If you’re walking through the woods in parts of the United States or Canada and hear a bloodcurdling screech, a Ringwraith isn't on your tail. It could be just a male elk in the mood.

At over 700 pounds (318 kilograms), the North American elk is one of the biggest deer species alive today.

Larger animals tend to have larger voice boxes and longer vocal tracts, which produce lower sounds—it’s why elephants roar while mice squeak.

Despite their large size, though, male elk produce shrill rutting calls known as bugles. (Follow Yellowstone elk's perilous journey in the latest issue of National Geographic.)

How elk make these distinctive sounds has puzzled scientists—especially since the bugles can reach pitches too high to be produced by the animals’ voice boxes.

“It sounds like a scream or shriek, or several animals vocalizing at the same time,” says David Reby of the United Kingdom's University of Sussex and co-author of a new study on elk bugles.

“If I was in the forest at night and heard this sound without knowing what animal it was, I would probably feel terrified.”

Now Reby and his colleagues have figured out that elk produce the eerie noises by whistling and roaring through their vocal cords at the same time.

Whistle While You Roar

In the lab, the researchers studied high-quality recordings of elk bugles taken at a farm in New Zealand.

Their analyses revealed two distinct components of the bugles, a low-pitched roar around 150 hertz (Hz) and a high-pitched whistle reaching up to 4,000 Hz. Adult human voices usually range between 85 and 250 Hz.

The two sounds can shift independently, sometimes even in opposite directions. One sound can even cut out before the other one stops. (Related: "These Animals’ Screams Will Chill Your Blood.")

All this suggested to the researchers that the two components involve independent sound sources in the elk's body—a phenomenon known as biphonation.

View Images A bull elk bugles in Canada's Jasper National Park. Photograph by Barrett Hedges, Nat Geo Image Collection

When they did a CT scan of the head and neck of a male elk that had died of natural causes, the team determined its vocal folds are the perfect size to produce the low-pitched roar.

But that didn't explain the high-pitched whistle.

However, while watching video of elk bugling, the scientists noticed the animals moved their lips and nostrils as they whistled.

“We think it is an aerodynamic whistle, produced either by flaring and contracting the nostrils or by air vibrating the soft palate, like a flute,” says Reby, whose study appeared April 20 in the Journal of Experimental Biology.

Since female elk can bugle but not whistle, the scientists restricted their study to males.

Tecumseh Fitch, a biologist at Austria's University of Vienna who was not involved in this research, says this type of sound production is unusual in mammals.

“This particular trick" of the elk combining a whistle with its normal call "is quite unique,” he says.

“I’d heard [elk] calls in the wild and never suspected a whistle mechanism. This was [an] innovative insight and I think a very solid and important one.”

Getting the Message Out

So why do the elk need such an elaborate sound system? Mating, of course.

The whistle component is very loud and has the ability to travel long distances, while the roar can be heard at short distances and reflects the animal’s size: A lower pitched roar indicates a larger male.

Reby and his colleagues speculate males bugle to communicate their size to nearby cows and rival males, while simultaneously advertising their presence to faraway females. (See "Wild Romance: Weird Animal Courtship and Mating Rituals.")

Wyoming’s Incredible Animal Migrations Revealed as Never Before In a first-of-its-kind mapping project, researchers track how elk, mule deer, moose, bighorn sheep, and pronghorn use Wyoming and Colorado wilderness areas.

Elk herds may be spread out over large distances of more than 15 square miles (about 40 square kilometers).

“It’s not just about choosing a mating partner but also finding where they are,” says Reby.

So while male elk bugles might sound demonic to us, to female elk they’re music to the ears.