If you've ever been within a mile of a dog when an ambulance or fire engine siren blares, you've probably heard the low, mournful sound of her howling in harmony. The trademark head back, chin up posture that accompanies a dog's howl is present in all breeds, though not all dogs howl.
Howling is a major means of communication among wolves, the ancestors of modern dogs. Howling in the wild is a wolf pack's version of a pager network, each member often checking in with the rest of the pack or alerting other members to food. Howling works well in the wild because howls can cover much more distance than barks. But despite centuries of domestication and breeding, the howling instinct still exists in modern canines.
Many sirens, particularly those set off by firehouses or fire engines sound quite a bit like a dog's howl -- slow to rise and fall and resonant over long distances. Dogs may interpret this sound as a communique from a member of the pack and are answering the call. Even dogs who do not belong to the same packs, such as dogs in different parts of the neighborhood, may howl in reaction to each other or to the siren.
The Sound of Music
It is not just sirens or other dogs howling that cause some dogs to respond in kind. A classic movie gag, in fact, is a dog's disconcerted howling alongside someone singing terribly. But dogs "sing along" with good music too. It may be that the notes hit by crooners or opera singers resemble the pitch of a howling pack mate. No one is truly sure, but it's a good bet that if music distressed a dog, she would leave and not sing along.
Chain of Command
Though howling exists in all dog breeds, not every dog howls when she hears a sound. Those who have two dogs may have noticed that one will howl at a siren while the other does not. While no formal research has identified why this is, popular theories suggest that individual dogs, like people, are just different and that only the more submissive dogs in a pack will howl after a sound.