Many dogs sleep curled up in a ball. Why is this happening?
Firstly, it is warmer for the dog. In the wild, dogs trampled grass to make a kind of nest where they curled up. By the way, the habit of many dogs is connected with this, circling in one place and stomping around before lying down.
There is a version that the dog, curled up in a ball, protects the internal organs from predators. However, this hypothesis seems doubtful. If a dog has already missed the approach of a predator dangerous for it, then the pose of a “kalachik” is unlikely to help it much to defend itself.
Just imagine: you were attacked, and at this time you are calmly dozing, curled up in a ball. Will this posture help you defend yourself against a predator armed with rather serious weapons in the form of teeth, fangs and the like? Very unlikely. Therefore, it seems obvious that dogs, nevertheless, rely on other ways of self-defense, much more active than the sleeping position. And they are very unlikely to sleep when the enemy approaches. And, waking up, they are unlikely to remain lying.
So this hypothesis seems untenable.
Finally, the simplest hypothesis (and the most likely) is just as convenient. After all, a dog can curl up in a ball both in hot weather and under a blanket, that is, keeping warm can be completely irrelevant. As a rule, in this position, the muscles are relaxed, and breathing is deep and even. Often dogs lie like this on soft surfaces and prefer to have an angle or some sort of back support, but not necessarily.