ASSOCIATION BETWEEN TYMPANIC BULLA MORPHOLOGY AND LOCOMOTION IN RODENTS: APPLICATION TO THE FOSSIL RECORD

For rodents, hearing is essential to survival; it enables predator evasion, prey detection, and conspecific recognition. It is also constrained by the physical environment. As such, we hypothesize the hearing system of rodents to be ecology-specific. The link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species; such analysis offers the opportunity to explore the ecological affinities of many fossil species only known from skulls. We used geometric morphometrics to quantify the shape of the auditory bulla of 203 specimens of modern rodent specimens representing 93 species from 17 families and four different locomotory modes (arboreal, semi-fossorial, fossorial, and terrestrial). We placed landmarks and semi-landmarks on photos of the ventral and lateral views of each specimen to capture characteristics of bullar inflation and external auditory meatus (EAM) extension. The results of our principal component analyses and canonical variate analyses demonstrate an association between bullar morphology and locomotion in rodents. Each locomotion has a distinct bullar shape. Thus, arboreal rodents have inflated bullae and a very short EAM whereas fossorial rodents have a laterally expanded EAM. The classification phase of the analysis correctly classifies 75.3% of extant species studied. Our pFDA demonstrates a weak phylogenetic effect concentrated in select taxa (Sciuridae, Muridae) and an a posteriori correct classification rate of over 85%. The application of our landmarking to select fossil rodents yields inferences consistent with prior analyses based on postcranial skeletons, particularly in the ventral view of the bullae. Fossils occupy a broader morphospace than extant taxa, suggesting the need to expand our training set to include well-documented fossil taxa that will be helpful in determining the ecology of more poorly-known species.