Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 29, Number 1;   January, 2002

Where have all the frogs gone?.

by Heather Brown

These are the kinds of questions that some researchers are beginning to ask. In recent years, a number of scientists have begun to notice that amphibians are declining at an alarming rate worldwide. This rapid decline is particularly disturbing because many of the affected species live in relatively pristine habitats, such as nature reserves, and are presumably removed from most human activity. Although the specific causes of amphibian declines are still unclear, some scientists are beginning to think that environmental chemicals, such as pesticides and industrial waste products, may be contributing to the declines by interfering with the actions of hormones in the amphibian's reproductive system.

Hormones are tiny chemical substances that circulate through the bloodstream and interact with specific targeted organs throughout the body. They are extremely important because they influence many aspects of behavior and development. The most widely popularized of the different types of hormones are the sex hormones which directly and indirectly influence sexual development and behavior. Although both genders have quantities of most sex hormones circulating throughout their bloodstream, males typically have higher levels of circulating androgens (testosterone and dihydrotestosterone) than females and females typically have higher levels of circulating estrogens than males. Altering the natural levels of sex hormones in either direction can lead to direct and indirect alterations in sexual development and behavior and may ultimately prevent individuals from mating successfully. In the last century, vast quantities of man-made chemicals have been released into the environment, either directly through pesticides or indirectly through industrial waste run-off. Recently, researchers such as Theo Colburn with the World Wildlife Fund have shown that exposure to extremely low doses of some commonly-used chemicals, doses that humans and wildlife are routinely exposed to, can also have subtle, yet extremely potent, effects on sexual differentiation and development and may pose serious, long-term health problems for wildlife and humans.

Numerous scientists have looked at the effects of exposure to some of these man-made chemicals on developing embryos and have found that if you alter circulating levels of either androgens or estrogens during critical periods of fetal development, the embryo will fail to develop normal male or female internal and external sex organs. One well-studied example of an environmental chemical altering male external sex organs is found in alligators living in a contaminated lake in Florida. Dr. Lou Guillette, a professor at the University of Florida in Gainesville, has found that alligators living in Lake Apopka, a lake known to contain high levels of pesticide and industrial waste run-off, have significantly smaller penises than alligators living in an uncontaminated lake in Florida. This reduction in penis size has been attributed to fetal exposure to a chemical known as DDE, which is a break-down product of the well-known pesticide DDT. The size of the testes and other male reproductive organs require exposure to circulating androgen during fetal life to develop properly. Exposure to DDE during critical periods in development blocks androgen from binding to the androgen receptor, which reduces the amount of circulating androgen in the bloodstream and causes male fetuses to be born with smaller penises.

It seems that most scientists have been focusing their attention on the developmental effects of exposure to hormone-disrupting chemicals. But what about behavioral effects? As a graduate student in the Department of Biology at Indiana University, I was interested in this question and began studying the effects of certain man-made chemicals on male frog mating behaviors. The very loud calls produced by males in many species of frogs have come to epitomize a warm summer evening for lots of people. But these calls also serve the very important function of attracting females who are ready to mate. Like most sexual behaviors, mate-calling in male frogs requires the presence of sex hormones, specifically androgens. If the levels of circulating androgen in a male frog decrease below a certain threshold because he is exposed to high levels of a chemical that blocks androgens, such as DDE, he will stop calling and it will be much more difficult for him to attract females with whom he can mate. If mating is disrupted enough, then populations could start to decline. The implications of this type of research are important from a conservation perspective and will hopefully encourage more scientists to begin looking at the behavioral effects of hormonally disruptive chemicals in our environment so that we can continue to enjoy the sounds of frogs well into the future.