Meeting Abstract
Squamate reptiles (lizards and snakes) are a morphologically and ecologically diverse clade, and within it fossoriality has evolved multiple times independently and defines the ecology of hundreds of species. Most of these species use their heads as the primary organ for penetrating the substrate. Nevertheless, little is known about what characteristics make an animal proficient at penetrating granular substrates with their heads, and no large-scale evolutionary functional morphological studies have been undertaken. Here we characterize head shape variation in 152 species of fossorial lizards along three fundamental axes of head shape: diameter, slope/taper, and pointiness. We then construct 28 mechanical models spanning the evolved variation, but manipulating these three measures independently, and drive these models into a variety of granular glass bead and natural rock substrates using a linear actuator, while measuring penetration force. Evolutionary correlation analysis shows weak negative relationships between pointiness and the other two measures, and a weak positive relationship between diameter and slope, but none of these are significant, likely showing that different fossorial animals use different strategies. Our mechanical model experiments show that the force needed to penetrate the substrate decreases in finer particles, and for narrower heads that have a shallower slope. Pointiness has the smallest effect, but this effect increases in natural rock substrates and for wider heads, situations where substrate jamming may be considerable. In these cases, pointier heads lead to lower penetration forces. Interactions among main effects were much larger in rock substrates than bead substrates, indicating a more complex behavior in natural substrates.
