I do not know for sure about dogs, but in humans fat burning occurs at certain heart beat levels. If the exetcise is of high output in short time, so high heart beat, preffered energy source are glycogens aka carbs followed by muscle, no fat burning at all. Later on fat could be used as energy source to replenish "fuel storage" but since this thread is about performance during working and not about dieting, it is a different subject itself.
In extreme low body fat situations, the body instead of metabolising fat and carbs throughout the day for energy, will burn muscle into glucose into energy etc to function.