|MadSci Network: Zoology|
Tom, Thanks for your question, it is interesting to compare the adaptations of bats and birds for flight. Firstly, nearly all bones from all classes of vertebrate except fish are hollow when studied as decomposed or thoroughly cleaned specimens (and some fish bones are, too). In other words, animals had discovered that tubes are structurally much more efficient than solid rods long before engineers did ! Bird bones are actually hollow (air filled) in the living animal, presumably for lightness, but air also circulates within their skeleton and may assist with respiration. Mammal bones, while 'hollow' in that the solid parts of limb bones are tubular, are usually filled with fatty or "white" marrow, which is different from the "red marrow" at the ends of the bones where blood cells are manufactured. The large quantity of fat within large mammal limb bones makes them very difficult to clean thoroughly. Some parts of mammal bones, such as the sinuses in the skull, are air filled. Bats have marrow in their bones, just like any other mammal, but the extreme narrowness of their bones means that the marrow-filled cavities are proportionally smaller than in other mammals, effectively giving them lighter bones than other mammals, an alternative adaptation for flight.
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