The auditory system of the minke whale (Balaenoptera acutorostrata) : a potential fatty sound reception pathway in a mysticete cetacean

Abstract

Despite widespread concerns about the effects of anthropogenic noise on baleen whales (suborder Mysticeti), we lack basic information about their auditory physiology for comprehensive risk assessments. Hearing ranges and sensitivities could be measured if customized equipment and methods were developed based on how baleen whales receive sound. However, sound reception pathways in baleen whales are currently unknown. This thesis presents an integrative approach to understanding hearing in baleen whales through dissections, biomedical imaging, biochemical analyses, and modeling sound propagation through a whale head using the Finite Element Method (FEM). We focused on the minke whale (Balaenoptera acutorostrata) because it is one of the smallest and most abundant mysticete species, reducing logistical difficulties for dissections and experiments. We discovered a large, well-formed fat body extending from the blubber region to the ears and contacting the ossicles. Although odontocetes, or toothed whales, are thought to use specialized "acoustic fats" for sound reception, no such tissues had been described for mysticetes to date. Our study indicates that the basic morphology and biochemical composition of the minke whale "ear fats" are very different from those of odontocete acoustic fats. However, the odontocete and mysticete fatty tissues share some characteristics, such as being conserved even during starvation, containing fewer dietary signals compared to blubber, and having well-defined attachments to the tympano-periotic complex, which houses the middle and inner ears. FE models of the whale head indicated that the ear fats caused a slight increase in the total pressure magnitude by the ears, and this focusing effect could be attributed to the low density and low sound speed of the ear fats in the models. Fatty tissues are known to have lower densities and sound speeds than other types of soft tissues, which may explain why they are an important component of the auditory system of odontocetes, and perhaps mysticete cetaceans as well. In an aquatic habitat where the pinna and air-filled ear canal are no longer effective at collecting and focusing so'und towards the ears, we propose that both odontocete and mysticete cetaceans have incorporated fatty tissues into their auditory systems for underwater sound reception.