One of the most incredible evolutionary adaptations in the animal kingdom is doubt-less the one that gave some animals the power to fly. While we generally consider feathers to be crucial to this success, it is no less true that significant physiological and skeletal changes were also selected in order to achieve this technical feat. Hence the skeleton of flying animals has to combine two contradictory characteris-tics. It needs to be as light as possible in order to reduce to a minimum the effort re-quired to overcome gravitational force. In addition, the skeleton must allow the pow-erful wing muscles to lift the body, meaning that it has to be especially resilient, since it will be subjected to immense forces.
So most avian bones are hollow and thus lighter, but they are reinforced with inter-nal partitions to give them rigidity. In some birds, the air sacs in the respiratory sys-tem even extend into the larger bones, further decreasing overall body density.
Some bones develop dramatically or incorporate apophyses, small outgrowths that provide a firm anchorage for the powerful muscles so they can function correctly.
These distinctive features are especially obvious in the enlarged breastbone (wish-bone) to which the powerful wing muscles are attached, or in radial apophyses in some birds.
Finally, some bones are fused together, increasing their rigidity and giving the skele-ton particular resilience. Hence the ribs are joined together by uncinate processes, which greatly strengthen the rib cage and prevent it from being crushed by the mov-ing wing muscles. Likewise, many vertebrae and the pelvic girdle bones are fused together, strengthening the tail.
Despite all these adaptations, flight would be impossible if the forelimbs hadn’t also undergone major anatomical changes. So forelimbs were transformed into wings, the prerogative of all flying animals, whether birds or bats.
In birds, the principal change involved the bones in the hand. Here birds only have three fingers, but the bones are arranged in such a way that the surface is increased considerably. These two features mean that the limb weighs less, and there is a larger area to accommodate feathers.
Finally, in chiropteran mammals i.e. bats, the only ones capable of active flight, the forelimb presents a series of changes related to the capacity for flying. The humerus is relatively short, making the radius the main bone in the forearm. Just like birds, the bones in the hand are highly modified. Although there is no reduction in the number of fingers, the metacarpal bones and phalanges have undergone extraordi-nary elongation. These bones, like the spokes of an umbrella, extend the patagium, which is the membrane of skin that forms the wing’s supportive surface.