Correlations between form, function and behaviour: the inner ears of birds and reptiles

The vertebrate inner ear is a complex structure that incorporates the organs of balance and hearing. Both senses are essential to the normal functioning of an organism: unsurprisingly, the inner ears of living vertebrates have been studied intensively in terms of their soft anatomy and their physiological/neurological mechanisms, and the behavioural and general biological implications of ear function have been investigated thoroughly. The soft tissues are housed in bony structures that form part of the side walls of the braincase at the back of the skull. Few studies have attempted to seek correlations between the properties of the bony structures (e.g. volume, maximum length/width, shape), hearing ability or sense of balance, and the resulting behavioural repertoire of an animal. From a palaeontologist's perspective, this is an unfortunate omission as, in most cases, fossils only yield information on the hard anatomy of an organism. Partly as a result of this situation, the inner ears of many extinct vertebrates have received little attention hitherto and detailed studies have only become possible recently with the advent of non-destructive imaging technology. We propose a proof-of-concept study to investigate the utility of inner ear morphology in assessing the behaviour and auditory performance of extinct reptiles and birds. We will test whether differences in reptile/bird inner ear anatomy can be explained by differences in behaviour, vocalisation, acoustic function and social organisation. This test will be based upon an analysis of the inner ear anatomy of 40 living reptiles and birds, which will be chosen to represent a wide range of vocal and auditory behaviours. Data on inner ear anatomy will be collected using X-ray Computerised Tomography (CT), which will allow us to build detailed 3-D virtual models of the ear's bony structures. Correlations will be sought between the dimensions of the lagena (that part of the inner ear housing the organ of hearing / the basilar papilla), the known auditory abilities of an animal (e.g. range of sound frequencies detectable, sensitivity), and the range of behaviours that the animal exhibits (e.g. those that use vocal communication versus those that do not). Combination of these different data sources (inner ear bony anatomy, ear function and animal behaviour) is a novel approach and will provide a rigorous basis for inferring behaviour in extinct animals. If no correlations exist between ear anatomy, hearing ability and behaviour this will allow us to falsify existing hypotheses of hearing function in extinct reptiles and birds. However, if correlations are identified this project will act as a pilot study for future investigations on hearing and behaviour in a variety of extinct species (including non-avian dinosaurs and early amniotes, for example). In addition, the project will provide a large amount of new anatomical information on the inner ears of living reptiles and birds, which will be of great value to auditory biologists, zoologists and systematists.