Moose are dying off across North America—and no one knows why.

The New York Times reports that the mass decline of moose has struck populations from Montana to New Hampshire to British Columbia. In Minnesota, only about 3,100 moose remain, down from about 12,000 in the 1990s. Is a moose plague to blame? Alien abduction?

Although it's hard to tell for sure, many scientists are blaming climate change. One major group of suspects is parasites that benefit from shorter winters and warm, moist environments. Tiny moose ticks, brain worms and liver flukes can be not only disgusting but also life-threatening for the giant creatures. Tree parasites also thrive in warm environments and are liable to kill off the forests that make up the moose's habitat. Fewer trees means higher visibility, which is good news for wolves and hunters but bad news for their huge prey. Or it could be the heat itself: animals that are adapted to freezing environments just might not be able to handle the increasingly warm weather.

But moose are hard to study. While all these theories might have merit, some possible causes might not even have been proposed yet. It's a mystery for now.

Although we're still kind of hoping it's aliens.

[image via AP]