A lot of what we understand about human development comes from some very early studies on the development of frog eggs. Many frog species are now extinct, Maxson says. They are disappearing because of habitat degradation. That's particularly true in South America and some places in Europe, Asia, and Australia, where humans are expanding, paving things over for homes, or removing forests for agricultural and lumbering purposes. However, the frogs closer to home, while perhaps less in danger of eradication, can serve as warning signals of imminent environmental problems, Maxson says.
Most species start out as aquatic tadpoles and metamorphose into land-living animals.
As tadpoles in an aquatic system, they are very sensitive to pollutants. So a number of my colleagues use tadpoles as monitors of water pollution. In the frog world, males are the vocal sex, calling to attract females, says Dr. Linda Maxson. What's more, the male frog is a single-minded swain, Maxson says. Most species have an external fertilization mode known as amplexus.