Hearing in Cetaceans: From Natural History to Experimental Biology

Abstract

1Sound is the primary sensory cue for most marine mammals, and this is especially true for 2 cetaceans. To passively and actively acquire information about their environment, cetaceans 3 have perhaps the most derived ears of all mammals, capable of sophisticated, sensitive hearing 4 and auditory processing. These capabilities have developed for survival in an underwater world 5 where sound travels five times faster than in air, and where light is quickly attenuated and often 6 limited at depth, at night, and in murky waters. Cetacean auditory evolution has capitalized on 7 the ubiquity of sound cues and the efficiency of underwater acoustic communication. The sense 8 of hearing is central to cetacean sensory ecology, enabling vital behaviors such as locating prey, 9 detecting predators, identifying conspecifics, and navigating. Increasing levels of anthropogenic 10 ocean noise appears to influence many of these activities. 11Here we describe the historical progress of investigations on cetacean hearing, with a 12 particular focus on odontocetes and recent advancements. While this broad topic has been 13 studied for several centuries, new technologies in the last two decades have been leveraged to 14 improve our understanding of a wide range of taxa, including some of the most elusive species. 15 This paper addresses topics including how sounds are received, what sounds are detected, 16 hearing mechanisms for complex acoustic scenes, recent anatomy and physiology studies, the 17 potential impacts of noise, and mysticete hearing. We conclude by identifying emerging 18 research topics and areas which require greater focus. 19 20 3

Baseline hearing abilities and variability in wild beluga whales (Delphinapterus leucas)

“…The low frequency limit is largely a function of the AEP methods; short-latency, rapidly rising AEP waves are not easily detectable with longer wavelength, low-frequency stimuli (Burkhard et al, 2007). Four kilohertz is often the lower limit for cetacean AEP studies (Mooney et al, 2012). The high-frequency cut-off is likely the hearing limit for each animal.…”

Hearing pathways in the Yangtze finless porpoise,Neophocaena asiaeorientalis asiaeorientalis

“…The AEP technique is increasingly utilized in marine mammals as a means to rapidly, passively and noninvasively investigate hearing (reviewed in Mooney et al, 2012;Nachtigall et al, 2007;Supin et al, 2001). The animal subjects, one male and one female, were originally from the wild, but have resided at the Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China, for the past 6 and 14 years, respectively.…”

“…Thus, physiological response amplitudes in the right ear are higher, but also faster, compared with those to lower amplitude sounds on the left (contra-lateral) side of the head. These physiological mechanism may be in part adaptions to sound speed in water (~5× faster than in air), compensating for the minimal intra-aural time and loudness differences of an aquatic medium (Mooney et al, 2012;Moore et al, 1995). Thus, finless porpoises, and likely other odontocetes, have multiple means to direct and shade sound within the head.…”

Acoustic communication in terrestrial and aquatic vertebrates

“…The ear canal is narrow, filled with cellular debris and most likely non-functional. Middle and inner ears are encased in a bony structure (the tympanic bulla), which is connected only by cartilage and connective tissue to the skull (Au and Hastings, 2008;Mooney et al, 2012). It is currently assumed that in toothed whales, acoustic energy is conducted through the fatty canal of the lower jaw directly to the tympanic bulla.…”