Two in every five amphibian species in the world, or around 40% of 8,000 species, are categorized as threatened—a much higher percentage than other classes of animals, a new assessment has found.

In a paper published in the journal Nature on Wednesday, researchers looked at 20 years’ worth of data on nearly every amphibian species on Earth, compiled by more than 1,000 scientists—a follow-up assessment to a similar one conducted in 2004.

The paper finds that amphibians around the world are “deteriorating rapidly”—and that climate change poses much more of a risk to them than it did in the first analysis, responsible for 39% of species getting closer to extinction compared to just 1% in the 2004 work.

“They don’t have any protection in their skin,” Patricia Burrowes, a professor of biology at the University of Puerto Rico, told NPR. “They don’t have feathers, they don’t have hair, they don’t have scales.”

(NPR, NBC, USA Today, Washington Post $, AP)