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This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

Frogs in Puerto Rico are croaking at a higher pitch due to global heating, scientists have found.

The frogs appear to be decreasing in size at warmer temperatures, which causes their croaks to become high-pitched. If the trends continue, the heat could become too much for the sensitive amphibians to survive successfully, researchers have said.

The call of the coquí frog is well known to most who have spent time in Puerto Rico. It is named for its two-note call “co-qui”, which rings out throughout the island every night.

Male coquí frogs use their distinctive call to mark their territory and warn off rivals, but scientists have noticed that it is changing.

Peter Narins of the University of California, Los Angeles, has been studying their croaks for 23 years. While recording the sounds along the slopes of El Yunque mountain in Puerto Rico, he and his team found that the calls changed depending on the altitude of the frogs.

Amphibians are very sensitive to changes in temperature, and the higher on the mountains they were, the cooler it was. The frogs on the mountain peaks, therefore, were found to be larger than ones sitting in warm valleys, and this meant their call was different.

Nairns said: “Coquí that produced short, high-pitched calls at high rates lived near the base of the mountain, while the calls of animals living near the mountain’s peak were longer, lower-pitched, and repeated less frequently.”

Two decades after first recording their sound, Nairns returned to the mountains with a colleague, Sebastiaan Meenderink. The pair found that every frog call, no matter where it was on the mountain, had become higher pitched.

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“In order to record a call with certain characteristics we had to move to a slightly higher altitude,” said Meenderink. “It was as if all the animals had moved up the mountain.”

The scientists are concerned that the frogs will keep crawling up the mountain to escape higher temperatures at the bottom, but they will eventually run out of room.

“For now, the consequences are not dire,” said Meenderink. “A barely perceptible change in frog body size and call has little impact on the environment. However, if left unabated, the temperature increase will eventually cause a collapse of the coquí population, which will be catastrophic for the Puerto Rican ecosystem.”