Title: The effect of head shape on penetration of granular materials: An analysis of lizard mechanical models.

Author (Talk): Philip Bergmann, Clark University

Abstract:

Many animals move not only on top of, but also through granular substrates. Sand swimmers move through loose granular substrates, where no tunnel is formed, while burrowers move through packed, often moist substrates and form a tunnel. Sand swimming and burrowing lizards have evolved many times and are represented by dozens of species. These species have various adaptations to the shape of the head to facilitate penetration and displacement of the substrate. We measured the heads of 525 lizards, belonging to 178 species, quantifying the maximum width, and the slope and pointiness of the snout to characterize the variation in head shape in these animals. We then fashioned aluminum models to mimic the range of evolved head shapes and plunged these into a series of granular media, recording the force needed to penetrate each substrate with a force transducer. We considered ten substrates varying in particle diameter and whether the particles were irregularly shaped, or glass spheres. We found that in lizard, head width, snout slope and pointiness were uncorrelated, so are free to evolve independently. From the model experiments, we found that thicker heads with steeper slopes and less pointy snouts required more force to penetrate the substrate, with the largest effect from head width. We also found that intermediate particle sizes required most force to penetrate when the substrate was in the sand range (<2000 microns), but gravel required much more force to penetrate by all models.