine disruptors," alter embryonic growth by mimicking or blocking the body's hormones. At what point is there enough scientific proof to take action, and what action should public health officials take?