What about those hard things that protrude through the scalp on moose and reindeer, through the feet on hoofed animals, and through the gums on most vertebrates? Are they bone? In the instance of teeth, no. Although teeth and bone are both hard and calcium-rich, they have entirely different chemistries and structures. Same for tusks, which are continuously growing front teeth. They are ivory.
What about hooves, claws, beaks, and maybe your mother-in-law’s fingernails?
Yes, all are potentially dangerous weapons and of definite protective value, but they are not bone. Rather they are all made of keratin, which is a fibrous protein with some similarities to the protein, collagen, that constitutes bone. Since keratin does not have any calcium crystals deposited into its mesh work, it is more flexible than bone and weighs less. For instance, we have a fine mesh work of keratin in our skin, which makes it a tough protector of our precious bones. Cow, bison, and antelope horns and turtle shells are bone covered with keratin. Certain ancient fish have scales made of bone, again covered with keratin. These scales are not connected to the fish’s bony skeleton but merely provide a flexible layer of bony armor.
A point of confusion surrounds the term whalebone. It can refer to the whale’s actual bone, or it can refer to baleen, which certain whales use to filter tasty krill from big gulps of seawater. Baleen is another exposed body part made of keratin. The long flexible strands of baleen were valued in the 19th century for making collar and corset stays, buggy whips, and umbrella ribs. Today, reinforcements for corsets, hoop skirts, and costumes are made strips of steel, plastic, or reed, but they are still called boning. I guess it is just easier to say than keratining.
Up to this point in the discussion, there is no naturally exposed bone in the animal kingdom. I suppose most animals disdain the thought of seeing their own bone. I certainly abhor the thought of ever gazing at mine.
There are two exceptions to the generalization that living bone is out of sight. The most obvious one is antlers. To me, moose look like slobs, but maybe they like seeing the reflection of their magnificent antler rack of exposed bone while standing chest deep in muck. Male moose, reindeer, and other members of the deer family grow and then shed antlers, usually annually. Antlers sprout from the skull and grow faster than any other mammalian bone. Just like most other bones, an antler grows longer from the area immediately beneath the tip’s cartilage cap. During growth, the antler is covered by a thin layer of velvety skin rich with blood vessels. These supply the necessary nutrients for rapid growth. Once growth is complete, the velvet gets rubbed off, and the bone dies since it no longer has a blood supply. The antlers are out there for everybody to gawk at until osteoclasts, the bone destroying cells, at the antler’s base destroy its attachment to the skull. Then no more exposed bone until next year. Left on the ground, the shed antlers are an excellent source of calcium for Earth’s little creatures, who gnaw on them happily.
The other exception to naturally exposed bone is the shield-like skin on some reptiles and frogs and on a few mammals. This specialized skin is called ostederm (literally, bone-skin). It provides a flexible defensive armor and accounts for the bumpy skin on a gila monster (reptile) and for the impressive shield on an armadillo (mammal). Crocodiles make two additional uses of their osteoderm. When the croc is out of the water on a cold day, its bone-skin, which has a great blood supply, can serve as a solar collector and bring heat to warm the owner’s innards. On a hot day, the heat reverses its course through the osteoderm. Or the beast may decide to totally submerge itself and wait for me. While holding its breath, it slows carbon dioxide accumulation and blood acidification by temporarily exchanging bad ions in the blood for good ions in its osteoderm. That way it can say down longer. What happens next can be either good or bad depending on one’s perspective.
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