Thousands of critically endangered frogs have been found dead in South America, leaving scientists scrambling to find the cause. The most likely culprit, they say: pollution.

The Titicaca water frog (Telmatobius coleus), one of the largest aquatic frogs in the world, goes by a unique nickname. It has “amazingly baggy skin, which gives it the common name scrotum frog,” says National Geographic explorer Jonathan Kolby, a PhD student who studies frogs in Latin America.

“Their wrinkly skin is an adaptation to help them absorb more oxygen from the water, possibly because they live at such high altitudes”—around Lake Titicaca along the Peru and Bolivia border, says Kolby.

“But they do look pretty funny,” Kolby adds about the large frogs. (The