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Virginia Morell, Published September 01 2013

The call of the wild: Why do wolves howl?

A wolf’s howl is one of the more iconic sounds of nature, yet biologists aren’t sure why the animals do it. They’re not even sure if wolves howl voluntarily or if it’s some sort of reflex, perhaps caused by stress. Now, scientists working with captive North American timber wolves in Austria report they’ve solved part of the mystery.

Almost 50 years ago, wildlife biologists suggested that a wolf’s howls were a way of reestablishing contact with other pack members after the animals became separated, which often happens during hunts. Yet, observers of captive wolves have noted that the pattern of howls differs depending on the size of the pack and whether the dominant, breeding wolf is present, suggesting that the canids’ calls are not necessarily automatic responses.

Friederike Range, a cognitive ethologist at the University of Veterinary Medicine in Vienna, was in a unique position to explore the conundrum. Since 2008, she and her colleagues have hand-raised nine wolves at the Wolf Science Center in Ernstbrunn, which she co-directs.

Although the center’s wolves don’t hunt, they do howl differently in different situations, Range says. “So we also wanted to understand these variations in their howling.”

The scientists have divided the wolves at the center into two packs. Range and her colleagues first determined each wolf’s position within the hierarchy of the pack. They have obvious, preferred partners that they play with, groom, and lie close to when sleeping,” Range says. The scientists then took each wolf out for three 45-minute walks, spread over several weeks.

In almost all cases, the pack began to howl within the first 20 minutes after a member was led away on a walk, Range says. But the one out for a stroll usually did not return the call. Those left behind howled in 26 of the 27 walking trials, but only two times during the control trials. The scientists kept careful track of which wolves were actually howling. Overall, the animals did most of their yodeling when the pack’s dominant member went for a walk. Individual wolves howled more when the wolf that was led away was his or her preferred pal. “It’s not a contagious response,” Range says. “Social relationships are very important to them, and the howling patterns reflect that.”

Thinking that the stress of separation likely triggered the wolves’ howls, the scientists tested the animals’ levels of the stress hormone cortisol by collecting saliva samples 20 minutes after each trial began. “We’ve trained them to let us put a stick with cotton on the end into their mouths and pull it around,” Range says. “I thought stress would be connected to the amount of howling, but that’s not always the case.”

The wolves’ cortisol levels spiked when the dominant animal was taken for a walk, but not when their preferred partner was led away. Despite their numerous howls in the latter situation, they were apparently not stressed. And that means the wolves’ howls aren’t like the robotic responses of Pavlov’s dogs, which salivated when the dinner bell rang. Instead of always being a simple physiological stress response, a wolf’s howl is at times more voluntary and driven by social factors, the team reported online last week

in Current Biology.

“It’s strategic, not emotional,” Range says. “They’re trying to contact individuals that are important to them and reform the pack. And they have some control over how much they howl.”