Sensory ability in the narwhal tooth organ system

Abstract

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high‐salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence. Anat Rec, 297:599–617, 2014. © 2014 Wiley Periodicals, Inc.

Cetacea

“…The canines of females can erupt and produce a shortened tusk. The porosity of the tusk suggests that the structure acts as a sensory device for measuring water salinity, temperature and pressure (Nweeia et al, 2014), as well as for social interactions (dominance) and hunting (striking) of prey species.…”

Evolutionary Tinkering of the Mandibular Canal Linked to Convergent Regression of Teeth in Placental Mammals

“…Previous studies proposed that mammalian teeth might play a sensorial role in detecting a wide array of external stimuli including pressure, proprioception, and percussion [26] in addition to their role for food intake. Our results suggest that the development of dorsal canaliculi might be linked to the presence of tooth loci/ vestigial teeth in both anteaters and baleen whales.…”

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

“…Monocular vision in conjunction with echolocation would more readily provide visibility of open water channels and breathing holes or cracks. The tusk of M. monoceros, although clearly a secondary sexual characteristic, has been found to be highly sensitive to changes in salinity and water temperature, and may therefore be used as an additional sensory apparatus (Nweeia et al 2014). Dissections showed a direct connection with the maxillary division of the trigeminal nerve (CNV) (Nweeia et al 2014).…”

“…The tusk of M. monoceros, although clearly a secondary sexual characteristic, has been found to be highly sensitive to changes in salinity and water temperature, and may therefore be used as an additional sensory apparatus (Nweeia et al 2014). Dissections showed a direct connection with the maxillary division of the trigeminal nerve (CNV) (Nweeia et al 2014). Correspondingly, larger trigeminal nuclei or maxillary portion of the trigeminal nerves may be expected on the endocasts of male M. monoceros or other individuals possessing tusks, as has been found in other mammals, such as the duck-billed platypus and its extinct relatives (Macrini et al 2006), which have increased sensory perception in the rostra.…”