Strange lights in the sky have been terrifying humans for millennia, as evidenced in Biblical accounts of flying chariots and ancient Indian texts describing battles in the heavens.
Now it's been shown UFOs also freak out animals as big and brave as American black bears.
According to research published today in scientific journal Current Biology, the bears – which can weigh up to 400kg and stand two metres tall – show physiological signs of distress in the presence of drones.
Scientists attached heart-rate and GPS monitors to four bears living in wild northwest Minnesota, and mapped the data with flight paths of drones in the location.
"Some of the spikes in the heart rate of the bears were far beyond what we expected," says Mark Ditmer of the University of Minnesota.
"We had one bear increase her heart rate by approximately 400 percent, from 41 beats per minute to 162 beats per minute. Keep in mind this was the strongest response we saw, but it was shocking nonetheless."
The bears didn't show any outward signs of stress, and rarely seemed startled or fled the scene.
"Without the use of the biologger, we would have concluded that bears only occasionally respond to UAVs."
Ditmer says the study shows drone operators should be careful when plotting flights in the vicinity of wildlife.
Next he plans to research whether bears can get used to drones, and if so, how long it takes.