Why did two-thirds of these weird antelope suddenly drop dead?  

In just three short weeks, some two-thirds of all the world’s saiga suddenly dropped dead. The big question is, how come?

The saiga is an endearing antelope, whose bulbous nose gives it the comedic air of a Dr. Seuss character. It typically wanders over large tracts of Central Asian grassland, but every spring, tens of thousands of them gather in the same place to give birth. These calving aggregations should be joyous events, but the gathering in May 2015 became something far more sinister when 200,000 saiga just dropped dead. They did so without warning, over a matter of days, in gathering sites spread across 65,000 square miles—an area the size of Florida. Whatever killed them was thorough and merciless: Across a vast area, every last saiga perished.

Richard Kock, a veterinarian and conservationist from the Royal Veterinary College, saw it all. He and his team were there on a routine monitoring trip to check the health of the population. “Mass mortality events are never nice things and I’ve experienced quite a few,” he says. “But the experience of the saiga was unprecedented, and unworldly. Even after 40 years of work, I just said: I don’t understand.”

The mega-death was all the more tragic because it struck at what should have been a time of celebration. After the collapse of the Soviet Union, poachers had brought the saiga to the edge of extinction, but governments and conservationists rallied to protect the species, and it rebounded. Saigas are good at that. Females can produce their first calf before their first birthday, and in every subsequent breeding season, most produce twins. So they recover quickly from disasters. By 2015, their population had quadrupled since the early 2000s, and it was predicted to do so again in a few years. “Everyone was saying: Oh great, we’ve really got them over,” says Kock. “They were beginning to talk about downgrading them off the endangered list. And then — bang — this happened.”