Thousands of species were exposed in 2024 to heat they had never experienced before.

On May 5, 2024, the first body was found on a cacao plantation in the Chontalpa region (Tabasco State, Mexico). In the following days, locals found more lying on the ground, some dying, others already dead. The peak number of deaths, with 80 individuals, occurred during the week of May 12-18. The last death occurred on June 23. In total, 286 bodies were counted in the Tabasco jungles, with another 64 found in neighboring states. In total, up to a third of the Mexican mantled howler monkey ( Alouatta palliata mexicana ) population in Tabasco, its main distribution area, had disappeared. The cause of so many deaths was an intense heat wave. A recently published study in PNAS estimates that thousands of species like these monkeys were exposed to a heat that, if it continues, jeopardizes their future.

“The symptoms observed [in the living] were muscle spasms, stiffness, difficulty walking, and breathing problems,” says Pedro Américo Días, a researcher at the Institute of Neuroethology of the University of Veracruz (Mexico). They also performed a dozen autopsies: “The necropsies showed hemorrhages and necrosis in the lungs, liver, kidneys, and brain,” Días adds. Everything fits with heat stroke as the cause, aggravated by coincidental factors, the researcher highlights: “Extreme heat waves; the deaths occurred during periods of exceptionally high temperatures that lasted two to four days. Dehydration; the monkeys obtain water mainly from their food, but the drought and extreme heat dried out the vegetation. And a limited diet: On the cacao plantations where they lived, they had access to less variety of foods.”

That 2024 event didn't wipe out the Mexican howler monkeys. But biologists fear they've incurred what's known as an ecological debt. One fact worries scientists: they recovered only a dozen offspring among the dead, as if death were preying on adults. This has left a generation orphaned, not only of their parents, but also of their teachings. It has also left a stress that must have weakened them both as individuals and as a species. For Días, "if the recently observed trends of rising ambient temperatures continue, there is a serious threat to the future of these primates."

Like howler monkeys, many living beings had to overcome the heat of 2024, officially the hottest year on record . Like these primates, thousands of vertebrate species (fish, reptiles, amphibians, birds, and mammals) were exposed to temperatures they hadn't been exposed to since 1940. The exact figure is that 5,638 terrestrial vertebrate species suffered thermal extremes never before experienced in at least a quarter of their territory. This is the main result obtained by a group of researchers who developed an algorithm to relate the geographical distribution of more than 33,000 species with the temperature in their territories. All their results have been published in the latest edition of the scientific journal PNAS .

"This algorithm allows us to see the thermal threshold of each species," says Josep M. Serra, a researcher at the Barcelona Institute of Botany (CSIC/MCNB) and co-author of the study. Until now, most studies and analyses on the impact of climate change have had time horizons of decades or even centuries, such as, "by 2100 x species will be extinct ," or they have shown the consequences of some of the indirect effects of warming, such as that of melting ice on polar bears, or the spread of fungi that paralyze the hearts of amphibians . But few studies have studied the direct effect of extreme heat events. "We have a set of mathematical algorithms that allow us to see, at certain temperatures, that a species is having a hard time," explains Serra.

By animal group, the most exposed were ectotherms, the cold-blooded ones, particularly a third of amphibians, which add heat to their many threats , and 21% of reptiles. By ecoregion, animals in rainforests, tropical scrub, and savannas were the most exposed. Geographically, wildlife in Central and South America, equatorial and northern Africa, and large islands like Australia suffered extreme heat.

The Mexican mantled howler monkey suffers from two of the conditions (humid rainforests and Central America) that have put its fate in jeopardy. But it also suffers from a third that is of greatest concern to biologists: its distribution area is very small. Although they have managed to adapt to cocoa plantations, only 3% of their original forests remain. This severely restricts their habitat. And what the authors of the PNAS study have found is that the smaller your territory, the less resilient you are to heat.

“Species with small ranges tend to be more exposed to climate change due to the reduced availability of refuges within them,” writes Mark Urban, a senior author of the paper from the Department of Biology and Evolutionary Ecology at the University of Connecticut (United States), in an email. He adds that species with shrinking territories “are already beginning to have smaller populations.” A reduced population in a small area not only has fewer places to escape the heat, but also tends to have less genetic variability , another risk factor.

Gerard Martínez de León, from the Institute of Ecology and Evolution at the University of Bern (Switzerland), studies the impact of thermal extremes on living beings . But he studies it in invertebrates. "They study it in vertebrates because they are the species for which there is the greatest resolution, the most data, but the ones that will really suffer the most are the invertebrates," he recalls. And these are at the base of many ecosystems. Another difference is that he analyzes a shorter time frame, that of heat waves .

"All species are adapted to their environment, which means they've already experienced more or less extreme episodes in the past. Therefore, they have a series of strategies to deal with very hot years or periods of heat," Martínez de León emphasizes. The problem is that things are moving too fast: "We are facing a context in which we are generating new conditions never seen before in evolutionary times; there have never been such rapid changes," he adds.

There's something that connects Martínez de León's work with that of the authors of the PNAS study, and even with Días, who studied the howler monkey die-off . It's the concept of ecological debt. A drought won't wipe out the monkeys, just as a year without ice won't wipe out polar bears. It's the succession of events, of thermal extremes, that can compromise the fate of both monkeys and bears.

“It's a concept from the late 1990s. It was developed for fragmented populations of species that are doing well in a fragmented habitat, but are so restricted in habitat availability that sooner or later they are known to become extinct,” Martínez de León recalls. Now the focus is on climate disturbances, such as heat waves, as generators of debt. “During a heat wave or after an extremely hot year, a species, the population of a species, seems to be doing well, but there are a series of processes behind it, physiological and others, that serve as indicators that sooner or later it will end up suffering.” And he gives a very specific example: “after a heat wave, there is a loss of the ability to reproduce.”