We believe that the flight and echolocation behaviors displayed by the Egyptian fruit bat (Rousettus aegyptiacus) may translate to more precise head movement coding in response to their unique sensorimotor demands. While there is evidence for a relationship between locomotive behaviors and structural adaptations in the bony labyrinth of birds and mammals, none exist in the bat’s bony labyrinth. The present study determines whether cellular adaptations that translate to increased capability for high fidelity head movement coding exist in the crista central zone of Rousettus. This was accomplished using immunohistochemical techniques comparing hair cell and afferent phenotypes within crista central zone of Rousettus and the mouse (Mus musculus). Central zone hair cell counts revealed an increased fraction of type I hair cells in Rousettus. Additionally, Rousettus exhibits increases in the proportion of higher order complex calyces within the central zone. The calcium binding protein OCM is also observed to have expression that is restricted to type I hair cells in the central zone of Rousettus, unlike in Mus where it is also expressed in type II hair cells. KCNQ4, a low-voltage activated potassium channel, exhibits a positive association with CALB2 expression in the central zone of both Rousettus and Mus, which suggests that their association may contribute to the unique response characteristics of calyx-only afferents that project to the central zone. These findings provide evidence that the crista epithelia of Rousettus exhibits cellular adaptations that are consistent with enhanced head movement coding that may serve to be advantageous to their unique behavioral niche.

Bats are highly agile animals by virtue of their flight and echolocation behaviors, and they are acknowledged to be “spatiotemporal integration specialists.” In order to avoid obstacles, scavenge food, and capture prey, bats must be able to navigate a three-dimensional space. In flight, bats must perform high speed navigation through obstacles using a blend of sensory modalities including vision and echolocation. While some bat species exhibit structural adaptations to their cochleae as an adaptation to the demands of echolocation, there is an absence architectural adaptation of the vestibular labyrinth that would accommodate the increased demands for high frequency head movement coding associated with agile movement. The present study tested the hypothesis that there may exist cellular adaptations within the vestibular epithelia consistent with increased capability for high fidelity head movement coding. This was accomplished using immunohistochemical analyses focusing primarily in the striolae of the utricles of Rousettus aegyptiacus and Mus musculus, regions bounded by the presence of calretinin (Calb2) positive calyces. Hair cell counts revealed increased proportions of type I hair cells (hair cells surrounded by calyces) in Rousettus when compared to Mus (72.6% � 3.5% and 57.2% � 2.7%, respectively, p<10-5). Additionally, Rousettus exhibits increases in the proportion of complex calyces in the striola when compared to Mus. Ocm, a calcium binding protein expressed in vestibular hair cells, is also observed to have expanded expression in Rousettus and Mus both in the striola as well as the extrastriolar regions. These novel findings indicate that vestibular epithelia of Rousettus exhibit cellular adaptations over Mus that are consistent with increases in head movement coding capability associated with agile movement in flight.