Meeting Abstract

P3.107  Sunday, Jan. 6  Differential vertebral growth produces variations in adult thoracolumbar proportions in half-bounding mammals JONES, K E*; GERMAN, R Z; Johns Hopkins University; Johns Hopkins University kjone108@jhmi.edu

Vertebrae are serially homologous structures that are tightly integrated through their development and evolution. However, in mammals the dorsal vertebrae are split into thoracic and lumbar vertebrae that are morphologically and functionally differentiated. The lumbar region tends to be specialized for locomotion via well developed epaxial musculature which may be used for producing thrust during quadrupedal jumping. Here we test the hypothesis that half bounding taxa exhibit a longer lumbar region than non-specialists. Further, the relative role of growth rate of individual vertebrae versus vertebral number in altering adult proportions is tested using longitudinal data. Lateral x-rays of 38 specimens from two half-bounding (Oryctolagus cuniculus and Chinchilla lanigera) and two non-specialized (Cavia porcellus and Monodelphys domestica) species of similar size were the source of centrum length measurements on individual vertebrae. The repeated measurements design included the same individuals soon after birth and again at adult size. The half-bounding species had both more lumbar vertebrae and a longer lumbar:thoracic region than the non-specialists. This correlates with suggestions that the lumbar epaxial muscles are important in producing thrust during saltatorial behavior. This relatively longer lumbar region was apparent in new-borns but longitudinal data indicated that the difference also increased during postnatal growth. Lumbar vertebrae consistently grew twice as fast as thoracic vertebrae in all taxa. Therefore, evolution of additional, fast-growing, lumbar vertebrae in half-bounding taxa is key to increasing the rate of post-natal growth relative to the thoracic region, and ultimately varying adult regional proportions.