Infrasonic and Ultrasonic Hearing Evolved after the Emergence of Modern Whales

• Published in: Current Biology
• Main Authors: Mourlam, Mickaël J., Orliac, Maeva
• Other Authors: Institut des Sciences de l'Evolution de Montpellier (UMR ISEM), École pratique des hautes études (EPHE), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Montpellier (UM)-Centre de Coopération Internationale en Recherche Agronomique pour le Développement (Cirad)-Centre National de la Recherche Scientifique (CNRS)-Institut de recherche pour le développement IRD : UR226
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2017
• Subjects: [SDU.STU.PG]Sciences of the Universe [physics]/Earth Sciences/Paleontology; Labyrinth; baleen whales; toothed whales
• Online Access: https://hal.archives-ouvertes.fr/hal-01894431; https://doi.org/10.1016/j.cub.2017.04.061

International audience Mysticeti (baleen whales) and Odontoceti (toothed whales) today greatly differ in their hearing abilities: Mysticeti are presumed to be sensitive to infrasonic noises [1, 2, 3], whereas Odontoceti are sensitive to ultrasonic sounds [4, 5, 6]. Two competing hypotheses exist regarding the attainment of hearing abilities in modern whales: ancestral low-frequency sensitivity [7, 8, 9, 10, 11, 12, 13] or ancestral high-frequency sensitivity [14, 15]. The significance of these evolutionary scenarios is limited by the undersampling of both early-diverging cetaceans (archaeocetes) and terrestrial hoofed relatives of cetaceans (non-cetacean artiodactyls). Here, we document for the first time the bony labyrinth, the hollow cavity housing the hearing organ, of two species of protocetid whales from Lutetian deposits (ca. 46–43 Ma) of Kpogamé, Togo. These archaeocete cetaceans, which are transitional between terrestrial and aquatic forms, prove to be a key for determining the hearing abilities of early whales. We propose a new evolutionary picture for the early stages of this history, based on qualitative and quantitative studies of the cochlear morphology of an unparalleled sample of extant and extinct land artiodactyls and cetaceans. Contrary to the hypothesis that archaeocetes have been more sensitive to high-frequency sounds than their terrestrial ancestors [15], we demonstrate that early cetaceans presented a cochlear functional pattern close to that of their terrestrial relatives, and that specialization for infrasonic or ultrasonic hearing in Mysticeti or Odontoceti, respectively, instead only occurred in fully aquatic whales, after the emergence of Neoceti (Mysticeti+Odontoceti).