Meeting Abstract
Whales and dolphins receive underwater sounds through a completely different mechanism than terrestrial mammals. Instead of hearing through the ear canal and the tympanic membrane, cetaceans hear through specialized fatty tissues leading to an acoustic funnel located just anterior to the tympanic aperture and the vestigial ear canal. We used X-ray computed tomography to trace the ontogenetic development of this unique feature in 57 fetal cetacean specimens from 10 different families of toothed (odontocete) and baleen (mysticete) whales, showing that the acoustic funnel arises from a V-shaped structure formed by the malleus and the goniale. This V-shaped acoustic funnel is one of the first features to develop in the ears and is a relatively well-ossified, prominent, and persistent feature of all early fetal cetaceans. Interestingly, in the echolocating odontocetes, the acoustic funnel develops into a cone-shaped feature facing anteriorly, directly into the intramandibular acoustic fats, which is likely related to their forwardly oriented, highly directional receiving beam. In contrast, the acoustic funnel of balaenopterid mysticetes rotates laterally later in fetal development, which is consistent with a recently hypothesized lateral sound reception pathway. Balaenid mysticetes and several fossil mysticete species retain a somewhat forwardly oriented acoustic funnel in the mature condition, indicating that the lateral sound reception pathway of balaenopterid mysticetes may be a later development both in the ontogenetic and evolutionary history of cetaceans.
