In Tecolutilla, Mexico, the brutal toll of a spring heatwave became shockingly visible when howler monkeys, parched and exhausted, began dropping from the treetops. With temperatures soaring above 43C (110F), many hit the ground already lifeless. Rescuers scrambled to save survivors with ice packs and intravenous fluids, but in Tabasco state alone at least 83 monkeys died — and veterinarians fear the true toll may have reached into the hundreds.
Scenes like this are becoming increasingly common across the globe as the climate crisis drives more frequent and severe heatwaves. In Australia, flying foxes have collapsed in the thousands; in Canada, billions of barnacles were baked alive in tide pools; and in laboratory studies, soaring temperatures have left male beetles nearly sterile. Scientists are only beginning to grasp how extreme heat affects wildlife and accelerates extinction risks.
Maximilian Kotz, a climate scientist at the Potsdam Institute for Climate Impact Research, notes that the danger is not just rising averages but the sharp increase in the number of extremely hot days. While researchers long expected warming to shift habitats and disrupt food supplies, intense heatwaves introduce an entirely new layer of stress.
Recent studies suggest these events can trigger sudden population crashes. Kotz and colleagues examined decades of data covering more than 3,000 bird populations alongside weather records. Their analysis revealed that while birds in temperate and colder regions showed little decline, tropical species suffered heavily. Over 70 years, extreme heatwaves slashed tropical bird populations by 25% to 38%, with songbirds hit hardest.
“Tropical species already live close to their heat tolerance limits,” Kotz explains. “They now face 10 times more hot days than in the past.” The team, writing in Nature Ecology & Evolution, suggests a mix of causes: direct deaths from overheating, disrupted reproduction, and loss of insect prey. Strikingly, they found that climate change has driven tropical bird declines more than human pressures such as deforestation or mining. Even protected reserves, Kotz warns, are not safe from greenhouse gas-driven heat extremes.
The devastating effects of heatwaves are not limited to the tropics. During the 2021 Pacific Northwest heatwave, when temperatures exceeded 46C (116F), University of British Columbia marine biologist Christopher Harley surveyed tide pools filled with dead mollusks and barnacles. His early estimate of a billion dead animals has since been revised sharply upward: closer to 10 billion barnacles and 3 billion mussels. Barnacles bounced back relatively quickly, but mussel populations — slower to reproduce — may take decades to recover.
Heat’s impact on insects appears mixed. Preliminary research shows honeybees coping fairly well, while aphids fared poorly. Globally, extreme temperatures have produced both population explosions and collapses, highlighting the complexity of the threat.
For mammals, the picture is still emerging. PJ Jacobs, an evolutionary biologist at the University of Pretoria, has studied small rodents in southern Africa. Their high surface area relative to body size makes them vulnerable, and under simulated heatwaves, fertility rates plunged. Jacobs warns that such reproductive stress could accelerate declines.
“Animals can’t remain active in extreme temperatures — they must rest, which reduces feeding and reproduction,” he says. Heat can trigger dehydration, disorientation, and collapse. Even when lab mice sought relief, neurological impacts persisted. “The brain is the body’s control center,” Jacobs explains. “When it falters, you see outcomes as stark as monkeys falling out of trees.”