University of Pittsburgh researchers said the weed killer Roundup, in sub-lethal and environmentally relevant concentrations, caused two species of amphibians to change their shape.
The study is the first to show that a pesticide can induce morphological changes in a vertebrate animal, biological sciences Professor Rick Relyea said in a university release Monday.
Roundup is a systemic, broad-spectrum herbicide produced by the U.S. company Monsanto.
The presence of predators can cause tadpoles to change shape by altering the tadpoles' stress hormones, Relyea said, causing them to grow bigger tails to better escape.
But similar shape changes seen after exposure to Roundup suggest the weed killer may interfere with the hormones of tadpoles and potentially many other animals, Relyea said.
"It was not surprising to see that the smell of predators in the water induced larger tadpole tails," Relyea said. "That is a normal, adaptive response.
"What shocked us was that the Roundup induced the same changes. Moreover, the combination of predators and Roundup caused the tail changes to be twice as large."
Because tadpoles alter their body shape to match their environment, having a body shape that does not fit the environment can put the animals at a distinct disadvantage, the researchers said.
"This discovery highlights the fact that pesticides, which are important for crop production and human health, can have unintended consequences for species that are not the pesticide's target," Relyea said.
"Herbicides are not designed to affect animals, but we are learning that they can have a wide range of surprising effects by altering how hormones work in the bodies of animals.
"This is important because amphibians not only serve as a barometer of the ecosystem's health, but also as an indicator of potential dangers to other species in the food chain, including humans."