Bats’ and birds’ evolutionary paths are vastly different

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 that behave like ostriches - long-legged creatures 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 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 author of research published 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 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.

"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?"

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."

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

Elodie Smith is a writer for the College of Veterinary Medicine.

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