Abstract
Abstract. Baleen whales (Mysticeti) are known to use low frequencies (LF; 200 Hz and below) and infrasound (< 20 Hz) for communication. The lowest hearing limits of toothed whales (Odontoceti), which are able to produce ultrasound (> 20 kHz), reach low frequencies. Researchers have tried to understand the evolution of LF and infrasonic hearing in mysticetes by linking the shape of the inner ear cochlea or individual cochlear measurements to known hearing frequencies and making inferences to extinct species. Using landmark-based shape analysis of complete cochlear coiling, we show that cochlear coiling shape correlates with LF and high-frequency (HF; > 10 kHz) hearing limits in cetaceans. Very LF (≤ 50 Hz) and infrasonic hearing are associated with, for example, a protruding second turn, a descending apex, and a high number of turns. Correlations between cochlear and cranial variables and cochlear and cranial shape indicate that low LF hearing limits are furthermore connected to longer cochleae and relatively larger cranial widths. Very LF hearing in Mysticeti appeared in the middle Miocene, and mysticete infrasonic hearing had evolved by the late Miocene. Complete cochlear coiling is suitable for estimating hearing limits in cetaceans, closely approximated by cochlear length times number of cochlear turns.
Highlights
In mammalian hearing, sound is usually transmitted by vibration of the eardrum or bone conduction into the air-filled middle ear, where it is amplified by the ossicles and further transmitted into the liquid-filled inner ear at the oval window (e.g., Fleischer, 1973)
Throughout the paper we use the following definitions based on human hearing: ultrasound is above 20 kHz, the term “audible range” refers to the human audible range between 20 kHz and 20 Hz, frequencies of 200 Hz and below are called low frequencies (LF), and infrasound is below 20 Hz (e.g., Møller and Pedersen, 2004)
High values on PC1 are associated with very regular cochlear coiling, with the basilar membrane steadily ascending until completion of the second turn
Summary
Sound is usually transmitted by vibration of the eardrum or bone conduction into the air-filled middle ear, where it is amplified by the ossicles and further transmitted into the liquid-filled inner ear at the oval window (e.g., Fleischer, 1973). Inside the cochlea sound waves propagate towards the apex via displacement of the basilar membrane, thereby stimulating the hair cells responsible for the conduction of the acoustic signal to the nervous system. Throughout the paper we use the following definitions based on human hearing: ultrasound is above 20 kHz, the term “audible range” refers to the human audible range between 20 kHz and 20 Hz, frequencies of 200 Hz and below are called low frequencies (LF), and infrasound is below 20 Hz (e.g., Møller and Pedersen, 2004). We use the term “very low frequencies” for hearing frequencies of 50 Hz and below within the audible range. Following Heffner and Heffner (2008), frequencies above 10 kHz are defined as high-frequency hearing
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