Dogs will howl for different reasons but howling is usually thought be a form of communication. Howling is a normal vocalization that wild dogs often make but it isn't exclusive to just coyotes and wolves. All breeds of dogs, even a little Pomeranian or a Chihuahua, have the ability to howl, but certain breeds are more well known for making this sound. Sometimes a howl is done out of fear, due to pain, from disorientation after waking up from anesthesia, or simply to make their presence known. It's important to know what your dog may be trying to communicate to you when they are howling.
A Medical Concern
Since a dog may howl if it is distressed due to being sick or hurt, howling can be an indication of a medical problem. Dogs may howl if they are in pain, if they are experiencing certain hormone abnormalities, if they are losing their vision or hearing, if they have certain neurological conditions, or for other medical reasons. One example is Canine cognitive dysfunction sometimes referred to as dog dementia. This is a medical condition that affects a dog's brain and can also cause a dog to howl. This is usually seen in older dogs and is similar to Alzheimer's in people. Dogs with canine cognitive dysfunction are often disorientated, confused, and forget routine parts of their life such as knowing to go outside to do their business, which side of the door opens, or how get around furniture in the house. This dysfunction is a neurological issue and since a dog may be confused, it may start howling because it is confused or scared. If you are concerned that your dog may be howling because of a medical problem, it is important to get it checked out by a veterinarian as soon as possible.
Fear
Just like a dog who is howling out of a confused state of fear, dogs who are in other fearful situations may howl too. Extremely scared puppies may let out a howl when in new situations, stray dogs may howl when caught by good samaritans, or dogs left alone in a new place may also howl out of fear. But regardless of the cause of the fear, if a dog is howling and in a situation that could be interpreted as scary, it is sadly a good indication that they are terrified. Happy, excited, or relaxed dogs do not typically howl.
A Behavioral Concern
Canine behavior has been studied extensively and while we are still learning how to best communicate with dogs, we do know that sometimes howling is an indication of a behavioral problem. Usually these behaviors are rooted in fear or anxiety of some kind, but it may take time and careful observation to figure out what the specific trigger is for your dog.
Dogs who have separation anxiety, for example, are commonly noted to howl while their owners are away, causing a noisy display. If you leave your dog at home and hear howling when you step out of the door, it's a good sign that your dog may be stressed, or scared. True separation anxiety will involve other behaviors along with howling, like destructive behavior, pacing, urinating or defecating when left alone, and/or clawing at a cage or door. Thankfully, there are things you can do to ease this anxiety and the howling behavior that is often associated with it, but it will take time, training, and sometimes the use of pheromones, supplements, drugs, and other training aids.
In Response to Other Noises
You may have a dog that howls when it hears a siren or other loud noise. This is most likely because your dog thinks the noise is actually a howl from another dog and howling can be a bonding activity or form of communication from dog to dog. If one dog starts howling then other dogs will likely join in. Siberian Huskies are often seen showing off this natural instinct of howling as a group.
To Get Attention
Some dogs are naturally more vocal than others, and since all dogs have different personalities, you may have a dog who likes to howl to communicate with you. Howling at a person to garner attention can mean "stay away" if the dog is protecting someone or something but it can also just mean "come play" if it's your dog or another friendly canine who wants your attention.