It seems to be a good time to post a research update coming from the auditory physiology side of the project. I’m currently wrapping up a research trip to Europe, where I worked on multiple different components of the project with the other team members in the UK and Netherlands. Olivier and I also had a productive visit to a bird hearing expert earlier this week in Oldenburg, Germany.
I have been reviewing the scientific literature on low frequency hearing in birds, including the various experimental studies and hypothesized detection mechanisms. There are indeed many factors that can control the detection of low frequencies by birds. Sound vibrations move through the middle ear, inner ear, and auditory hair cells, ultimately becoming encoded into a neural signal at the end of the transmission line. At each of these levels, there are distinct structure-function relationships that will affect signal transmission. There is likely no single anatomical marker that will precisely reflect infrasonic hearing abilities, but the relevant anatomical parameters and how they might be expected to affect low frequency transmission is becoming clearer.
Multiple building blocks need to come together to pursue the big question, “do seabirds detect infrasound?”. One of these building blocks is a quantification of the large-scale patterns of auditory anatomy in seabirds. For this goal, I have been collecting microCT scans of seabird heads and skulls. In general, not a lot has been described on the auditory anatomy of seabirds.
The microCT images are quite cool to view and analyse. Below are microCT images of white-chinned petrel, Procellaria aequinoctialis (A). This species is commonly caught as bycatch from long-line fisheries vessels operating in South African waters, and that was my source for this particular specimen (thanks to folks at Capfish and BirdLife). You can even see a fish still in the throat and a bit of the hook apparatus on the lower right. Here are some additional images showing the reconstruction process for this technique. You can see clearly on this image the columella (middle ear bone of birds) and inner ear (B). After some analysis, the 3D models of specific components can be reconstructed (C,D).
One possible further application of the comparative microCT data is to use the 3D surface models to simulate acoustic transmission properties of the ear. This can be an option for animals for which direct hearing assessments are difficult, such as whales. The simulation models are then verified by direct vibrometric observation of middle ear. This technique might be a useful method for testing between the performance of different routes of acoustic stimulation. For example, the vibration transmission performance of the middle ear in tetrapod animals (including birds) generally declines towards low frequencies. Alternatively, sound at these very low frequencies might effectively couple to the whole body of the bird itself.
After some delay with getting permits to collect seabird heads (partly due to restrictions related to the bird flu striking earlier in the year), we have the permissions we need and I hope to be examining many more species soon.