Dead Monkeys Are Falling From Trees Amid Brutal Heat in Mexico

Gilberto Pozo, a biologist, was monitoring a small forest in the town of Cunduacán, in southern Mexico, in early May when two mantled howler monkeys fell from a tree in front of him with a thud.

“They were dehydrated and received treatment,” he said. “But they didn’t survive.”

At first, Dr. Pozo and his team at Cobius, a nonprofit conservation group, thought the monkeys had been overcome by smoke from fires set by farmers clearing land nearby.

But, as temperatures soared over 100 degrees Fahrenheit in recent weeks, dozens of reports of dead monkeys started popping up. Residents were finding groups of 10 or more dead at a time, many also showing signs of dehydration. As of Wednesday, 147 monkeys have died in the states of Tabasco and Chiapas in southern Mexico.

The deaths of dozens of mantled howler monkeys in Mexico may be the latest sign of the danger extreme temperatures pose to wildlife around the world. As global temperatures have shattered records, scientists have recently documented a die-off of Amazon dolphins and a mass bleaching event in the world’s coral reefs.

“The animals are sending us a warning, because they are sentinels of the ecosystem,” Dr. Pozo said of the monkeys. “If they are unwell, it’s because something is happening.”

Scientists investigating the deaths still don’t know exactly what caused them. But they hypothesize that warmer temperatures may have combined with a confluence of other factors — including fires, deforestation and logging — that have cornered the monkeys in smaller areas of forest with little shade, food or water. The scientists haven’t yet ruled out pathogens, but a recent necropsy on one of the monkeys showed no signs of influenza, including bird flu, or Covid-19, Dr. Pozo said.