Periodic Reporting for period 1 - HeadStrong (On the head of snakes: form, function and adaptation to life in dense media)
Période du rapport: 2022-04-04 au 2024-04-03
Context
Animals that live in dense media such as water, sand or soil undergo strong mechanical forces, especially during locomotion. Despite this, many snake species can burrow or swim efficiently. Because snakes lack limbs, their heads must face the environmental constraints associated with locomotion and this makes the snake head is an excellent system in which to study how physical characteristics of the environment shape phenotypic adaptation. Several studies on different aspects of snake head morphology have demonstrated similarities between fossorial and aquatic snakes. Yet, none of these studies have explored the hypothesis of a functionally driven convergence in response to a shared environmental constraint. Overall, the link between snake morphology and functional demands imposed by the physical properties of their habitat has been largely understudied. Most previous comparative work found a relationship between the shape of head structures in snakes and their ecology, but none has investigated functional hypotheses that could explain these results. On the other hand, the ability of snakes to move efficiently has fascinated engineers and physicists who have studied the detailed kinematics of snake locomotion in different substrates. Yet these functional studies lack a comparative morphological context because they focused on only a small range of species and largely not from an evolutionary perspective. This project aims to fill the gap between these two worlds by revealing the links between head morphology and the biomechanical requirements of environments for snakes. HeadStrong will test the explicit hypothesis that substrate density has driven the evolution of the head and skull shape of snakes.
Objectives
The main goal of this project is to compare the shape of the skull of many species of snakes that inhabit various substrates and to highlight the specific adaptations related to locomotion in these substrates. To achieve this, HeadStrong has been designed with four Research Objectives:
1) Is the shape of the braincase related to locomotor substrate in snakes?
The first objective of HeadStrong is to characterize the link between substrate density and skull shape.
2) Are the inner and outer layers of the braincase independent in shape?
The braincase of snakes plays two major roles; providing support for locomotion and hosting and protecting the brain. It is thus composed of an outer layer -the exocranium—the shape of which should be related to locomotor abilities while the inner layer –the endocranium—hosts and protects the brain. The second objective is to evaluate whether the inner and outer layer covary in shape more strongly when the animals undergo a strong mechanical constraint.
3) Does the force needed to penetrate a substrate depend on shape and substrate properties?
Once the links between shape and locomotor substrate are highlighted, to demonstrate that these differences in shapes are the results of an adaptation to the mechanical constraints related to each substrate, I will measure the efficiency of each shape to penetrate the different substrates. This will allow an assessment of the adaptation of the skull to locomotion in dense substrates.
4) How good is the braincase at protecting the brain?
Because the skull has a second crucial role –protecting the brain—I will compare the efficiency of each braincase shape to reduce and distribute the stress and strain on the brain and soft sensory tissues using mechanical simulations.
Animals that live in dense media such as water, sand or soil undergo strong mechanical forces, especially during locomotion. Despite this, many snake species can burrow or swim efficiently. Because snakes lack limbs, their heads must face the environmental constraints associated with locomotion and this makes the snake head is an excellent system in which to study how physical characteristics of the environment shape phenotypic adaptation. Several studies on different aspects of snake head morphology have demonstrated similarities between fossorial and aquatic snakes. Yet, none of these studies have explored the hypothesis of a functionally driven convergence in response to a shared environmental constraint. Overall, the link between snake morphology and functional demands imposed by the physical properties of their habitat has been largely understudied. Most previous comparative work found a relationship between the shape of head structures in snakes and their ecology, but none has investigated functional hypotheses that could explain these results. On the other hand, the ability of snakes to move efficiently has fascinated engineers and physicists who have studied the detailed kinematics of snake locomotion in different substrates. Yet these functional studies lack a comparative morphological context because they focused on only a small range of species and largely not from an evolutionary perspective. This project aims to fill the gap between these two worlds by revealing the links between head morphology and the biomechanical requirements of environments for snakes. HeadStrong will test the explicit hypothesis that substrate density has driven the evolution of the head and skull shape of snakes.
Objectives
The main goal of this project is to compare the shape of the skull of many species of snakes that inhabit various substrates and to highlight the specific adaptations related to locomotion in these substrates. To achieve this, HeadStrong has been designed with four Research Objectives:
1) Is the shape of the braincase related to locomotor substrate in snakes?
The first objective of HeadStrong is to characterize the link between substrate density and skull shape.
2) Are the inner and outer layers of the braincase independent in shape?
The braincase of snakes plays two major roles; providing support for locomotion and hosting and protecting the brain. It is thus composed of an outer layer -the exocranium—the shape of which should be related to locomotor abilities while the inner layer –the endocranium—hosts and protects the brain. The second objective is to evaluate whether the inner and outer layer covary in shape more strongly when the animals undergo a strong mechanical constraint.
3) Does the force needed to penetrate a substrate depend on shape and substrate properties?
Once the links between shape and locomotor substrate are highlighted, to demonstrate that these differences in shapes are the results of an adaptation to the mechanical constraints related to each substrate, I will measure the efficiency of each shape to penetrate the different substrates. This will allow an assessment of the adaptation of the skull to locomotion in dense substrates.
4) How good is the braincase at protecting the brain?
Because the skull has a second crucial role –protecting the brain—I will compare the efficiency of each braincase shape to reduce and distribute the stress and strain on the brain and soft sensory tissues using mechanical simulations.
Work accomplished
1) Is the shape of the braincase related to locomotor substrate in snakes?
