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Flight as a means of locomotion for certain species has evolved more than once. This ought to be clear, for example, in considering the differences between birds and flying insects, since vertebrates and arthropods diverged in evolution long before either animal type evolved the ability to fly. But even within a the phylum of vertebrates, flight has evolved more than once, specifically, birds and bats. (You can throw in flying squirrels as a third separate evolution of flight among vertebrates.)
However, if one considers the sorts of niches occupied by birds with those occupied by bats, it becomes apparent that birds have a far more diverse roster of modes of survival than do bats. After all, while most birds do indeed make their living while flying, be that while hunting or seeking nectar, others, such as ostriches, kiwis, and emus, have evolved to forego flight in favor of foraging or hunting on land. Penguins have also lost flight, but in favor of making their living in the oceans. Meanwhile, while bat species are also have diverse modes of survival, there are no bat species that have evolved to forego flight entirely.
The first birds evolved in the late Cretaceous period about 65 million years ago ( somewhat before the extinction of the dinosaurs by a meteor), while the earliest evidence of bat fossils dates to about 50 million years ago. As such, both animal groups have been around for comparable amounts of time, but the birds have diversified far more than the bats.
A recent report comparing correlations in bird and bat anatomy provides clues as to how (if not why) this difference has come about.
The team measured the wing and leg bones of 111 bat species and 149 bird species from around the world. Their dataset included X-rays of museum specimens and about a third of the new X-rays of bat specimens stored at the Cornell University Museum of Vertebrates.
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They observed in both bats and birds that the shape of the bones within a species' wing (handwing, radius, humerus), or within a species' leg (femur and tibia) are correlated—meaning that within a limb, bones evolve together. However, when looking at the correlation across legs and wings, results are different: Bird species show little to no correlation, whereas bats show strong correlation.
Bat legs form the bottom anchor of their wings, and so it shouldn’t come as a big surprise that their leg size correlates to their wing size (see figure above). Bird wings, on the other hand, are independent of their legs, and so wing size and leg length need not have anything to do with each other. Thus, the evolutionary processes of these two groups of flying vertebrates have been very different, despite their apparent similarity.
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