Unique Turbinal Morphology in Horseshoe Bats (Chiroptera: Rhinolophidae)

Abstract

The mammalian nasal fossa contains a set of delicate and often structurally complex bones called turbinals. Turbinals and associated mucosae function in regulating respiratory heat and water loss, increasing surface area for olfactory tissue, and directing airflow within the nasal fossa. We used high-resolution micro-CT scanning to investigate a unique maxilloturbinal morphology in 37 species from the bat family Rhinolophidae, which we compared with those of families Hipposideridae, Megadermatidae, and Pteropodidae. Rhinolophids exhibit numerous structural modifications along the nasopharyngeal tract associated with emission of high duty cycle echolocation calls via the nostrils. In rhinolophids, we found that the maxilloturbinals and a portion of ethmoturbinal I form a pair of strand-like bony structures on each side of the nasal chamber. These structures project anteriorly from the transverse lamina and complete a hairpin turn to project posteriorly down the nasopharyngeal duct, and vary in length among species. The strand-like maxilloturbinals in Rhinolophidae were not observed in our outgroups and represent a synapomorphy for this family, and are unique in form among mammals. Within Rhinolophidae, maxilloturbinal size and cross-sectional shape were correlated with phylogeny. We hypothesize that strand-shaped maxilloturbinals may function to reduce respiratory heat and water loss without greatly impacting echolocation call transmission since they provide increased mucosal surface area for heat and moisture exchange but occupy minimal space. Alternatively, they may play a role in transmission of echolocation calls since they are located directly along the path sound travels between the larynx and nostrils during call emission. Anat Rec, 300:309-325,

Maxilloturbinal Aids in Nasophonation in Horseshoe Bats (Chiroptera: Rhinolophidae)

“…In addition to this impressive set of traits, we recently discovered that the morphology of the delicate maxilloturbinal bones inside the nasal chamber is unique in Rhinolophidae (Curtis and Simmons, ). In most mammals, including other bats, maxilloturbinals articulate with the internal (nasal) surface of the maxillary bone and range from scroll shaped, or plate shaped, to branched in form (Negus, ; Bhatnagar and Kallen, ; Giannini et al, ; Smith et al, ).…”

“…In horseshoe bats, the maxilloturbinals are modified into two pairs of bony strands that project anteriorly from the ventral base of the olfactory recess (transverse lamina), complete a hairpin turn to project posteriorly down the nasopharyngeal duct (Fig. ) (Grosser, ; Curtis and Simmons, ). These bony strands appear to be completely freestanding within the nasopharyngeal duct, and they are covered in a thin epithelium.…”

“…These bony strands appear to be completely freestanding within the nasopharyngeal duct, and they are covered in a thin epithelium. Interestingly, members of Hipposideridae (the sister taxon to horseshoe bats) share a similar suite of morphological adaptations for echolocation, but altogether lack or have extremely reduced maxilloturbinals (Nelson et al, ; Curtis and Simmons, ). Previous histological work on Megaderma lyra , a more distantly related nasophonating member of superfamily Rhinolophoidea, shows very small, plate‐shaped maxilloturbinals (Smith et al, ).…”

“…Within Rhinolophidae, length of the maxilloturbinal strands and cross‐sectional shape varies considerably among species, and is at least partly correlated with phylogeny (Curtis and Simmons, ). While rhinolophid maxilloturbinals all share a similar general shape, the more lateral strand of the maxilloturbinal can vary from a short strand that is completely contained within the margins of the hard palate to a long strands that project posteriorly past the margin of the hard palate into the nasopharyngeal duct (Fig.…”

“…Dorsoventrally flattened maxilloturbinal strands are shared by sister taxa Rhinolophus luctus and R. trifoliatus . However, short maxilloturbinals appear to have evolved independently multiple times in Rhinolophidae (Curtis and Simmons, ). The functional significance of these differences in length and cross‐sectional shape remains to be explored.…”