Patterns of variation in the tympanic bulla of tuco-tucos (Rodentia, Ctenomyidae, Ctenomys)

As subterranean rodents live in burrows and are constrained by the physics of their environment to vocalize, mainly in low frequencies, their expanded middle ear cavities are associated with enhanced lower-frequency hearing. Previous literature has widely acknowledged inflated tympanic bulla as a character to be found in the majority of the Ctenomys species. To explore the morphology of Ctenomys tympanic bulla, we studied a sample of 669 skulls, obtained from 21 species, for tympanic bulla size, volume, and internal structure. The study determined that bullar inflation does not seem to be the rule in Ctenomys and that the relationship between bullar size (volume) and skull size do not correspond to the phylogeny based on cytochrome b sequences thus probably being a species-specific adaptive characteristic. We also found that the internal bullar structure differs between taxa, depending on the relative contributions of cancellous (alveolar) and septate patterns to the partitioning of the bulla.