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.
Jason Koski/Cornell University
Unlike an intelligent designer, the process of evolution can't go back to basic and start again. It is normally an additive process that has no control over what it has to work with and simply refines and improves on what is there. That's not to say new structures can't evolve but they do so by enlarging or remodelling something that was already there - the membrane of a bat's wing, for example is the webbing that exists in the tetrapod embryo between the fingers and toes, while the feathers of a bird's wing are highly modified scales. Both those structures evolved out of tissues that were already there. It would have been impossible for a bat to grow wing feathers instead of a membrane, for example, because the earliest mammals had lost their scales and evolved fur.
But of course, that would not have been a problem for an omnipotent intelligent designer who, having designed one wing would not need to set about designing another way to do the same thing.
So, constrained as evolution was by what it could use, it's not really surprising that birds and bats evolved on two different trajectories, with the only thing in common being flight (and of course the basic vertebrate skeletal body plan).
Evolution of birds and bats and why they haven't converged on the same wing design. The evolution of birds and bats offers a fascinating look at how different evolutionary paths can lead to similar functions—in this case, powered flight—while maintaining distinct anatomical differences. Birds and bats evolved wings independently, through a process called *convergent evolution*, which means they developed similar abilities due to similar selective pressures but not from a shared ancestor with wings. This independent evolution has resulted in significant differences in wing structure due to their distinct lineages.Now a group of researchers at Cornell University have shown that the evolutionary trajectory of bats was also constrained by the fact that, unlike birds, their hind legs are an integral part of the wing design, and this has inhibited the way the legs have evolved. In birds, the legs play no part in flight, so they were free to evolve in different ways, for perching, wading, running (as in the ostrich) or grasping prey, as in the raptors.
- Origins and Lineages
- Birds evolved from theropod dinosaurs around 150 million years ago, with early ancestors like *Archaeopteryx* showing both avian and dinosaur traits. Their wings evolved from forelimbs, with feathers that initially likely served for insulation or display before adapting to aid in flight.
- Bats, on the other hand, evolved from small, nocturnal mammals roughly 50–60 million years ago. Bats’ wings formed as their forelimbs elongated, with skin stretching between the fingers and down the body, allowing for a wing membrane rather than feathers.
- Wing Structure and Mechanics
- Bird Wings are primarily composed of feathers attached to a relatively rigid arm structure. The feathers play a crucial role in generating lift and controlling airflow. Birds use powerful chest muscles to move their wings, enabling efficient gliding and sustained flapping flight.
- Bat Wings are made up of a thin skin membrane, or *patagium*, stretched across elongated finger bones. This flexible membrane allows bats to manoeuvre exceptionally well, especially in confined spaces, by altering the shape of their wings to change direction quickly. Each finger is independently controlled, giving bats finer movement control.
- Constraints and Advantages of Each Design
- Bird Wings are ideal for efficient, high-speed flight and long-distance gliding due to the lightweight structure of feathers and the rigid wing frame. However, this design limits manoeuvrability at low speeds and in tight spaces.
- Bat Wings provide greater agility and manoeuvrability, making bats adept at navigating dense forests and caves. However, the flexible membrane is more prone to injury, and bats are generally less efficient at long-distance flying compared to birds.
- Environmental Adaptations and Niche Specialization
- Birds and bats occupy different ecological niches due to these anatomical and functional differences. Birds, with their diverse feather structures, are found in a wide range of environments, from open skies to grasslands, while bats are adapted to nocturnal life and often rely on echolocation for hunting, which works well with their manoeuvrable wing design.
- Evolution has shaped their wings according to these distinct lifestyles and habitats. For example, large birds like albatrosses have long, narrow wings for efficient soaring over open water, while small birds like hummingbirds have specialized wings for rapid, hovering flight. Bats, however, rarely reach the same level of aerodynamic efficiency as birds for sustained high-speed flight due to the mechanics of their flexible wings.
- Why They Haven't Converged on a Single Wing Design
- Convergence on an identical wing structure wasn’t possible because the evolutionary starting points—theropods for birds and small mammals for bats—imposed unique structural constraints on each lineage. The development of a feather-based wing versus a membrane-based wing was influenced by these evolutionary origins, and once established, each design further specialized within its own context.
- Additionally, each group’s wing structure offers unique advantages that align with their ecological needs. Birds’ feathered wings allow for a high lift-to-drag ratio, aiding in long flights and gliding, while bats’ membrane wings provide the precision needed for nocturnal hunting in cluttered environments.
In summary, birds and bats exemplify how evolution can produce similar solutions to environmental challenges—flight—through very different designs, each suited to the unique needs and constraints of the organisms involved. This diversity in wing design reflects the adaptive versatility of life and the specific ecological niches these animals fill.
The Cornell team has recently published their findings in Nature Ecology & Evolution and in an article by Elodie Smith, of the College of Veterinary Medicine, in Cornell Chronical.
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.
Dr. Andrew Orkney, co-corresponding author.
College of Veterinary Medicine
Department of Biomedical Sciences
Cornell University, Ithaca, NY, USA.
Andrew Orkney, a researcher in the laboratory of Assistant Professor, Brandon Hendrick. Both are authors 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.
Assistant Professor, Brandon P. Hendrick, co-corresponding author.
College of Veterinary Medicine
Department of Biomedical Sciences
Cornell University, Ithaca, NY, USA.
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. What was the secret to their evolutionary success?
Dr. Andrew Orkney.
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.
Dr. Andrew Orkney.
Some of the measurements for this study were taken at the imaging facility of the Cornell Institute of Biotechnology.
AbstractJust as the TOE predicts then, starting with different materials, but responding to the same environmental selection pressures, the wings of the two types of flying vertebrates, bats and birds, the solutions were different, and, because the wing membrane of bats was literally tied to their legs, bats' legs were not free to evolve independently so have limited functions while birds have evolved several different functions for their legs.
Bats and birds are defined by their convergent evolution of flight, hypothesized to require the modular decoupling of wing and leg evolution. Although a wealth of evidence supports this interpretation in birds, there has been no systematic attempt to identify modular organization in the bat limb skeleton. Here we present a phylogenetically representative and ecologically diverse collection of limb skeletal measurements from 111 extant bat species. We compare this dataset with a compendium of 149 bird species, known to exhibit modular evolution and anatomically regionalized skeletal adaptation. We demonstrate that, in contrast to birds, morphological diversification across crown bats is associated with strong trait integration both within and between the forelimb and hindlimb. Different regions of the bat limb skeleton adapt to accommodate variation in distinct ecological activities, with flight-style variety accommodated by adaptation of the distal wing, while the thumb and hindlimb play an important role facilitating adaptive responses to variation in roosting habits. We suggest that the wing membrane enforces evolutionary integration across the bat skeleton, highlighting that the evolution of the bat thumb is less correlated with the evolution of other limb bone proportions. We propose that strong limb integration inhibits bat adaptive responses, explaining their lower rates of phenotypic evolution and relatively homogeneous evolutionary dynamics in contrast to birds. Powered flight, enabled by the membranous wing, is therefore not only a key bat innovation but their defining inhibition.
Orkney, A., Boerma, D.B. & Hedrick, B.P. Evolutionary integration of forelimb and hindlimb proportions within the bat wing membrane inhibits ecological adaptation. Nat Ecol Evol (2024). https://doi.org/10.1038/s41559-024-02572-9
© 2024 Springer Nature Ltd.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
No such constraints would have mattered to an intelligent, omnipotent designer, however.
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