Auditory papillae from three species of bird (pigeon, starling, and chick), and two species of European lizard (Podarcis murolis and Podarcis sic&) were examined by scanning electron microscopy. Hair bundles from all papillae showed tip links oriented along the direction of gradation in heights of the stereocilia (i.e. parallel to the hair-cell axis of bilateral symmetry, and so parallel to the
excitato~-i~bito~ axis for m~hanotransduction). This orientation was seen irrespective of the overall orientation of the hair bundle within the papilla. The stereocilia formed columns, joined by the tip links, which ran parallel to the hair-cell axis of bilateral symmetry. The stereocilia within the same column tended to stay together, while those in different columns tended to separate during preparation, In many columns ail the stereocilia tended to be a little taller, or a little shorter, than the equivalent stereocilia in adjacent columns, suggesting that all the stereocilia within one column had been affected by a common height determinant during development. In addition, links running laterally between stereocilia were seen, in a band near the base of the stereocilia. The results are consistent with the hypothesis that tip links are a universal feature of mechano-transducing acousticolateral hair cells, and that
they are involved in sensory transduction. The results also support suggestions that the tip links may play a role in determining the heights of the stereocilia during development.