I sit watching one of my dogs chewing on a raw beef shank bone and wonder at how blissful the experience seems to be for him. There appears to be no better canine sedative than a bone to gnaw on. However the bone that I gave him had very little meat on it, and those few clinging meat scraps had disappeared quickly. Yet despite the absence of meat he is still chewing on the bone, scraping it or sometimes crushing it when he can get it far enough back into his mouth to work on with his molars. Ultimately he will actually eat most of the bone and that is the scientific puzzle. Why would a dog, or any other carnivore, seem to want such an apparently non-nutritious food source to such a degree that it is willing to spend hours working, crushing and grinding it so that it can be consumed?
Strangely enough we get our first inkling as to what is going on by looking at research on the diet of humans. For example, John D. Speth of the University of Michigan excavated some sites in New Mexico that contained the bones of bison that had been killed around 1450 AD. However there was something strange about these particular deposits. It seems that these ancient hunters left most parts of the female bodies to rot at the butchering site yet dragged home as much of the male carcasses as they could carry. So what was wrong with these female bison?
The clue as to what was happening comes from the season. While most known prehistoric bison kills happened in fall and winter, this New Mexico site contained animals killed in springtime. What makes female animals so unappetizing during the spring turns out to be fat, or rather the lack of it. Pregnant and nursing cows are often severely stressed in the spring because they are carrying a nearly full-grown fetus or nursing a calf, and it is well before there is enough vegetation to use for adequate foraging. As a result they have to live off of their own fat reserves and thus their bodies get fat-depleted. Similar fat depletion can occur when animals are near starvation during cold or dry seasons. At such times an animal's body fat can drop to only a few percent of their total weight which is far less than what appears in even the leanest cuts of beef. It may surprise many people to learn that a diet made up of almost pure protein actually contains too few calories for adequate nutrition and can even lead to protein poisoning. Thus it appears likely that these hunters rejected the meat of the female bison because of its low fat content.
To see how inadequate a high protein diet is in the absence of fat, we can look a historical incident which occurred in the Wyoming during the winter of 1857. A military officer named Randolph Marcy ran out of food and had to march his men all the way to Santa Fe, New Mexico in order to find adequate provisions. His troops survived by eating their pack animals. Unfortunately the poor quality of the meat nearly killed the men. Marcy reported that, "We tried the meat of horse, colt, and mules, all of which were in a starved condition, and of course not very tender, juicy, and nutritious. We consumed the enormous amount of from five to six pounds of this meat per man daily, but continued to grow weak and thin, until, at the expiration of twelve days, we were able to perform but little labor, and were continually craving for fat meat."
This brings us to the importance of bones in the evolution of carnivores. Seasonal changes swinging between warm and cold in the mid-latitudes and wet and dry in the tropics affects the availability of the vegetable matter used as food by the animals that meat eaters depend upon as their prey. The last reservoir of fat in an animal undergoing hard times is in the bones. Bone marrow is particularly rich, with more than half of its composition being fat. In addition, bonded to the calcium making up the bone itself is the so-called bone grease, which, although less digestible and concentrated is still a substantial source of fat. If you are a predator and for some reason your prey is in very poor condition for part of the year, then you will greatly increase the value of the meat that you have if you can get some fat with it. The fat serves as a sort of nutritional multiplier. This means that the ability of carnivores to reach the bone marrow of their prey, and their desire to work at grinding down and consuming the bulk of a bone to access the bone grease could mean the difference between life and death.
A number of carnivorous species, including the hyena and some extinct canines, like the dire wolf, display specialized bone crushing teeth, and powerful jaw muscles to facilitate eating bones. Our domestic dogs have to work harder because they do not have those specialized teeth. On the other hand they do have very strong jaws, and even a small dog can work up a bite strength of around 700 pounds per square inch which can gradually wear down the largest of bones. Most importantly, evolution has left dogs with the desire to work at getting this source of fat. Evolution uses the trick of making necessary behaviors for survival of the individual or species pleasurable (like eating or sex) and so it has made the bone chewing eating behavior dogs such a great satisfaction for them.
One caution-if this article has motivated you to give your dog a bone, make sure that it is a raw bone. Cooking sweats out the bone grease and often melts away the fat in the bone marrow, thus making cooked bones less desirable. Furthermore cooked bones are much more brittle and eating sharp bone splinters can injure your dog. For the most part, the raw bones and the fat bonded to them are safely ground down and consumed by dogs.