If you've ever watched dogs and cats eat, you'll know that dogs mostly stand to eat -- unless it's a bone, for which they'll often lay down -- while cats usually eat in a hunched position. A dog's eating posture is most important if he's prone to bloat, which is a medical emergency.
For the most part, the reason dogs stand up while they eat is instinctive. Historically, canines were competitive scavengers who often had to protect the scarce food they could forage. Therefore, standing up while eating allows a dog to defend his prized sustenance. Should a competitor arrive on the scene, he can move away to another kill, move his food away from others or even become aggressive with intruders if need be.
When dogs are relaxed they may, like cats, lay down to eat. This is often the case in domestic situations when dogs have bones or other chew toys, such as a Kong or rawhide, which take considerable time to eat. Laying down allows them to hold onto the item, keeping it steady, with their paws.
For some dogs, standing up while eating may help with certain health conditions. Elevated bowls are often recommended by veterinarians for dogs who have neck or back problems, since a raised bowl provides for better posture. Dogs with mega-esophagus, an enlarged esophagus that affects peristalsis needed to push food down into the stomach, also benefit because it is easier for them to swallow standing up rather than with their heads down.
Caveat: Gastric Problems
Some people have been told that heightened bowls help protect dogs from bloat or gastric dilatation, a medical emergency wherein a dog's stomach twists and fills with gas. Typically, dogs who bloat are large or giant breeds, middle-aged or older, or whose who inhale food, although any dog can get gastric dilation. Contrary to popular opinion, a study published in the Journal of the American Veterinary Medical Association revealed that raised bowls may contribute to gastric dilation in dogs.