Possible age-related hearing loss (presbycusis) and corresponding change in echolocation parameters in a stranded Indo-Pacific humpback dolphin

Abstract

SUMMARYThe hearing and echolocation clicks of a stranded Indo-Pacific humpback dolphin (Sousa chinensis) in Zhuhai, China, were studied. This animal had been repeatedly observed in the wild before it was stranded and its age was estimated to be ~40years. The animal's hearing was measured using a non-invasive auditory evoked potential (AEP) method. Echolocation clicks produced by the dolphin were recorded when the animal was freely swimming in a 7.5m (width)×22m (length)×4.8m (structural depth) pool with a water depth of ~2.5m. The hearing and echolocation clicks of the studied dolphin were compared with those of a conspecific younger individual, ~13years of age. The results suggested that the cut-off frequency of the high-frequency hearing of the studied dolphin was ~30-40kHz lower than that of the younger individual. The peak and centre frequencies of the clicks produced by the older dolphin were ~16kHz lower than those of the clicks produced by the younger animal. Considering that the older dolphin was ~40years old, its lower high-frequency hearing range with lower click peak and centre frequencies could probably be explained by age-related hearing loss (presbycusis).

Echolocation signals of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis) in Sanniang Bay, China

“…Although the sounds of Indo-Pacific humpback dolphins have been investigated in a number of studies (Schultz and Corkeron, 1994;Van Parijs and Corkeron, 2001a,b;Goold and Jefferson, 2004;Nanayakkara et al, 2007;Li et al, 2012;Sims et al, 2012;Li et al, 2013;Lin et al, 2013;Wang et al, 2013;Kimura et al, 2014), the present study represents the first documentation of characteristics of the onaxis clicks of Indo-Pacific humpback dolphins in their natural habitat, obtained using a broadband and high-frequency acoustic sampling system. The five-hydrophone array recording system made it possible to determine which clicks were on axis.…”

“…5(b). BW rms ranged from 23.1 to 38.3 kHz, with an average of 28.5 kHz; 55.8% of the BW rms TABLE I. Descriptive statistical analysis of click parameters from wild Indo-Pacific humpback dolphins and a comparison with previously calculated statistics from two humpback dolphins in captivity (Li et al, 2013 had a peak distribution between 25 and 30 kHz, and only 3.9% of the BW rms was over 35 kHz.…”

“…Beluga whales have been shown to produce clicks with higher SLs when moved to a noisier environment (Au et al, 1985). Table I shows that f c of clicks from the wild IndoPacific humpback dolphins were lower than those of the clicks from a young conspecific in captivity and higher than those of the clicks from an old conspecific in captivity (Li et al, 2013). One possible explanation is that the investigated dolphin group in the wild consisted of individual of varying ages, and the old dolphins in the wild may also produce clicks with lower f c values when compared with those of young dolphins.…”

“…b) Electronic mail: wangd@ihb.ac.cn (A-tag), which only records high-frequency phonating events and peak pressures, but not waveforms of the high-frequency sounds Kimura et al, 2014). Ultrasound recordings of Indo-Pacific humpback dolphins with frequencies higher than 100 kHz have only been obtained from a single hydrophone system under opportunistic and uncontrolled recording conditions in the field (Goold and Jefferson, 2004) or in rescue scenarios of few stranded individuals in captivity (Li et al, 2012;Li et al, 2013) by high speed digital sampling. When using a single hydrophone in the wild, the position of the phonating animals with respect to the hydrophone is uncertain.…”

Apparent source level of free-ranging humpback dolphin,Sousa chinensis, in the South China Sea

“…Echolocation clicks are considered to be broadband sounds with a high peak frequency of more than 100 kHz (Goold & Jefferson, 2004;Li et al, 2012Li et al, , 2013, and are similar to those produced by the Delphinidae family. In the present study, we estimate the SLs of on-axis echolocation signals, when free-ranging humpback dolphins manoeuvre their biosonar beam to focus on hydrophone arrays.…”