Amphibious hearing in spotted seals (Phoca largha): underwater audiograms, aerial audiograms and critical ratio measurements

Abstract

Spotted seals (Phoca largha) inhabit Arctic regions that are facing both rapid climate change and increasing industrialization. While little is known about their sensory capabilities, available knowledge suggests that spotted seals and other ice seals use sound to obtain information from the surrounding environment. To quantitatively assess their auditory capabilities, the hearing of two young spotted seals was tested using a psychophysical paradigm. Absolute detection thresholds for tonal sounds were measured in air and under water over the frequency range of hearing, and critical ratios were determined using octave-band masking noise in both media. The behavioral audiograms show a range of best sensitivity spanning four octaves in air, from approximately 0.6 to 11 kHz. The range of sensitive hearing extends across seven octaves in water, with lowest thresholds between 0.3 and 56 kHz. Critical ratio measurements were similar in air and water and increased monotonically from 12 dB at 0.1 kHz to 30 dB at 25.6 kHz, indicating that the auditory systems of these seals are quite efficient at extracting signals from background noise. This study demonstrates that spotted seals possess sound reception capabilities different from those previously described for ice seals, and more similar to those reported for harbor seals (Phoca vitulina). The results are consistent with the amphibious lifestyle of these seals and their apparent reliance on sound. The hearing data reported herein are the first available for spotted seals and can inform best management practices for this vulnerable species in a changing Arctic.

Amphibious hearing in ringed seals (Pusa hispida): underwater audiograms, aerial audiograms and critical ratio measurements

“…The audiograms for the two subjects are in good agreement from 0.1 to 25.6 kHz, above which the adult male (Natchek) exhibits apparent high-frequency hearing loss. The thresholds of the young female (Nayak) are quite similar to those reported recently for harbor (Kastelein et al, 2009;Reichmuth et al, 2013) and spotted seals (Sills et al, 2014) across the hearing range. In contrast to historical data, these ringed seal audiograms are consistent with the hypothesis of a functional hearing group for northern seals.…”

“…Regardless, the in-air thresholds measured for these ringed seals are among the lowest reported for marine mammals. When compared with available data for ice-living seals, these ringed seal audiograms -along with recent data for spotted seals (Sills et al, 2014) -show significantly better sensitivity to airborne sounds than measured previously for one harp seal (Terhune and Ronald, 1971). While others have suggested that the harp seal thresholds were elevated as a result of noise (Watkins and Wartzok, 1985;Moore and Schusterman, 1987), the reported ambient noise levels and CRs (Terhune and Ronald, 1971) suggest that masking was not a relevant factor.…”

“…This study with ringed seals parallels and complements a set of experiments recently conducted with spotted seals (Sills et al, 2014) as part of a larger effort to describe hearing, and the effects of noise on hearing, in ice-living seals. The methods and testing environments used for the two species were nearly identical.…”

“…The audiograms obtained in this study demonstrate acute aerial hearing sensitivity for ringed seals that is comparable to that of spotted (Sills et al, 2014) and harbor seals (Reichmuth et al, 2013). Although they forage and travel extensively at sea, ringed seals rely on sea ice as a substrate for resting, whelping and molting, and experience terrestrial predation pressure from polar bears.…”

“…The northern seals (subfamily Phocinae) include the ringed, Baikal (Pusa sibirica), Caspian (Pusa caspica), spotted (Phoca largha), harbor (Phoca vitulina), grey (Halichoerus grypus), ribbon (Histriophoca fasciata), harp (Pagophilus groenlandicus), hooded (Cystophora cristata) and bearded seals (Erignathus barbatus). Among these species, hearing thresholds are currently available for spotted (Sills et al, 2014), harbor (Møhl, 1968a;Terhune, 1988Terhune, , 1991Kastak and Schusterman, 1998;Wolski et al, 2003;Southall et al, 2005;Kastelein et al, 2009;Reichmuth et al, 2013), Caspian (Babushina, 1997), harp (Terhune and Ronald, 1972) and ringed seals (Terhune and Ronald, 1975a). Of note relative to the present study, however, is that sensitivity data for ringed, harp and Caspian seals do not extend to the low frequencies, and ringed seal hearing has only been studied in water.…”