Hypothesis: A helix of ankyrin repeats of the NOMPC-TRP ion channel is the gating spring of mechanoreceptors

rock increases local field strength enough to alter map readings but not compass direction, should leave the birds reoriented or disoriented. Indeed, at smooth gradients they are reoriented, whereas at irregular ones they are ludicrously disoriented (Figure 3). Pigeons, like most animals with maps, possess enormous numbers of magnetite grains in the ethymoid sinus. There are species in which magnetic sensitivity may be mediated in other ways, for example magnetic induction in elasmobranchs, or paramagnetic effects in the visual pigments of silvereye birds, but magnetite-based compasses seem to fit the behavioral data in nearly all cases. In species using a map for navigation, there are far too many of these grains for a mere compass, but enough to easily determine map location at least at an order of magnitude better than the 2 km estimated from returns of visually impaired pigeons. Pigeons subjected to gradually increasing and decreasing intense magnetic fields should have their magnetite grains remagnetized. If the map sense is directional, like a typical hand compass, the birds should be disoriented. On the other hand, if the map is axial, that is, if it cannot distinguish between north and south, the orientation should be better. Birds treated with magnetic fields are indeed better oriented. However, if the birds are subjected to a sharp intense pulse, either kind of map organ should be severely impaired, which is exactly what is observed. Whichever cues the map sense involves, it seems likely that it is this information which allows low-resolution first-time migrants to achieve high-resolution returns—and high-resolution southward trips in future years. Far and away the biggest question in long-distance navigation is the nature of the map sense. Second is the issue of recalibration, which is still puzzling. The interaction of these specific learning programs doubtlessly holds many magnificent secrets. Sensory reception by mechanically sensitive cells is mediated by an ion channel coupled to a molecular spring [1]. This gating spring is thought to transmit an external force (e.g. from a sound wave or a tactile stimulus) to the ion channel. The force alters the probability of the ion channel being open. This produces an electrical signal that is conveyed to the central nervous system [2]. One such channel has recently been identified as a member of the NOMP-C subfamily of TRP channels [3,4]. However, the molecular identity of the gating spring remains unknown. We propose that the gating spring is …