One organism’s flesh is another’s meat – there are even insects that can digest wood – but no animal has ever evolved that can derive any nutritional value from bones, AFAIK. (Tasmanian devils ingest bones but I’m pretty sure they pass through undigested.) Why is that?
Wood is made of carbohydrates that can be broken down by bacteria living in the gut of insects. The last bit of bone that does not get digested is mineral, and has no nutritional value.
ETA: Hyenas are probably digesting the tissue inside the bones, not the entire bone.
Anecdotal, but on a recent paleontological dig in Utah in the Morrison (Jurassic, near Arches N.P.) hosted by BYU, a museum director and professor pointed out borings in dinosaur bones and how the fact they worked from the underside up could be used to reconstruct the original orientation before a period of fluvial redeposition. Here’s an article that alludes to something similar.
“Osteocallis mandibulus n. igen. and isp. is described in dinosaur bones from continental deposits of the Upper Cretaceous Maevarano Formation of Madagascar and the Upper Cretaceous Kaiparowits Formation of southern Utah. O. mandibulus consists of shallow, meandering surface trails, composed of numerous arcuate grooves, bored into compact (cortical) bone surfaces, and is tentatively interpreted as a feeding trace.”
Sardines are a good source of calcium because people eat them bones and all.
Another contrarian datapoint: There were no remains found with, on nor near the wreck of the Titanic. Nor associated with other bathyspheric wrecks that have been investigated since then: Bismark, Hood, the Midway ships. My gut reaction is that something had to have eaten them, but I don’t think that there’s any evidence for what that mechanism may have been.
BTW: Great Dave, unless I’m misremembering it, my copy of Young’s Mammology does claim that Hyenas take nutrition from the bones themselves, not just the tissues in the bone. I’ll dig it out to check.
Interesting. Might the pressure at those depths have a softening effect on the bone, thus making it more palatable to sea critters?
The answer to every question beginning “Why don’t they…” is always “Money”. 
Or to put it another way, there are lots of other sources of calcium that are easier to ingest. So generally the investment in maintaining a biological system that can digest bones is greater than other ways of getting calcium is not worth the trouble.
However, just last night I saw an episode of “Blue Planet” that showed the bones of a whale at the bottom of the ocean covered with colonies of bacteria or very small animals working away at the bones.
I don’t believe it’s that. Forex ISTR that when the wreck of the Edmund Fitzgerald was being explored recently there had been a report that they’d found human remains near the ship. But the remains couldn’t have been any of the crew - because it was wearing lifepreserving equipment from the 1880s. That implies to me that deep fresh water environments lack the ecosystem to disgest bones that an ocean has.
Which isn’t unreasonable when you consider that most deep water fresh water bodies are relatively young, and would need a lot of radiative evolution to fill those niches - since transport of the ocean varieties would be percluded by the huge hazard of osmotic differences between living in fresh water and living in salt water. Plus the difficulty in having a deep water creature surviving to get to or near surface pressures, then adapting back down to the deep water again.
Oh, I was wrong it’s Vaughan’s Mammology I’ve got, not Young’s. Okay, a quick run through has a discussion of the Hyenas being adapted to crushing and eating large bones, but not specifying that they’re taking nourishment from the bone matrix, or just the tissues distributed through the matrix.
Do deer antlers count as bone? I know that mice and other rodents gnaw on shed antlers, but I’ll admit that I don’t know if they actually “eat” them.
Reptiles definitely ingest and to some degree digest bones. Based on a fair amount of experience with the feces of rodent-eating snakes, I would say that they have more trouble digesting hair than bones. I’m not sure how much nutritional benefit reptiles derive from the bones they eat, but I believe that they get calcium from them.
It may be the case that some deer eat baby birds for the calcium in their bones.
This article was mentioned in another thread, on hooved predators. It would appear the deer are specifically eating the bones of the chicks for their calcium content. It’s still being researched and isn’t fully understood yet, though.
ETA: Dangit. Beaten to it.
There’s a couple of things here.
Bones contain both minerals and protein. Once the protein is gone you can’t get anything more than minerals from what’s left over.
Plenty of animals eat bones, and they digest the protein fraction. Then they excrete the leftover minerals, because they don’t need them. There’s only so much calcium a hyeana can use.
Small bones are almost always eaten…tail bones, vertebrae, ribs, parts of the skull, phalanges, and so forth. But heavy limb bones like the femur, tibia and humerus might be too strong for most predators to crack. They’ll suck out as much as possible, but can’t crush the bone itself. And of course, small animals are eaten whole.
So when you come across a bone out in the forest, there’s usually nothing digestible left except the mineral fraction. And nothing will eat that, unless they need those particular minerals. So it’s not correct that bones don’t get eaten, they do. But only the digestible portion of the bones can be digested. Wood is made of cellulose and lignin. There are organisms that can digest cellulose, and even lignin, but you can’t digest calcium and use it as an energy source.
Just out of curiousity - how much of the shape of the bone is due to the structure imparted by the proteins? What I mean, it seems to me that if bones are calcium heavy protein matrices, as long as there’s a recognizable shape, there’s bound to be still some protein fractions available there.
Granted, for most aninamls, because of the difficulty of breaking that matrix into smaller digestable pieces is hard, there are easier (and less energy intensive) sources of protein fractions. But that doesn’t mean that they don’t have any nutritional value.
Hyenas? Even a housecat can easily chew its way through a small rodent. I had always been curious about the exact process a cat follows when eating prey, so I looked it up on YouTube. Surprisingly, if the prey is small enough, the cat’s likely to just eat it head to tail, the way you or I might eat a hotdog.
Are antlers made of the same stuff as bones? Because there is some critter in the forest that eats fallen moose antlers in Alaska. If you don’t find them soon after they are shed, they will likely be gnawed on.
Try a Google search with “whale bones monterey” for articles and pictures of worms that are devouring the bones of a deceased whale - here’s an example article: “Whale carcass yields bone-devouring worms”
Wasn’t “The Octopus” a book about how Hormel used animal bones in various products. Ground fine enough, the bones add nutritive calcium to hot dogs.
The protein is mostly in the marrow, not the bone tissue itself. The bone tissue has very few, if any, calories to extract.