UV-pollutant combo hits tadpoles hard.

Many of the studies documenting a global decline in amphibians have linked the shrinking populations with exposure to excessive ultraviolet (UV) sunlight or to pollutants, especially ones with a hormonal effect. Biologists now find that slightly elevated UV exposure reduces the chance that tadpoles will become frogs. That chance declines even more with coincident exposure to an estrogen-mimicking pollutant.

Maxine Croteau's team at the University of Ottawa exposed leopard frogs to UV radiation for 8 months. Exposures started at hatching and lasted 12 hours a day at doses emulating what would occur 50 centimeters below the water surface at mid-day in May in northern North America. In the wild, only frogs in ditches or in small, evaporating ponds--and therefore without access to shielding plants--encounter such a constant UV exposure.

Ordinarily, between 6 and 11 percent of leopard frog tadpoles survive and metamorphose into adults, notes coauthor Vance L. Trudeau. In contrast, just 2 to 4 percent of the UV-exposed tadpoles reached adulthood, and they took at least a month longer to do so than did frogs raised in the lab but not exposed to excessive UV. The pollutant 4-t-octylphenol, an estrogenic breakdown product of popular surfactants in detergents, soaps, and other products, didn't affect metamorphosis--except in frogs getting daylong UV. In those groups, metamorphosis rates plummeted to a mere 1 or 2 percent, even when the 4-t-octylphenol concentration was 0.2 parts per billion, an amount found in the environment.--J.R.