RESEARCH ARTICLE Early Development and Orientation of the Acoustic Funnel Provides Insight into the Evolution of Sound Reception Pathways in

Whales receive underwater sounds through a fundamentally different mechanism than their close terrestrial relatives. Instead of hearing through the ear canal, cetaceans hear through specialized fatty tissues leading to an evolutionarily novel feature: an acoustic funnel locat-ed anterior to the tympanic aperture. We traced the ontogenetic development of this feature in 56 fetal specimens from 10 different families of toothed (odontocete) and baleen (mysti-cete) whales, using X-ray computed tomography. We also charted ear ossification patterns through ontogeny to understand the impact of heterochronic developmental processes. We determined that the acoustic funnel arises from a prominent V-shaped structure established early in ontogeny, formed by the malleus and the goniale. In odontocetes, this V-formation develops into a cone-shaped funnel facing anteriorly, directly into intramandibular acoustic fats, which is likely functionally linked to the anterior orientation of sound reception in echo-location. In contrast, the acoustic funnel in balaenopterids rotates laterally, later in fetal de-velopment, consistent with a lateral sound reception pathway. Balaenids and several fossil mysticetes retain a somewhat anteriorly oriented acoustic funnel in the mature condition, in-dicating that a lateral sound reception pathway in balaenopterids may be a recent evolution-ary innovation linked to specialized feeding modes, such as lunge-feeding