The residents of Tecolutilla, Mexico, knew the heatwave was bad when they heard the thuds. One by one, the town’s howler monkeys, overcome with dehydration and exhaustion, were falling from the trees like apples, their limp bodies smacking the ground as temperatures sizzled past 43C (110F) in spring last year.

Those that survived were given ice and intravenous drips by rescuers. At least 83 of the primates were found dead in the state of Tabasco, though local veterinarians estimated hundreds throughout the region probably perished.

Howler monkeys lie dead on the forest floor in Comalcalco, Mexico, after falling from the trees. Photograph: Reuters
Episodes such as this are unfolding across the world as the climate crisis delivers harsher and more frequent heatwaves. Flying foxes have tumbled from trees in Australia; billions of barnacles have baked in tide pools in Canada; male beetles have been virtually sterilised by soaring temperatures.

Beyond these local die-offs, ecologists are only just beginning to grasp the full threat that extreme heat poses to the world’s wildlife populations, and how quickly it can drive species towards extinction.