What are the limits on whale ear bone size? Non-isometric scaling of the cetacean bulla

Ecological correlates of the morphology of the auditory bulla 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 likely to be constrained by the physical environment. The resulting hypothetical link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species before. Such link may enable the determination of the ecological affinities of many fossil species only known from partial skulls. In this study, we used geometric morphometrics to quantify the shape of the auditory bulla of 197 specimens representing 91 species from 17 families of extant rodents across four different locomotory modes. We used landmarks and semi-landmarks on the ventral and lateral views of the skull to capture morphological characteristics of the bulla and external auditory meatus (EAM). Our results demonstrate an association between bullar morphology and locomotion in rodents. Bullar shape enables the correct classification of 76% of the species in our training set. Fossorial taxa, in particular, show a characteristic morphology including an asymmetric bulla with a dorsally located and laterally expanded EAM that has a small opening diameter. A phylogenetically informed flexible discriminant analysis shows a weak phylogenetic effect on tympanic morphology. There is no evidence for differences in bullar hypertrophy across locomotory categories. The application of this approach to select fossil rodents from the Oligo-Miocene shows broad agreements with prior studies and yields new locomotory inferences for 14 fossil species, including the first proposed locomotion for members of the family Florentiamyidae. Such results call for the timing of burrowing diversification in rodents to be reevaluated.

Developing echolocation: distinctive patterns in the ontogeny of the tympanoperiotic complex in baleen and toothed whales (Cetacea)

Cetaceans (baleen and toothed whales) present a unique set of adaptations for life in water. Among other abilities, the two living groups can hear and produce different sound frequencies: baleen whales use low frequencies primarily for communication, whereas toothed whales acquired the ability to echolocate using high-frequency sounds. Both groups exhibit modifications to the morphology of the ear bones (tympanic bulla and periotic) that closely track their behaviour and ecology. The evolution of sound reception in whales is being investigated thoroughly, whereas the changes in prenatal development (ontogeny) that generate these disparate ear bone morphologies remain mostly unknown. In this study, we characterize the ontogeny of the ear bones in Cetacea by looking at the progression of ossification and associated changes in morphology using a combination of traditional measurements and an innovative landmark-free method to quantify shape on a newly assembled three-dimensional dataset spanning the ontogeny and phylogeny of extant Cetacea. We have found that the two groups of Cetacea share some aspects of ear ontogeny, such as a common growth trajectory of the periotic. However, differences in ossification, allometry and growth trajectory, particularly in the periotic bone, reflect their divergent inner ear morphology and hearing abilities.