EVOLUTION AND FUNCTIONAL MORPHOLOGY OF THE MAMMALIAN FEEDING APPARATUS

         As a Research Associate in the Santana Lab in the Department of Biology at The University of Washington, I am working with a team of researchers from multiple institutions on an NSF-funded project to understand the evolution and function of the mammalian feeding apparatus. We are specifically interested in how skull shape and the muscles that operated the jaws during feeding are related to what mammals eat, and testing for common patterns in craniomuscular evolution that are linked to diet. To do this, we are compiling a large comparative 3-dimensional dataset of skull and jaw muscle anatomy in bats, carnivores, and primates using a combination of non-destructive CT scanning and traditional dissections. These three groups represent three of the most taxonomically diverse mammalian orders and include species that independently evolved a variety of diet types ranging from herbivores to omnivores to carnivores, etc.

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On the left, is a video fly-through of the head of a bat facing with its nose to the right in which the muscles and other soft tissues were stained using Lugol's iodine, which makes them visible. The video ends at approximately the midline of the head so you can see the brain and tongue. On the right, is a model of a bat skull (facing with the nose toward the left) and the muscles that open and close the jaws. These models were created using specialized imaging software to digitally dissect the skull and muscles from CT scans like the one to the left.

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         We CT scan museum specimens to obtain digital 3-dimensional replicas of bat, carnivore, and primate skulls, as well as to image the jaw muscles using staining techniques that make soft tissues visible in scans. This allows us to reconstruct the skulls with jaw muscles as they would look in living taxa, from which we can measure size, shape, and orientation of muscles among mammals with different diets. We also perform traditional dissections to measure muscle microstructures, such as muscle fiber lengths and muscle mass, from which we hope to build more accurate models to estimate bite forces, as well as to compare with measurements taken from CT data.

 

 

 

 

 

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Biomechanical models showing distribution of stresses (hot colors = high stress, cool colors = low stress) across the skull while biting with both (bilateral) or one (unilateral) canine teeth based on a CT scan of the skull of a hoary bat that has a large, naturally occurring cleft palate. Using CAD software, we were able to digitally fill the cleft. We then modeled the forces and attachment sites for the jaw-closing muscles on both models and compared distributions of stress (shown), strain, and ability to produce bite force using an engineering method called finite element analysis. This allowed us to test how presence of a cleft between the teeth impacts feeding performance in bats, in which this trait has evolved at least eight times independently.

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         Compiled across bats, carnivores, and primates, these datasets should reveal the diversity of craniomuscluar morphology like never before, and whether there are common craniomuscular traits that evolve in tandem with similar diets. Do all species that eat hard foods show similar trends in skull and muscle morphologcial evolution? Are there multiple morphological solutions to the same diet? Do other things, like echolocation in bats, also drive craniomuscular evolution? We are answering these, and many other questions, with the data we are collecting!

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