Why Bats Can’t Walk: The Evolutionary Lock That Keeps Them Flying

Bats and birds showcase a fascinating contrast in their evolutionary paths, with recent research revealing bats’ wings and legs evolve in unison, limiting their ecological roles compared to birds, whose limbs evolve independently.

This discovery offers new insights into why birds occupy a wider range of ecological niches and poses questions about the broader implications for other flying species like the diverse pterosaurs.

Unique Traits of Bats and Birds

Bats are remarkably diverse creatures, capable of climbing onto animals to drink their blood, snatching insects from leaves, or hovering to sip nectar from tropical flowers. Each of these behaviors relies on uniquely adapted wing designs.

But have you ever wondered why there are no flightless bats, like ostriches that wade along riverbanks for fish, or bats that roam the open seas, akin to the wandering albatross?

Scientists may now have the answer: Unlike birds, bats have evolved with a strong connection between the development of their wings and legs. This evolutionary coupling likely limits their ability to adapt to a wide variety of ecological roles, as birds have.

“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 in the Department of Biomedical Sciences, in the College of Veterinary Medicine.

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

Analyzing Bone Structures

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 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.

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.

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.

Implications for Pterosaur Diversity

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

The team’s findings raise 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,” Orkney said. “What was the secret to their evolutionary success?”

Ongoing Research in Avian Evolution

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

Reference: “Evolutionary integration of forelimb and hindlimb proportions within the bat wing membrane inhibits ecological adaptation” by Andrew Orkney, David B. Boerma and Brandon P. Hedrick, 1 November 2024, Nature Ecology & Evolution.
DOI: 10.1038/s41559-024-02572-9

Some of the measurements for this study were taken at the imaging facility of the Cornell Institute of Biotechnology.

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