Many species of animal use vocal communication to indicate danger, the availability of food, or other interesting events to their friends. While much attention has been given to the significance of these calls, what the trigger is for these vocalisations remains a mystery. As most animals have a severely limited repertoire of vocal calls, many researchers think that the vocalisations of animals, in contrast to those of humans, are simply a result of involuntary muscle movements. However, few studies have yet been conducted to directly test this hypothesis. Do animals communicate as a reflex, perhaps in response to stress or other internal states? Or is it a more voluntary behaviour, based on and modified by how the animal understands its environment? Now, a team of researchers from Italy, Switzerland and Austria have addressed this question in a study recently published in Current Biology by investigating what makes a wolf howl.

When their pack gets fragmented, wolves howl to help them reconnect with stray members. While it is obvious that the howling behaviour is caused by the separation of one or more members of the pack, the immediate trigger is unknown. On the one hand, the physiological stress that separation causes could provoke howling, suggesting the behaviour is more like a reflex, while on the other hand, more flexible social mechanisms could play a role. The team worked with a pack of nine captive wolves to try and understand what mechanisms underlie this behaviour, by firstly focusing on the stress levels of howling animals and secondly on the social relations within the pack.

To begin with, they removed one wolf at a time from the pack, taking it for a stroll. As expected, this physical separation induced howling in the remaining wolves. All howls were recorded, after which the concentration of a stress hormone, cortisol, was measured in the saliva of each of the wolves. They found that the remaining wolves indeed have higher levels of cortisol, suggesting that the physical separation is a stressful event. However, crucially, the authors found no correlation between the stress levels of an individual and the number of times it howled. This suggests that howling is not necessarily simply a consequence of the levels of stress an animal experiences.

Before doing the experiment, the team had determined the social dynamics of the pack: which wolves are more social towards each other, and what the hierarchical structure is within the pack. When they looked at which wolves were howling in response to the separation of a given individual, they found that those wolves that had displayed the greatest affection towards it were howling most frequently. These results suggest that there is a clear link between the social interactions within wolf packs and their vocal communication, perhaps allowing individuals to promote contact with those animals that are important to them.

This study sheds light on how animals use vocal communication in their social interactions, and suggests that, contrary to previous suggestions, animal vocalisations are not just involuntary muscle movements. Furthermore, it is good news for lost travellers in dark forests around the world: those wolves they're hearing are just looking for their friends.

S. W.
Wolf howling is mediated by relationship quality rather than underlying emotional stress
Curr. Biol.