Sam van Wassenbergh: aerodynamics behind lizards resistance to hurricanes

In our latest post, Sam van Wassenbergh from the University of Antwerp discusses his latest work ‘An aerodynamic perspective on hurricane-induced selection on Anolis lizards’. He presents the importance of functional traits trade-offs in species adaptations, highlights the need to use multidisciplinary approaches in science and shares his pride on working with his student.

Sam van Wassenbergh at the computer at University of Antwerp, Department of Biology.
Sam van Wassenbergh at the computer at University of Antwerp, Department of Biology.

About the paper

In our paper, we wanted to understand why some lizards are easily blown out of the trees in hurricane winds, while others do not. To do so, I studied the aerodynamics of lizards that are shielding themselves from hurricane-level winds by hiding on the leeside of their perch.  We performed computational fluid dynamics (CFD) simulations in the same type of software that engineers use to design airplanes, cars, turbines etc.

In 2018, my former colleagues at the Museum of Natural History in Paris published an amazing article in Nature in which they showed that after a hurricane has passed islands in the Caribbean ocean, Anolis lizards that live in the trees have, on average, larger too pads, longer forelimbs, and shorter hindlimbs.  So there is a strong selection by hurricanes on the anatomy of these lizards.  Some results were logic: the larger the toepad area, the more gripping force, and longer forelimbs can help to grasp around perches.  But why did the hindlimbs shorten in populations after hurricanes?  This was still a mystery.

Two of the authors from the Nature paper, Colin Donihue and Anthony Herrel, were my colleagues at the Museum of Natural History in Paris.  They knew I had experience with performing fluid dynamic simulations. They came to my office one day, explained me their questions on the aerodynamics of lizards in hurricanes, and we quickly had a plan for this follow-up study.  As I didn’t have enough time to do the practical work all by myself, I teamed up with Biology Master student Shamil Debaere.

Results from the aerodynamic modelling (computational fluid dynamics) showing streamlines and pressures on the body of the lizard while grasping onto a cylindrical branch in hurricane-level winds.
Results from the aerodynamic modelling (computational fluid dynamics) showing streamlines and pressures on the body of the lizard while grasping onto a cylindrical branch in hurricane-level winds.

The results showed that the lizards generally experience high aerodynamic drag forces on the hindlimbs and hip region.  As long as they manage to keep their body very close to the perch, shielding by the perch is good.  As soon as there is a gap between the perch and the hip, for example when the hindlimb become stretched, the wind has a frontal impact on the thighs.  Animals with stretch-out, long hindlimbs catch the most wind.  This explains the selection for shorter hind limbs in Colin and Anthony’s paper.

So in short, our simulations showed that high, local aerodynamic pressures explain why the hindlimbs are the weakest link for lizard to hold onto their perches in hurricane winds.  This suggests that evolution of shorter hindlimbs is strongly driven by selection for drag reduction. It is the first study to look at the aerodynamics of perched lizards in storm winds.

It remains puzzling, both from a biomechanical and evolutionary perspective, why the hindlimbs are the weakest link for lizards when resisting wind forces.  For example, why didn’t they evolve larger toe pads or hindlimb adductor muscles to increase their gripping forces to counter the increased drag forces on this region?  Still fairly little is known on the form-function relationships of the performance of grasping on perches.

Video image of Anolis during a leafblower experiment by Donihue and co-workers (left) together with the 3D surface reconstruction by Shamil Debaere used in the study (right).
Video image of Anolis during a leafblower experiment by Donihue and co-workers (left) together with the 3D surface reconstruction by Shamil Debaere used in the study (right).

About the research

This line of research changes our view on how the anatomy of small animals is adapted to their ecology in case they are faced with dangerous winds.

My student Shamil Debaere, who did most of the practical work, tacked a number of technical challenges.  Getting the complex 3D-shape of the lizard ready for computer simulations of airflow around it, was quite difficult.  He had to adjust his protocol several times before finally managing to complete the first simulation.

Unravelling how exactly the anatomy of animals like lizards is a compromise between the different functions it needs to fulfil, is the ultimate goal.  Since our study is the first to show that a previously overlooked factor like aerodynamic drag reduction is also important, there is still a lot of work to do to reach this goal.

About The Author

I am an evolutionary biologist who specialised in the study of biomechanics of animals. My current projects focus on the biomechanics and functional morphology of feeding in birds and fishes. This is different from the topic of this paper, but the interaction between the animal and its surrounding fluids has been a common theme in my work as a post-doctoral researcher. I am proud that my student Shamil Debaere and I managed to tackle the technical challenges to be able to analyse the aerodynamics on realistic 3D-morphologies of perching lizards.

Read the article in full here

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