Utilizing Micro-CT to Study the Evolution of Bats

Vaibhav Chhaya
PhD Candidate
Department of Biology, University of Washington, Seattle

Sharlene Santana, PhD
Professor and Curator of Mammals
Department of Biology and Burke Museum of Natural History and Culture
University of Washington, Seattle

Figure 1: 3D reconstruction of an Intermediate Roundleaf bat head (Hipposideros larvatus), Micro-CT (Skyscan 1172) scan at a voxel size of 13.4μm.

Bats are quite diverse in appearance, comprising more than 1,400 species, and thereby provide an excellent opportunity to study how evolutionary forces have shaped their morphology. At the University of Washington’s Department of Biology and Burke Museum, Dr. Sharlene Santana and PhD Candidate Vaibhav Chhaya are studying the diversity among bats using high resolution 3D micro-CT scans of the skulls of numerous bat species. By analyzing morphological changes that evolved over millions of years and correlating these changes with specific events in bat evolution, they have identified echolocation and diet as key forces shaping bat skull diversity.

Can you expand briefly on your focus in your research?

Our research focuses on the evolutionary biology of bats, the second-most diverse group of mammals. Like any other organism, bats need to use their anatomy to accomplish tasks or functions to survive and reproduce, and we study how the performance of these tasks is determined by the morphology of the underlying anatomical parts. Specifically, our current research project examines how the internal and external structure of the bat skull –particularly the snout– affects their ability to feed, smell, echolocate, and moisten and warm inhaled air.

You mention the performance of the tasks the bat performs is governed partly by the morphology of the skull — can you expand on that?

It means that the shape of the skull and the bones within the nasal cavity determine how well a bat can conduct a specific task. For example, a bat with a short face can produce a greater bite force for its size, but that could also mean that its nose has less space for olfactory and respiratory structures.

Figure 2: 3D reconstructions of an Intermediate Roundleaf bat head (Hipposideros larvatus), Micro-CT (Skyscan 1172) scan at a voxel size of 13.4μm.

Can you explain more about bat echolocation — what is it used for, why do you study that, and how might that apply to biomimicry for the development of sound propagation?

Bats possess one of the most advanced echolocation systems in the animal kingdom. They are not only able to fly and maneuver in the dark through cluttered environments such as dense forests but are also able to locate (and in some cases even classify!) their food just by using sound. It is extremely remarkable to us how these capabilities can be packed into a cute little fuzzball, and how such sophistication can arise as an outcome of natural evolutionary processes.

Besides evolutionary insight, studying bat echolocation can inspire the development of improved sonar systems and sound sensors that have a wide range of applications, particularly in robotics (examples: https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1006406; https://phys.org/news/2015-05-robotic-sonar.html).

What is important about this research today, and what are you hoping to achieve through your work?

Studying these structure-function relationships gives us crucial insight into the evolutionary processes that generate diversity in bats and life on Earth in general. Broadly speaking, our field of research helps us to understand how organisms are able to adapt and survive in response to changing environments, and this knowledge can inform conservation efforts, especially with the onset of rapid climate change. Finally, form-function relationships may offer clues to complex engineering problems and inspire technological innovations.

How are you using micro-CT technology in your research?

We use micro-CT technology to obtain detailed three-dimensional information about anatomical structures in bats, particularly the skull. Our research involves scanning dry and fluid-preserved museum specimens, both of which allow us to study different aspects of craniofacial anatomy. For the fluid specimens, we use Lugol’s iodine as a stain to enhance the radiodensity of soft tissues such as mucosa, muscles, and other integumentary structures. From these CT scans, we are then able to generate three-dimensional models of anatomical structures, study their morphological properties such as shape, size, and surface area, and simulate their performance in different behaviorally relevant scenarios using computational biomechanical models.

Is there anything you can say about micro-CT vs. other methodologies, or how you use micro-CT in tandem with other technologies?

Bat skulls are generally small, and the structures we are studying are inside the nasal cavity. Therefore, micro-CT scanning is one of the few methods that allows us to visualize them while keeping the specimens intact. For a subset of specimens, we are coupling micro-CT and contrast-enhanced micro-CT with histological studies of nasal epithelia to more finely assess the function of the turbinal bones inside the nasal cavity.

What has been the most important discovery or unexpected outcome in your research?

In one of our projects, we have been studying the morphology of the turbinal bones, which are located inside the nasal cavity and are crucial for olfaction and respiratory conditioning. After comparing the surface areas of these bones across bat species, we have found that they may additionally contribute to echolocation! We are currently working on simulating how sound propagates through bat nasal cavities that have varying turbinal morphologies to understand the effect of these bones on echolocation sounds.

Figure 2: 3D reconstructions of nasal cavity morphology in a Lesser short-nosed fruit bat. Cynopterus brachyotis (left) and a Greater horseshoe bat, Rhinolophus ferrumequinum (right), Micro-CT (Skyscan 1172) scan at a voxel size of 13.4μm.

Do you have any key publications you suggest we cite for further reading?

Arbour J.H., Curtis A.A. and Santana S.E. 2021. Sensory adaptations reshaped intrinsic factors underlying morphological diversification in bats. BMC Biology 19: 88.

Arbour J.H., Curtis A.A. and Santana S.E. 2019. Arbour J.H., Curtis A.A. and Santana S.E. 2019. Signatures of echolocation and dietary ecology in the adaptive evolution of skull shape in bats. Nature Communications 10: 2036.

Contact information:
Sharlene Santanassantana@uw.edu
Vaibhav Chhaya: chhaya@uw.edu

If you are interested in being featured in a future Researcher Spotlight, please contact ann@microphotonics.com. We love to hear how our instruments are being used in the field!

 

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