Fear instinct

Frogs see common herbicide as predator

By Dinsa Sachan
Published: Tuesday 15 May 2012

instinctIT IS known that herbicides, chemicals used to kill weeds, affect amphibians. A study has for the first time shown that these chemicals force amphibians to change shape. The changes are similar to those induced in the presence of predators. Amphibians—frogs, toads and salamanders—have dwindled at an alarming rate since the 1980s; a third of the world’s amphibian species are either threatened or extinct.

Researchers led by Rick Relyea from University of Pittsburgh in the US studied the impacts of Roundup, most widely used herbicide in the world and manufactured by Monsanto, on amphibians. They replicated wetland conditions in water tanks. Tadpoles of wood frogs, leopard frogs and American toads were allowed to grow. The team then observed the reaction of tadpoles to various doses of Roundup for three weeks. The herbicide induced wider and larger tails among the tadpoles. When a predator was present along with the herbicide, the tail would grow twice as much. The larger tail allows better swimming ability, while deflecting predator strikes away from the body. If the predator grabs the tail, it can tear away and regenerate. But such a morphology comes at the cost of less efficient feeding. “We suspect that the herbicide is altering the stress hormones,” says Relyea. The study was published in the April 2 issue of Ecological Adaptations.

But Vasudha Jha, spokesperson of Monsanto India, is defiant. “We are aware of the study. In initial review we have found that he tested a direct overspray to a very shallow six-inch (15 cm) water depth, which is not an approved use of the product. Consequently, the study creates a serious misperception.”

Relyea defends: “The comments show that Monsanto has not even read the paper. The water in the tanks was 15 inch (38 cm) deep, which is typical of wetlands that have amphibians.”

Till now, a complex interplay of climate change, habitat destruction and disease has been held responsible for the decline in amphibian population. Over the years, research has added another culprit: pesticides.

In 2005, a study by Relyea had found that Roundup causes 70 per cent decline in amphibian biodiversity and 86 per cent decline in total mass of tadpoles in a pond. But Roundup is not the only culprit. “Studies have shown that herbicides like atrazine mimic estrogen, the female sex hormone and turn male tadpoles into females,” says Karthik Vasudevan, biologist at Wildlife Institute of India in Dehradun.

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