There’s something strange going on near Lake Titicaca with its scrotum frogs (and it didn't happen on April 1).

At least 10,000 of these fat, wrinkly, and very rare frogs have mysteriously died in Peru. Thousands of the frogs were discovered floating in the river Coata by members of the Committee Against the Pollution of the Coata River. The river flows into Lake Titicaca, the highest navigable lake in the world, which straddles the border between Peru and Bolivia.

Although this die-off occurred on the Peruvian side of the lake, similar events have also been seen on the Bolivian side.

Sometimes referred to as the Titicaca Water Frog, the frog is only found in the waters in and around Lake Titicaca. The IUCN Red List declares this species as “critically endangered” and it’s believed the highly fragmented populations are all in decline.

They received their nickname “Titicaca scrotum frog” from their characteristic skin folds. They might make them look silly, but they’re actually a highly specialized adaptation. The frogs are effectively able to remove oxygen from water through their skin, so these folds actually help to increase this amount by increasing the surface area.

The Committee Against the Pollution of the Coata River argues that the deaths were caused by unchecked pollution, which has been reported on for several years, including a previous spate of frog mass-deaths, but remained overlooked by the authorities. Last week, it became harder to ignore, with demonstrators bringing 100 of the dead frogs to the nearby Puno city square in protest.

“I've had to bring them the dead frogs. The authorities don't realize how we're living,” protest leader Maruja Inquilla told AFP. "They have no idea how major the pollution is. The situation is maddening."

^{The Titicaca Water Frog (Telmatobius culeus) seen here in Bolivia. Arturo Muñoz/Bolivian Amphibian Initiative}

Peru's National Forestry and Wildlife Service (SERFOR) has released a statement that says they’re now evaluating the situation and preparing to launch an investigation. 

The statement said that in a sampling point of about 200 meters (660 feet), 500 dead specimens were found. It added, “It is believed that more than 10,000 frogs were affected over about 50 kilometers (30 miles).”

Speaking to IFLScience, Arturo Muñoz of the Bolivian Amphibian Initiative explained what was behind previous mass-deaths in Bolivia between May and April 2015. 

"We found sulfide levels were very high in the lake," Muñoz told IFLScience. He added that heavy rains and strong winds could have released sulfides from the bottom of the lakes and rivers, which subsequently might have killed the frogs.

"In December 2014, there was a bloom of algae that turned the water in that area completely green. The bloom of the algae also causes an unbalance in all water parameters, [sic] such as oxygen levels... In other words, water with no oxygen, and the fish and frogs die.

"We think that these two main aspects were the cause for the massive deaths," he concluded.