Why so Horn-y?: Teasing Apart Influences on Morphological Variation in Horns and Skulls of Horned Lizards (Phrynosoma)

Abstract

Morphology is the way that most organisms interact with their biotic and abiotic environment, by allowing them to move through space, reproduce, eat, and defend. Horned lizards of the genus Phrynosoma are distinguished from other lizards by their unique morphology, diet, and life history traits. Despite a long history of morphological analysis in horned lizards, only a few studies have simultaneously evaluated the effects of multiple factors influencing the spectacular variation present in the cranial horns. In this study, I teased apart the competing influences on cranial horn shape variation from multiple factors, including habitat, life history and phylogenetic contingency, across 16 species of Phrynosoma. Shape variation of skull and horns was captured using Type 1 landmarks on 3D volumetric models. Habitat variables, life history traits and the effect of phylogeny were characterized from occurrences data and previously published work. Phylogenetic path analysis was used to partition shape variation between Phrynosoma species among multiple factors. Principal component analysis of shape indicated that 83% of morphological skull variation between species was contained within the first four axes of variation. Most of that variation (PC1) was associated with elongation of second parietal and first squamosal horns and widening of the rostrum. Path analysis revealed that skull and horn shape is strongly influenced directly by habitat and indirectly via habitat’s effect on reproductive mode. Reproductive mode had a strong effect on shape, aligning with previous studies that have shown a relationship between climate and reproductive mode in horned lizards. The results in this study suggest that even after accounting for phylogenetic history, variables associated with habitat had the greatest effect on skull and horn shape but only when mediated through diet and reproductive mode. The results of this work have implication for understanding the distribution of morphological variation are likely influenced by bioclimatic variables and the availability of suitable microhabitats within their range.