Why didn't bats evolve to walk on land like birds?

Bats have evolved to display a variety of different skills, such as plucking insects off leaves or hovering over flowers to drink nectar. Despite their diverse behaviors, one question has always lingered: why don’t we see any flightless bats?

There are birds that waddle along riverbanks like ostriches, or spend their lives by the sea like the wandering albatross. But why not bats?

The answer to this question might have just been found by a group of researchers at Cornell University.

Bat-bird evolution paradox

It is important to understand that the evolutionary pathways of bats and birds are markedly different.

The evolution of bats’ wings and legs is tightly coupled, which seems to have limited them to fewer ecological niches than birds. A surprising finding was that the opposite is true for our feathered friends.

“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 Professor Brandon Hedrick.

Flight evolution in birds and bats

The concept of wings and legs evolving independently makes intuitive sense, considering they perform different functions.

Prior research had suggested that the origin of flight in vertebrates required forelimbs and hindlimbs to evolve independently, thereby allowing them to adapt to their distinct tasks more effectively.

So, comparing bats and birds presented an ideal situation to test this idea. Since they do not share a common flying ancestor, birds and bats are independent examples to study flight evolution.

Bats evolve differently

The research team measured the wing and leg bones of 111 bat species and 149 bird species from around the world. The analysis included X-rays of museum specimens and new X-rays of bat specimens stored at the Cornell University Museum of Vertebrates.

A stark contrast in the evolution of these two flight capable creatures was noted. In both bats and birds, the shape of the bones within a species’ wing (handwing, radius, humerus) or leg (femur and tibia) was found to be correlated – meaning that within a limb, bones evolve together.

However, the correlation across legs and wings was significantly different in bats and birds. While bird species showed little to no correlation, bats displayed a strong one.

Essentially, in bats, forelimbs and hindlimbs did not evolve independently. When the wing shape changes (either increases or decreases) the leg shape follows the same trend.

Ecological implications of bat morphology

The coupled evolution of bats‘ wings and legs has profound ecological implications. Bats are predominantly adapted to aerial niches, which constrains them mainly to environments where flight is advantageous.

This is in contrast to birds, whose limb independence allows them to occupy diverse habitats, ranging from arid deserts to aquatic environments.

The morphological constraints on bats mean that evolutionary pressures have favored adaptations which enhance aerial agility and efficiency, such as echolocation and varied wing shapes.

Consequently, bats have become specialized fliers, excelling in niches that demand proficient maneuverability and speed.

However, this specialization may come with trade-offs, limiting their ability to exploit non-aerial ecological niches compared to the more versatile birds.

Future research directions

The discoveries from this research open avenues for further investigation into the evolutionary biology of flight.

Understanding how bone morphology influences ecological roles can provide deeper insights into the evolutionary constraints and drivers that shape species diversity.

Future studies could explore the genetic and developmental factors underpinning the tight coupling between bat wings and legs. Additionally, research could focus on comparative studies across more vertebrate groups to dissect the underlying evolutionary mechanisms that enable or restrict limb independence.

Such inquiries not only enhance our comprehension of evolutionary processes but also aid in conservation strategies by predicting how species may adapt to changing environments.

Bats evolved to stay airborne

The researchers acknowledge that the coupled evolution of wing and leg in bats could limit their capacity to adapt to new ecologies. However, this discovery has stirred interest in other areas.

For instance, it raises questions about the evolution of pterosaurs, an extinct group of flying reptiles that had membranous wings akin to those of bats.

With pterosaurs being significantly more diverse than either birds or bats, this research might shed some light on the factors behind their evolutionary prowess.

Following their discovery, the researchers have now redirected their focus to the evolution of bird skeletons.

“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.

The study is published in the journal Nature Ecology & Evolution. 

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–