section_path: Other animals
section_name: Other animals
paragraph_text: Other animals
  
  Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation. Many bird bones have little marrow due to them being hollow.
  
  A bird's beak is primarily made of bone as projections of the mandibles which are covered in keratin.
  
  Some bones, primarily formed separately in subcutaneous tissues, include headgears (such as bony core of horns, antlers, ossicones), osteoderm, and os penis / os clitoris. A deer 's antlers are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed.
  
  The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws.
  
  The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine turtles, or in various Mesozoic marine reptiles, such as ichthyosaurs, among others. This proportion can vary quickly in evolution; it often increases in early stages of returns to an aquatic lifestyle, as seen in early whales and pinnipeds, among others. It subsequently decreases in pelagic taxa, which typically acquire spongy bone, but aquatic taxa that live in shallow water can retain very thick, pachyostotic, osteosclerotic, or pachyosteosclerotic bones, especially if they move slowly, like sea cows. In some cases, even marine taxa that had acquired spongy bone can revert to thicker, compact bones if they become adapted to live in shallow water, or in hypersaline (denser) water.
  
  Many animals, particularly herbivores, practice osteophagy —the eating of bones. This is presumably carried out in order to replenish lacking phosphate.
  
  Many bone diseases that affect humans also affect other vertebrates—an example of one disorder is skeletal fluorosis.