Inferences of hearing capabilities and audition-related behaviours in extinct reptiles and birds have previously been based on comparing cochlear duct dimensions with those of living species. However, the relationship between inner-ear bony anatomy and hearing ability or vocalization has never been tested rigorously in extant or fossil taxa. Here, micro-computed tomographic analysis is used to investigate whether simple endosseous cochlear duct (ECD) measurements can be fitted to models of hearing sensitivity, vocalization, sociality and environmental preference in 59 extant reptile and bird species, selected based on their vocalization ability. Length, rostrocaudal/mediolateral width and volume measurements were taken from ECD virtual endocasts and scaled to basicranial length. Multiple regression of these data with measures of hearing sensitivity, vocal complexity, sociality and environmental preference recovered positive correlations between ECD length and hearing range/mean frequency, vocal complexity, the behavioural traits of pair bonding and living in large aggregations, and a negative correlation between ECD length/rostrocaudal width and aquatic environments. No other dimensions correlated with these variables. Our results suggest that ECD length can be used to predict mean hearing frequency and range in fossil taxa, and that this measure may also predict vocal complexity and large group sociality given comprehensive datasets.

The hearing organ of the inner ear was the last of the paired sense organs of amniotes to undergo formative evolution. As a mechanical sensory organ, the inner-ear hearing organ's function depends highly on its physical structure. Comparative studies suggest that the hearing organ of the earliest amniote vertebrates was small and simple, but possessed hair cells with a cochlear amplifier mechanism, electrical frequency tuning, and incipient micromechanical tuning. The separation of the different groups of amniotes from the stem reptiles occurred relatively early, with the ancestors of the mammals branching off first, approximately 320 million years ago. The evolution of the hearing organ in the three major lines of the descendents of the stem reptiles (e.g., mammals, birds-crocodiles, and lizardssnakes) thus occurred independently over long periods of time. Dramatic and parallel improvements in the middle ear initiated papillar elongation in all lineages, accompanied by increased numbers of sensory cells with enhanced micromechanical tuning and group-specific hair-cell specializations that resulted in unique morphological configurations. This review aims not only to compare structure and function across classification boundaries (the comparative approach), but also to assess how and to what extent fundamental mechanisms were influenced by selection pressures in times past (the phylogenetic viewpoint).T he hearing organs of modern amniotes (reptiles, birds, and mammals) show an almost bewildering variety of morphologies. This variety provides the functional morphologist with an exciting natural experiment to assess the functional consequences of structural diversity in an organ whose function largely depends on just this structure. To do this presupposes an understanding of (i) the evolutionary history of the amniotes, (ii) the extent and nature of the morphological variation, and (iii) those mechanisms of hearing that are directly influenced by morphology. This review emphasizes cochlear mechanisms that may have been influenced by morphological changes during amniote phylogeny. As is now customary, the term cochlear will be used loosely for the mechanisms involved in the hearing organ of the cochlear duct (Scala media) of all amniotes.The present discussion is based on certain assumptions, the most important being that the hearing organs of all amniotes are homologous (1). They have a common ancestry, share a common structure, and develop from the same genetic substrate, and their position in the organisms' Bauplan is the same. Thus the functional units of the hearing process-the hair cells and their innervating nerve fibers-are also homologous.A second assumption, based on the comparative anatomy and physiology of putatively primitive amniote hearing organs, is that the archetypal auditory papilla was short (Ϸ1 mm) with only a few hundred hair cells. It is assumed that these hair cells (i) had inherited electrical tuning from their vestibular-system ancestors, (ii) were innervated by both afferent and efferent ...

The last two decades have produced a great deal of evidence that in the mammalian organ of Corti outer hair cells undergo active shape changes that are part of a "cochlear amplifier" mechanism that increases sensitivity and frequency selectivity of the hearing epithelium. However, many signs of active processes have also been found in nonmammals, raising the question as to the ancestry and commonality of these mechanisms. Active movements would be advantageous in all kinds of sensory hair cells because they help signal detection at levels near those of thermal noise and also help to overcome fluid viscosity. Such active mechanisms therefore presumably arose in the earliest kinds of hair cells that were part of the lateral line system of fish. These cells were embedded in a firm epithelium and responded to relative motion between the hair bundle and the hair cell, making it highly likely that the first active motor mechanism was localized in the hair-cell bundle. In terrestrial nonmammals, there are many auditory phenomena that are best explained by the presence of a cochlear amplifier, indicating that in this respect the mammalian ear is not unique. The latest evidence supports siting the active process in nonmammals in the hair-cell bundle and in intimate association with the transduction process.

No abstract

SUMMARY Lizards have highly sensitive ears, but most lizard heads are small (1-2 cm in diameter) compared to the wavelengths of sound of frequencies to which they are most sensitive (1-4 kHz, wavelengths 34-8.5 cm). Therefore, the main cues to sound direction that mammals use - binaural time and intensity cues due to arrival-time differences and sound shadowing by the head - will be very small in lizards. The present work shows that acoustical coupling of the two eardrums in lizards produces the largest directionality of any terrestrial vertebrate ear studied. Laser vibrometric studies of tympanic motion show pronounced directionality within a 1.8-2.4 kHz frequency band around the best frequency of hearing, caused by the interference of ipsi- and contralateral inputs. The results correspond qualitatively to the response of a simple middle ear model,assuming coupling of the tympana through a central cavity. Furthermore,observed directional responses are markedly asymmetrical, with a steep gradient of up to 50-fold (34 dB) response differences between ipsi- and contralateral frontal angles. Therefore, the directionality is easily exploitable by simple binaural subtraction in the brain. Lizard ears are the clearest vertebrate examples of directionality generated by tympanic coupling.