Intrapopulational variation in head shape correlates with soil structure heterogeneity in a head-first burrowing amphisbaenian, Trogonophis wiegmanni

Morphological traits of animals have evolved to solve ecological requirements, the optimization of locomotion in each environment being one of the most frequent selective forces shaping morphology. Amphisbaenians are strictly fossorial reptiles that have evolved various snout shapes for burrowing head-first underground, yet the evolutionary and ecological origins of these different morphologies are little known. Here, we used a geometric morphometric approach to investigate head shape intrapopulational variation in the round-snouted checkerboard worm lizard (Trogonophis wiegmanni). We took 2D photographs of live animals in a North African island population. At each capture site, we also measured microhabitat characteristics and soil compaction, and took soil samples to analyze soil physical structure. While we detected no signal of sexual dimorphism in head shape and overall head disparity was low, we found significant relationships between the head shape of T. wiegmanni and some characteristics of vegetation and soil. Relatively more tapered snouts and narrower heads occurred in sites with taller bushes and soils containing higher amounts of clay (i.e. harder substrates that are more difficult to excavate). We hypothesise that differences in head shape due to microhabitat and soil variation might be considered a scenario under which different amphisbaenian head morphologies initially evolved.