Investigation into the possible role of dolphins' teeth in sound reception

Abstract

Odontocetes use active sonars for echolocation, navigation and socialisation. These sonars are characterised by narrow transmission and reception directivity patterns, over a variety of ranges. There is physiological and behavioural evidence to suggest that dolphins hear the echoes of their high-frequency clicks through their lower jaws. Current theory suggests that sound is transmitted through a thin region at the base of the jaw into a waveguide leading to the ear. The angular precision predicted by this theory is however much less than dolphins have been observed behaviourally to be able to do. A novel hypothesis is that the teeth of the dolphin's lower jaw act as an end-fire sonar array. This paper will start by putting these competing hypotheses into their context, presenting bioacoustics in the ocean, dolphin echolocation physiology and acoustic behaviour, and the mechanical properties of bones and teeth. This information is used to model the reception of different sounds by bottlenose dolphins (tursiops truncatus) with each possible mechanism, investigating the variations with nerve delay, the role of the jaw relative to the teeth, and the resulting changes in frequency sensitivity and directivity. The results can then be compared with observations of behavioural patterns.

Investigation on acoustic reception pathways in finless porpoise (Neophocaena asiaorientalis sunameri) with insight into an alternative pathway

“…Relative high hearing sensitivities on lower jaw tip were also reported in Risso's dolphins (Grampus griseus) and a Yangtze River finless porpoise (Neophocaena phocaenoides asiaeorientalis) (Mooney et al 2014(Mooney et al , 2015. Even the teeth overlying the lower jaw are considered as a periodic structures system to play roles in sound reception of the odontocetes and as a passive resonator system (Goodson and Klinowska 1990, Dobbins 2007, Graf et al 2008, Dible et al 2009. These studies broaden our view of sound reception in odontocetes as well as raise new points for additional research.…”

“…These studies extracted and treated teeth as an independent periodic system (Dible et al 2009, Graf et al 2008. The surrounding tissues and mandible were ignored in their models.…”

“…The influence of teeth on reception was not examined here. Teeth overlying the mandible have been suggested to function as transducer arrays to create acoustic band gaps for coming sounds and guide sounds in many studies (Goodson and Klinowska 1990, Dobbins 2007, Graf et al 2008, Dible et al 2009 although this is often debated. These studies extracted and treated teeth as an independent periodic system (Graf et al 2008, Dible et al 2009.…”

“…Even the teeth overlying the lower jaw are considered as a periodic structures system to play roles in sound reception of the odontocetes and as a passive resonator system (Goodson and Klinowska 1990, Dible et al 2009, Dobbins 2007, Graf et al 2008. These studies broaden our view of sound reception in odontocetes as well as raise new points for additional research.…”

Numerical-modeling-based investigation of sound transmission and reception in the short-finned pilot whale (Globicephala macrorhynchus)

“…2(b) and 2(c)] systems were assigned in the numerical model. Alongside soft tissues, the model contained skull and air components, whose compressional sound-wave speeds and densities were set as 3380 m/s and 2035 kg/m 3 , and 343 m/s and 1.12 kg/m 3 (Graf et al, 2008;Song et al, 2016). The shear wave speed of the solid skull structures was set to 2200 m/s.…”