Variability and constraint of vertebral formulae and proportions in colugos, tree shrews, and rodents, with special reference to vertebral modification by aerodynamic adaptation

Background: The aim of the present study is to provide the first large data set on vertebral formulae and proportions, and examine their relationship with different locomotive modes in colugos (Dermoptera), tree shrews (Scandentia), and rodents (Rodentia), which have been considered less variable because they were thought to have a plesiomorphic number of 19 thoracolumbar vertebrae. Materials and methods: The data included 33 colugos and 112 tree shrews, which are phylogenetically sister taxa, and 288 additional skeletons from 29 other mammalian species adapted to different locomotive modes, flying, gliding, arboreal, terrestrial, digging, and semi-aquatic habitats. Results: The following results were obtained: (1) intra-/interspecies variability and geographical variation in thoracic, lumbar, and thoracolumbar counts were present in two gliding colugo species and 12 terrestrial/arboreal tree shrew species; (2) in our examined mammals, some aerodynamic mammals, such as colugos, southern flying squirrels, scaly-tailed squirrels, and bats, showed exceptionally high amounts of intraspecific variation of thoracic, lumbar, and thoracolumbar counts, and sugar gliders and some semi-aquatic rodents also showed some variation; (3) longer thoracic and shorter lumbar vertebrae were typically shared traits among the examined mammals, except for flying squirrels (Pteromyini) and scaly-tailed squirrels (Anomaluridae). Conclusions: Our study reveals that aerodynamic adaptation could potentially lead to strong selection and modification of vertebral formulae and/or proportions based on locomotive mode despite evolutionary and developmental constraints.