Inner ear anatomy is a proxy for deducing auditory capability and behaviour in reptiles and birds

Abstract

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.

Halcyornis toliapicus(Aves: Lower Eocene, England) indicates advanced neuromorphology in Mesozoic Neornithes

“…The relatively large olfactory lobes of Halcyornis suggest that this species had a reasonable reliance on its sense of smell. The heightened sensory capabilities of Halcyornis are further illustrated by our estimates for hearing sensitivity, which are far higher than the values calculated by Walsh et al (2009) for Odontopteryx and Prophaethon. Since the vocalization range of most birds falls within the lower half of their hearing sensitivity (Konishi 1970), Halcyornis is likely to have had a relatively wide and high vocalization frequency range.…”

“…The endosseous cochlear duct is slightly curved and relatively long, with a mean length of almost 5.0 mm measured from the base of the pars vestibularis. Using the methods of Walsh et al (2009) to estimate mean hearing range and sensitivity from the scaled and transformed endosseous cochlear duct length, we calculate that Halcyornis would have had a mean hearing range of approximately 5900 Hz and a mean hearing sensitivity of approximately 3400 Hz Figure 5. Hearing sensitivity of Halcyornis toliapicus (A, hearing range; B, mean hearing) based on regressions calculated from data derived from living bird and reptile species (see Walsh et al 2009; reproduced with permission of Royal Society Publishing).…”

“…Here we use a new technique for estimating volume and surface area of brain regions as a percentage of total brain volume. This method is an extension of that used by Walsh et al (2009) for endosseous cochlear duct analysis, and involves digitally isolating the external expression of a particular region to allow surface area and volume measurements of that region to be collected. In the same way that regional dissections of Nissl-stained brains do not include undifferentiated tissue (Iwaniuk & Hurd 2005), this approach does not take into account the internal volume of undifferentiated regions, and may also include volume that was sinusoidal rather than neural tissue in life.…”

Assisted walking in Malagasy dwarf chamaeleons

“…The inner ear of Brookesia is morphologically unique and comparative data suggest that it may be optimized to detect weak accelerations, probably important to ensure postural stability. Indeed, the semicircular canals are very different from those observed for arboreal chamaeleons, such as C. tigris or other arboreal lizards such as A. carolinensis, and resemble those of other slow-moving terrestrial vertebrates such as turtles and tuatara (Walsh et al 2009). Our observations suggest distinct morphological adaptations to the tail and sensory systems allowing these animals to exploit terrestrial habitats, despite the presence of specialized grasping feet.…”

New tyrannosaur from the mid-Cretaceous of Uzbekistan clarifies evolution of giant body sizes and advanced senses in tyrant dinosaurs

“…The cochlear duct is elongated, such that it extends far ventral to the brain endocast and is approximately as long dorsoventrally as the depth of the semicircular canals. A long duct is a synapomorphy of tyrannosauroids (20,26), different from the much shorter ducts of most other theropods, and would have increased sensitivity to lower-frequency sounds (37,38) and may possibly have been related to complex vocalizations and sociality (38).…”