Dog's evolution has been fast-tracked by their association with man over the last x-thousand years, and to be selected, both by nature and by us, they needed to obey and to survive. So they needed to be able to keep going when it was asked of them, by keeping topped up - otherwise they would have been the next meal, thus removing their DNA from the gene pool. I doubt if wolves top up like domestic dogs - correct me if I am wrong. Of course we usually know how long we will be out for, and can plan accordingly. Military scenarios, where we know we don't know how long we will be out, are different, and the keeping topped up strategy applies there as well.