The world’s most popular weed killer, Monsanto’s Roundup, a systemic, broad-spectrum herbicide, can induce morphological changes in vertebrate skeletal animals, say U.S. biologists studying its effect on amphibians. A study by University of Pittsburgh researchers says the poison, tested in environmentally relevant concentrations, caused the shapes of two species of amphibians to change. The study is the first to show these dangerous consequences.

The presence of predators can cause tadpoles to change shape by altering their stress hormones, 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. The development is important because amphibians not only serve as a barometer of an ecosystem’s health, but also as an indicator of potential dangers to other species in the food chain, including humans.