Evolutionary paths vastly differ for birds, bats

"We initially expected to confirm that bat evolution is similar to that of birds, and that their wings and legs evolve independently of one another. The fact we found the opposite was greatly surprising," said Andrew Orkney, postdoctoral researcher in the laboratory of Brandon Hedrick, assistant professor biomedical sciences.

Both researchers are co-corresponding authors of research published on Nov. 1 in Nature Ecology and Evolution.

Because legs and wings perform different functions, researchers had previously thought that the origin of flight in vertebrates required forelimbs and hindlimbs to evolve independently, allowing them to adapt to their distinct tasks more easily. Comparing bats and birds allows for the testing of this idea because they do not share a common flying ancestor and, therefore, constitute independent replicates to study the evolution of flight.

The researchers observed in both bats and birds that the shapes 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.

This means that, contrary to birds, bats' forelimbs and hindlimbs did not evolve independently: When the wing shape changes -- either increases or shrinks, for example -- the leg shape changes in the same direction.

"We suggest that the coupled evolution of wing and leg limits bats' capability to adapt to new ecologies," Hedrick said.

Following their discovery, the team began re-examining the evolution of bird skeletons in greater depth.

"While we showed that the evolution of birds' wings and legs is independent, and it appears this is an important explanation for their evolutionary success," Orkney said, "we still don't know why birds are able to do this or when it began to occur in their evolutionary history."