1. Marine mammals depend on their hearing for every aspect of their lives including locating, and communicating with, other belugas, and to find food. As a result, it is important to understand how man-made sounds are affecting marine mammals such as beluga whales.
2. Little is known about the morphology of the inner ears of beluga whales and what the normal morphology looks like. By making a series of measurements of the sensory cells (hair cells) within the cochlea of the inner ear we can determine what normal baselines are, and in turn understand more about beluga hearing.
3. We found that the width of the hair cells, the spacing between hair cells and hair cell density, varies from the inner to outermost portion of the cochlea.
4. Knowing this information about the inner ear morphology can help us identify damaged inner ears, which can then help us determine how noise is affecting these animals.
Hearing is crucial for marine mammals to function in their daily lives and it is important to understand how humans are affecting the hearing of these mammals, such as beluga whales. Environmental change and decreasing ice cover in the Arctic are making new areas accessible to animals and also humans that generate noise. In order to further understand how man-made, or anthropogenic noise, affects these animals, we need to know what normal morphology (or a normal baseline) looks like.
Studying the inner ear morphology found within the cochlea of belugas, particularly the organ of Corti (the hearing organ in mammals), can help with this. Relatively little is known about the inner ear morphology of belugas. The apex is the centremost point of the cochlea where low frequency sounds are detected, and the base is the region closest to the stapes (or exterior of the ear) where high frequencies are encoded. Between the apex and the base, we found there are differences in the width of the sensory cells, the spacing of these cells, as well as the number of cells per millimetre.
Cell width differed between the two regions, from 5.8 µm in the apex to 8.4 µm in the base. The distance between the inner hair cells (IHC) and third row of outer hair cells (OHC), which is the width of the organ of Corti, decreased by 19 µm from the apex to the base of the cochlea. Average outer hair cell density changed from 148 cells/mm in the apex to 117 cells/mm in the base.
These results reveal variation throughout the cochlea, providing further insight on hearing function in belugas. Establishing what normal morphology is allows us to recognize damage if or when we find it and allows for further comparison among populations of beluga to better understand how noise affects these animals.