Below are examples of results from Dr. Hayes' bioassays showing various degrees of feminisation (development of spots) in male reed frogs caused by exposure to various chemicals. Chemical names are below each example. Notice that exposure to some chemicals produced little or no change. Photos courtesy of Dr. Tyrone Hayes.
	Control	17ßE2	17-ethynyl E2	Estrone
	11-OH-E2	3-Methoxy E2	2,3-dimethyl E2	3-gluc E2
	17-gluc E2	3-SO4 E2	17-SO4 E2	3,17-diSO4 E2
	Estriol	Estetrol	2-OH E2	4-OH E2

Because of their sensitivity to human sex hormones, the first pregnancy tests were developed using frogs and not rabbits. The African reed frog, in particular, can also help screen for substances that may act like estrogen, such as PCBs, DES, and DDT, says Professor Hayes. These chemicals are simply introduced into the water; within five to seven days, if the juvenile male reed frogs develop spots, you've found an estrogen-like substance -- and a potential human carcinogen.

With this frog model, also called a bioassay, Hayes and his students can test dozens of compounds on thousands of frogs at once. Because the color changes are so striking and so fast, the reed frogs are an economical first step for screening cancer-causing agents before going to rats and other models.

One of the most disturbing findings of his research are that the doses of hormone needed to make the males develop female spots are vanishingly small. "This is a controversial issue among toxicologists," says Hayes. "Normally, they look for death and deformity to determine the lethal dose. But with our frogs, there's no minimum dose; it just takes longer to see the effects."