I found that the shape of the braincase of snakes is indeed related to their locomotor substrate, with what appears to be a streamlining from the denser to the less dense substrates (Figure 1). Size also plays an important role in shaping the braincase, with larger species having a long and slender braincase while smaller species’ braincase is short and slender. Size is also related to substrate density; larger species are found in less dense substrates and smaller species in dense substrates. To sum up, the morphology of the braincase of snakes is driven by substrate density: the denser the substrate, the smaller and more streamlined the braincase, which suggests a potential mechanical advantage that is being tested in part 3 and 4.
2) Are the inner and outer layers of the braincase independent in shape?:
The inner and outer layer of the braincase strongly covary in shape, but this covariation does not seem to be related to the density of the substrate. In part 4, I am testing whether this covariation provides a mechanical advantage to the species to protect the brain and
sensory tissues.
3) Does the force needed to penetrate a substrate depend on shape and substrate properties?
This part was developed at the Ecole Polytechnique Fédérale in Lausanne (EPFL), in the BioRobotics lab led by Professor Auke Ijspeert. The experimental setup has been developed with the help of a postdoctoral researcher, Qiyuan Fu and a student in engineering, Thibault Golaz. The setup allows to mimic headfirst penetration in different substrates and allows the mechanical properties of each substrate to be measured. The experiments were not completed before the end of this fellowship but are currently in progress.
4) How good is the braincase at protecting the brain
I went to University College London to run the simulations, in the Moazen lab, led by Professor Mehran Moazen, in collaboration with Emily Rayfield (University of Bristol). I found that the shape of the braincase impacts both the distribution and intensity of both stress and strain. The shapes associated with denser substrates distribute better the stress and strain both on the outer and inner layer of the braincase which suggest an adaptation to burrowing (Figure 3).
1) Is the shape of the braincase related to locomotor substrate in snakes?
I found that the shape of the braincase of snakes is indeed related to their locomotor substrate, with what appears to be a streamlining from the denser to the less dense substrates (Figure 1). Size also plays an important role in shaping the braincase, with larger species having a long and slender braincase while smaller species’ braincase is short and slender. Size is also related to substrate density; larger species are found in less dense substrates and smaller species in dense substrates. To sum up, the morphology of the braincase of snakes is driven by substrate density: the denser the substrate, the smaller and more streamlined the braincase, which suggests a potential mechanical advantage that is being tested in part 3 and 4.
2) Are the inner and outer layers of the braincase independent in shape?:
The inner and outer layer of the braincase strongly covary in shape, but this covariation does not seem to be related to the density of the substrate. In part 4, I am testing whether this covariation provides a mechanical advantage to the species to protect the brain and
sensory tissues.
3) Does the force needed to penetrate a substrate depend on shape and substrate properties?
This part was developed at the Ecole Polytechnique Fédérale in Lausanne (EPFL), in the BioRobotics lab led by Professor Auke Ijspeert. The experimental setup has been developed with the help of a postdoctoral researcher, Qiyuan Fu and a student in engineering, Thibault Golaz. The setup allows to mimic headfirst penetration in different substrates and allows the mechanical properties of each substrate to be measured. The experiments were not completed before the end of this fellowship but are currently in progress.
4) How good is the braincase at protecting the brain
I went to University College London to run the simulations, in the Moazen lab, led by Professor Mehran Moazen, in collaboration with Emily Rayfield (University of Bristol). I found that the shape of the braincase impacts both the distribution and intensity of both stress and strain. The shapes associated with denser substrates distribute better the stress and strain both on the outer and inner layer of the braincase which suggest an adaptation to burrowing (Figure 3).
Progress in the field
HeadStrong is the first project evaluating the impact of the mechanical properties of the environment on the evolution of species. The preliminary results suggest that it is important to step back from our classical interpretation of “ecology”, using categorical variables (aquatic, fossorial, arboreal…), and to use a more quantitative approach to characterize the relationship between organism and their environment to better understand evolutionary processes, and especially adaptation.
Impact
HeadStrong has been an opportunity to bridge gaps between scientific fields, namely evolutionary biology, biorobotics and mechanical engineering. This project has created a new network between prestigious academic institutions, the Natural History Museum, the Ecole Polytechnique Fédérale de Lausanne, University College London and University of Bristol.
Given the high quality of the results of this action, I am preparing a major manuscript for submission to a leading scientific journal. I will present the results of this action during two talks at the World Congress of Herpetology in August 2024.
During this action, I was involved in various outreach activities, namely two videos led by the Digital Media team at the NHM and several events in high schools during which I presented scientific career paths, the project, and its main results.
HeadStrong is the first project evaluating the impact of the mechanical properties of the environment on the evolution of species. The preliminary results suggest that it is important to step back from our classical interpretation of “ecology”, using categorical variables (aquatic, fossorial, arboreal…), and to use a more quantitative approach to characterize the relationship between organism and their environment to better understand evolutionary processes, and especially adaptation.
Impact
HeadStrong has been an opportunity to bridge gaps between scientific fields, namely evolutionary biology, biorobotics and mechanical engineering. This project has created a new network between prestigious academic institutions, the Natural History Museum, the Ecole Polytechnique Fédérale de Lausanne, University College London and University of Bristol.
Given the high quality of the results of this action, I am preparing a major manuscript for submission to a leading scientific journal. I will present the results of this action during two talks at the World Congress of Herpetology in August 2024.
During this action, I was involved in various outreach activities, namely two videos led by the Digital Media team at the NHM and several events in high schools during which I presented scientific career paths, the project, and its main results.