Dual sound reception apparatus in protocetid whales

Modern cetaceans dwell in an underwater world of sound. Due to the specific physico-acoustic conditions inherent in the aquatic environment, sound reception pathway in modern whales drastically differs from that of land mammals and implies deep modification of their external acoustic apparatus. To fathom the implementation of this underwater hearing system, the rare data on the auditory region of early whales are paramount. Among them, previous studies on protocetid auditory region highlighted the presence of two potential acoustic portals on the lateral wall of the bulla: a tympanic ring and a tympanic plate. Through an anatomical survey, I explore the external sound reception apparatus of a protocetid whale and discuss the functionality of these two sound portals. The study of the tympanic ring, allow me to propose a reconstruction of the tympanic membrane of this early whale, suggesting that this structure was functional for aerial hearing. 3D investigation of the bone thickness of the bulla reveals the presence of homologous areas of reduced thickness within the tympanic plate of protocetid and modern cetaceans, highlighting a common functioning of this structure for underwater hearing. Thus, this detailed anatomical survey of the lateral wall of a protocetid tympanic bulla confirms the functionality of the two contiguous acoustic portals and sheds new light on the sound transmission mechanism in these early whales.

Gentoo penguins ( Pygoscelis papua ) react to underwater sounds

Marine mammals and diving birds face several physiological challenges under water, affecting their thermoregulation and locomotion as well as their sensory systems. Therefore, marine mammals have modified ears for improved underwater hearing. Underwater hearing in birds has been studied in a few species, but for the record-holding divers, such as penguins, there are no detailed data. We played underwater noise bursts to gentoo penguins ( Pygoscelis papua ) in a large tank at received sound pressure levels between 100 and 120 dB re 1 µPa RMS. The penguins showed a graded reaction to the noise bursts, ranging from no reactions at 100 dB to strong reactions in more than 60% of the playbacks at 120 dB re 1 µPa. The responses were always directed away from the sound source. The fact that penguins can detect and react to underwater stimuli may indicate that they make use of sound stimuli for orientation and prey detection during dives. Further, it suggests that penguins may be sensitive to anthropogenic noise, like many species of marine mammals.

Silent sounds in the Andes: underwater vocalizations of three frog species with reduced tympanic middle ears (Anura: Telmatobiidae: Telmatobius)

Underwater vocalization in anurans is restricted to a few, distantly related species. In some of them, sound is transmitted through tympanic and extra-tympanic pathways. Members of the Andean genus Telmatobius Wiegmann, 1834 lack a tympanic membrane, and earlier reports assumed the absence of vocalizations in the genus. We recorded underwater vocalizations and examined the middle-ear morphology in three species of Telmatobius with different lifestyles: Telmatobius oxycephalus Vellard, 1946 (semiaquatic, riverine); Telmatobius hintoni Parker, 1940 (markedly aquatic, riverine); Telmatobius culeus (Garman 1876) (fully aquatic, lacustrine). Males emit underwater calls, which in the three species are simple and stereotyped; they consist of a repeated train of notes, with a low fundamental frequency (309–941 Hz). In each of the three species, the tympanic membrane is absent and the tympanic cavity is extremely reduced or absent, whereas the opercular system is well developed. Our data, along with prior knowledge in other species of anurans, suggest that the species examined here probably perceived sound through extra-tympanic pathways. Given the limited knowledge about underwater calling in anurans, Telmatobius seems a logical candidate to study the functional and evolutionary bases of underwater hearing and tympanic middle-ear reduction in anurans.