Multi-level analysis of morphological variation in the vertebral column of lagomorph mammals

Characterizing morphological variation along the vertebral column of mammals is commonly investigated at a broad phylogenetic scale, leaving within-species variation understudied due to the requirement of larger sample sizes. This leads to a knowledge gap of how variation within species relates to morphological diversity among species. Here, we overcome these limitations and examine the morphological variation at the within-species level in the vertebral column of 4 species-equivalent groups of rabbits and hares. We then expanded to the among-species levels of the family Leporidae, the order Lagomorpha, and broadly among terrestrial placentals. We sampled 9 vertebrae along the vertebral column of each specimen. Using a geometric morphometric approach, we calculated the Procrustes variance of vertebrae shapes and used this as an index for the extent of morphological variation of each vertebra along the vertebral column, which we call the profile. We find that the profile of morphological variation along the column differs among species and between phylogenetic levels; among-species variation is not simply a scaled-up profile of the within-species level. We highlight that by adopting the multi-level analysis, we can better understand how the mammalian vertebral column can evolve.