The Eurasian beaver is a conservation success story and has been brought back from the brink of extinction, but it seems the animal is "haunted" by the predators—namely, us—that almost wiped it out.  

The beavers were once widespread across Europe and Asia, from Great Britain to Mongolia, but nearly went extinct due to overhunting by humans. By the early 1900s, there were only 1200 beavers left in eight small populations. Protection against hunting and reintroductions to their former homes have brought their numbers up, and today, there are over a million beavers in the wild. 

These rebounded populations are nocturnal, and scientists have been struggling to figure out why. The night life just doesn’t suit beavers or make much sense for them: Their eyes aren’t well adapted to seeing in the dark, and being active during the warmer daytime would make it easier for them to maintain their body temperature and keep their energy needs down. What’s more, they eat plants, which are available around the clock, so there’s no reason for them to limit themselves to foraging at night. 

If they’re not cut out for it, why do the beavers only come out at night? Biologist Kristijn Swinnen thought that fear of predators like wolves, bears, and lynx might have caused the beavers to shift their schedule. To test that idea, Swinnen and other researchers from the University of Antwerp set up camera traps in 34 beaver territories in Belgium, where the animals are legally protected from hunting and have no natural predators. 

The camera footage revealed that the beavers in the protected areas are active at night, just like those in unprotected zones with predators. They’re busier on nights with bright moons, which makes for easier foraging. This suggests that the beavers can alter their behavior in response to external factors, but the lack of predators hasn’t relaxed them into coming out during the day.  

The researchers suggest it’s not a fear of current predators that makes the beavers nocturnal, but “the ghosts of predators past,” as they write.

Animals’ behavior, they explain, is not only the product of their current environment, but pressures that existed deep in their evolutionary past. If these pressures were strong enough, their influence can persist for generations and long stretches of time. Take, for example, the American pronghorn, which is far faster than any of its modern-day predators. Scientists have attributed its incredible speed to pressure from much faster predators that chased it down hundreds of thousands years ago. Those predators are now extinct, but the pronghorn’s adaptation to them never went away.

Swinnen thinks that something similar is going on with the beavers. Eurasian Beavers have been hunted by humans for millennia, which almost caused their extinction. While modern hunters mainly used traps, prehistoric hunter-gatherers would have hunted by day using hand-held weapons, the researchers say, putting strong pressure on beavers to become nocturnal and reduce their chances of encountering humans. Even though the modern world is relatively safe, the ghosts of these predators keep the beavers in the dark.