There are some strange urban myths out there. One of these concerns Pitbulls, and goes along the lines that Pitbulls have locking jaws that once clamped down on an object, are impossible to loosen. 

In fact, this is a myth and there is nothing unusual about Pitbull jaws. There is no mysterious locking mechanism, which means that once attached it doesn't require any effort for the dog to keep holding on. 

The canine jaw is a hinge joint to allow the mouth to open and close. For the jaw to lock in place would require a ratchet mechanism that just doesn't exist. So the short answer to 'Can dog jaws lock?' is "No", but there's also a longer answer that goes "Kind of..."

When a dog bites down on something and holds on, refusing to let go, this is a result of mental tenacity rather than a physical mechanism. These dogs may engage in a game of tug and decide the prize is so great they don't want to give it up. A dog determined to hang onto the tugger will clamp down with their jaws but it is muscular effort and determination that keep the mouth shut, rather than a true lock. 

These dogs will typically growl as they tug. In order to get a better purchase, they'll also sink backward on their haunches, using all their body weight to pull backward. The dog often pulls rhythmically, shifting their weight to-and-fro, to try and get a better purchase on pulling. 

The dog often concentrates quite hard, which often means they stop wagging and their tail goes straight. During a full-on game of tug, the dog may well be caught up in the moment. Never try and pry the dog's jaws open as this will only make the dog more competitive and determined to hold on. Also, in a worst-case scenario, in the heat of the moment, the dog may try to bite the hand attempting to remove the object. 

Working dogs have made a huge contribution to the welfare of mankind. The canine talents for guarding, protecting, herding, tracking, or serving have been used for the benefit of mankind from those earliest prehistoric days when wild dogs were attracted to campfires. 

Indeed, man has deliberately selected dogs with certain characteristics, such as an ability to follow a scent trail, to perform a particular task. By breeding dogs together that had an excellent sense of smell, the breeds we know under the umbrella of 'scenthounds' came into being. 

Another example are the bully breeds. These dogs were bred to have powerful biting jaws that could hang onto prey until the hunter caught up with them. This is were the reputation for certain breeds having locking jaws came from.

In fact, that fearsome bite is down not just to anatomy but to a state of mind, where the dog is determined to hang on at all costs. This unwillingness to let go gives the appearance of a locking jaw, whereas its all in the mind rather than the jaw! 

There are medical conditions which can affect the jaws, but rather than locking the dog's bite onto an object, these conditions make it difficult for the dog to fully open their mouth. So an affected dog may find it difficult to yawn or pick up a ball because they are unable to open the mouth wide enough. Two conditions are most prevalent, and these are Westie Jaw and a myopathy. 

Westie Jaw is so called because it most commonly happens in West Highland White Terriers. It affects growing puppies and results in inflammation of the jaw joint, which causes extra bone to be deposited. This extra bone gets in the way of the hinging of the jaw, and prevents the mouth opening properly. This is a painful condition, so pain-relieving medication is essential. 

The second condition affects the muscles of the cheek which open and close the jaw. These muscles contract but don't relax, which makes it difficult for the dog to open their mouth. No one is quite sure why this happens and treatment with steroids and pain relief is necessary to prevent permanent damage to the muscles. 

When a dog cannot fully open their mouth this is surprisingly disabling. Depending on the size of the dog, they may not be able to open the mouth wide enough to pick up a ball. Indeed, for the worst affected dogs it's too painful to yawn or bark, and even eating or drinking can be a challenge. 

As a responsible owner, the first thing you should do is to seek veterinary help. 

As already mentioned, these conditions can be painful, so a minimum requirement is pain relief. In addition, myopathy affecting the muscles can benefit from steroid therapy. This may help to switch off the inflammatory reaction that causes the muscles to lock, and it also helps to minimize scar tissue formation. The latter is important as it helps reduce the risk of permanently restricted jaw movement. 

If the dog is having difficulty eating, then offer soft food or a slurry. This is easier for the dog to lap, without having to open their mouth wide. Also, it's more comfortable for the dog not to have to chew.