On the narwhal's tusk,

. . .a novel hypothesis was proposed in 2005 by Martin Nweeia of the Harvard School of Dental Medicine and his colleagues. You can read many reports of this hypothesis on the internet (and the abstract is viewable here) but, so far as I can tell, their study still hasn't been published as a full paper. Noting the presence of over 10 million tiny nerve tubules connecting the tooth's pulp cavity to its external surface, Nweeia and colleagues have argued that the tusk must be hyper-sensitive and thus act as a hydrodynamic sensor that can detect changes in temperature, pressure and salinity gradients. Even more remarkably, Nweeia et al. contend that the tusk isn't stiff as assumed but with flexible outer layers that provide it with resilience and flexibility. In fact its structure indicates that it can bend about 30 cm in either direction, apparently. Nweeia et al.'s hypothesis is, obviously, fascinating and incredible.