Bats are incredibly diverse animals: They can climb onto other animals to drink their blood, pluck insects from leaves, or hover to drink nectar from tropical flowers – all of which require distinctive wing designs. But why aren’t there any flightless bats like ostriches, long-legged bats that wade along riverbanks for fish like herons, or bats that spend their lives at sea like the wandering albatross?

Researchers may have just found the answer: Unlike birds, the evolution of bats’ wings and legs is tightly coupled, which may have prevented them from filling as many ecological niches as birds.

“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 Dr. Andrew Orkney, postdoctoral researcher in the laboratory of Dr. Brandon Hedrick, assistant professor in the Department of Biomedical Sciences. Both researchers are co-corresponding author of the paper that was published in Nature Ecology and Evolution on Nov 1.

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 to test this idea because they do not share a common flying ancestor and therefore constitute independent replicates to study the evolution of flight.

The team measured the wing and leg bones of 111 bat species and 149 bird species from all over the world. Their dataset included X-rays of museum specimens, and about a third of new X-rays from bat specimens stored at the Cornell University Museum of Vertebrates (CUMV).

They observed that in both bats and birds, 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 a 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,” said Hedrick.

The team’s findings open questions about the evolution of pterosaurs, an extinct group of flying reptiles that had membranous wings similar to those of bats. “Pterosaurs were a lot more diverse than either birds or bats, ranging from tiny insectivores to giraffe-sized Goliaths that rivaled the dinosaurs,” said Orkney. “What was the secret to their evolutionary success?”

Following their discovery, the team started 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, we still don’t know why birds are able to do this or when it began to occur in their evolutionary history,” said Orkney.

For this work and their upcoming research, Orkney and Hedrick both say that the importance of historical specimen collections cannot be understated. “We are deeply grateful for the support we received from the CUMV and other institutions at Cornell, such as the imaging facility of the Cornell Institute of Biotechnology, and we thank Casey Dillman, Vanya Rohwer, and Teresa Porri, in particular.”

Written by Elodie Smith

A modified version of this story appears in the Cornell Chronicle.