A wolf’s howl is one of the most 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 that 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 also 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. “We started taking our wolves for walks when they were 6 weeks old, and as soon as we took one out, the others would start to howl,” she says. “So immediately we became interested in why they howl.”

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 dominance hierarchy in its pack and the animals’ social relationships. The captive wolves do not have families as wild wolves do, and so they form hierarchies. “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. They removed the wolves in random order, so that the animals could not predict which one in their pack was going to leave. The researchers also set up a control situation by placing each of the wolves in an adjoining holding area again on three occasions for 45 minutes each time. The rest of the pack could not see the wolf in this area, but because he or she was nearby in a familiar place, there was no need for the animals to communicate.

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 also howled more when the wolf that was led away was his or her preferred pal—which means that the wolves aren’t simply howling because others are. “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 that 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 reports online today 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.”

“The paper provides the first experimental evidence … that the main reason [for howling] is to help the pack assemble after a long hunt,” says Dave Mech, a wolf biologist at the University of Minnesota, Twin Cities. He proposed the notion in 1966 after witnessing a pack of 15 wolves hunting. At the end of that hunt, the wolves were widely dispersed, he says, but “after howling, the pack was able to assemble again.”

But John Theberge, a wildlife biologist emeritus from the University of Waterloo in Canada, and Mary Theberge, who study wolf howls in wild populations, point out in an e-mail message that “extrapolating conclusions from penned animals to … wild ones is dangerous. The role of howling may be quite different in the wild where an animal is free to silently follow the scent trail of others.” They also suggest that hormones other than those associated with stress may be involved in the wolves’ howls. “Perhaps some other emotion was involved, such as a general level of excitement,” they write, noting that wild wolves also howl as they get up after long periods of rest. “No stress is evident,” but the wolves are “aroused” and howling. And that means there’s plenty left to explore in the howl of a wolf.