Crocodiles are amphibious animals that go back and forth between air and water. Hence, their acoustic world is shared between these two mediums that strongly differ by their physical properties. Although the number of studies on crocodilian acoustic communication and hearing has recently increased, most experiments and observations have focused on crocodilian aerial hearing abilities and our knowledge about their underwater acoustic world remains scant.
How crocodiles localize sounds in both air and water appears to be a central question. Previous studies have brought evidence for directional hearing in crocodiles, suggesting the existence of specialized adaptations. For instance, anatomical data show the presence of air space in the skull providing connections between the ears resulting in a pressure difference receiver. Experiments and observations suggest that this may improve directional hearing but it remains unclear how this anatomical particularity influences the cues that are classically used by other animals and humans for sound localization (ITD, ILD and the monaural cues in the HRTF). Finally, although theoretical considerations predict that locating a source of water-borne sound should be even more difficult than in the air, two pilot studies demonstrated that crocodilians seem able to localize an underwater sound source. The few experimental studies available in crocodilians thus appeals for further research.
The aim of the PhD project will be to investigate how crocodiles navigate in their amphibious acoustic world. Using ethological experiments coupled to physical measurements aimed to characterize the acoustic cues available for sound location, we will mainly focus on the crocodiles’ ability to localize and discriminate between sound sources, two important aspects of the auditory scene analysis.
Outcome of the project: There is currently a great interest in understanding the evolution of the archosaurian lineage. Crocodilians diverged from birds more than 240 million years ago and they are of primary interest in the reconstruction of ancestral archosaurian biology. In this perspective, increasing our knowledge about crocodilian biology is a necessary step. More broadly, the research performed during this PhD project will bring new knowledge about auditory scene analysis, a universal problem faced by all animal species living in complex environments.
Logistics. Experiments on captive individuals will be performed at the ENES lab (Saint-Etienne) and in various zoos (e.g. La Ferme aux Crocodiles, Pierrelatte). Field experiments will probably be performed in Brazil (Pantanal). The focal species will mainly be the Nile crocodile and the spectacled caiman.
Supervision. Nicolas Grimault (CNRL CNRS UMR 5292) and Nicolas Mathevon (Université de Saint-Etienne, ENES/Neuro-PSI CNRS UMR 9197).