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.
Mantled howler monkeys are one of the largest primates in Mexico and Central America, measuring around 25 inches on average. Covered in thick black fur, they are known for their low, guttural calls. They eat fruit and leaves, which are also one of their main sources of water. Scientists suspect that the drought dried up leaves and streams, making it harder for the monkeys to hydrate.
The species, which is found as far south as Peru, is considered vulnerable by the International Union of Conservation of Nature. But the Mexican subspecies is in worse shape and has been classified as endangered.
In Mexico, the heat has helped unleash drought in much of the country and the capital is running out of water. Environmental changes have very likely put more stress on Mexico’s small mammals. Tabasco State is home to much of the country’s cattle and is one of the most heavily deforested states in Mexico. As farms have expanded in the region, the tropical forests where monkeys live have shrunk.
“In general, howler monkeys are very resilient to those conditions and can survive for long periods of time,” said Liliana Cortés Ortiz, a primatologist at the University of Michigan and the vice chair for the primate specialist group at the International Union for Conservation of Nature.
Videos of large groups of dead monkeys on the ground, or of local residents with the limp bodies of baby monkeys, have spread through social media in recent weeks. “Please whoever is setting fires, stop,” one person said in a video posted on Facebook.
On Monday, the deaths prompted a reaction from President Andrés Manuel López Obrador, when he told reporters his government was looking into how to help. “It’s been very hot,” he said. “I’ve never felt it this bad,” in recent visits to some states, he added.
This isn’t the first time this species of howler monkeys has been in trouble. In 2016, a similarly hot and dry year, mass die-offs of howler monkeys were also reported in Nicaragua. At the time, scientists estimated at least 280 animals died in three months, though they were unable to pinpoint the cause.
Now, scientists from the region are forming a working group to put together protocols of what people should or shouldn’t do if they find monkeys in distress. They are also trying to attract funding to do more research into the causes of the deaths.
Dr. Cortés Ortiz said she worried about what could be happening to other species that people aren’t as likely to notice.
Though species have evolved to adapt to different conditions, things are now changing “so fast, that it’s going to be very difficult for many species to adapt,” Dr. Cortés Ortiz said. “There is not enough time.”
For now, nonprofit and academic groups in Mexico are caring for the monkeys they can find. More than a dozen are in clinics being hydrated and treated. Some are recovering in the city Cunduácan, where Dr. Pozo first saw animals fall from a tree. But this Wednesday, he said, “sadly, one of them has died.”