At some point in our lives most of us will have resorted to shouting during a conversation with an elderly relative. Unfortunately, as we age, sounds have to be louder to be heard and high-pitched sounds are no longer perceived. But are humans alone in suffering auditory decline with age? Certainly in captive dolphins, older members of the family do show signs of age-related hearing loss, but the same has never been observed in the wild. However, when a male Indo-Pacific humpback dolphin approximately 40 years old was rescued after it had stranded itself in an inland river near Foshan, China, in March 2012, Songhai Li, from Sanya Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences, China, and his colleagues seized the opportunity to find out (p. 4144).
The team began by giving the rescued dolphin a hearing test. To do this, three EEG (electroencephalograph) suction-cup electrodes were placed on the dolphin's head and back, to measure electrical activity in response to a range of different sounds. The researchers varied both the frequency (kHz) of the sounds and the loudness to find the lowest threshold of hearing, namely the frequency of the quietest detected sound. For the 40-year-old dolphin this corresponded to a sound at 38 kHz. By comparing this with a hearing test from a younger 13-year-old male dolphin that had been stranded in a similar spot back in August 2007, the team could see that in the younger dolphin this threshold was at a higher frequency. Overall, the results showed that the older dolphin had more difficulty hearing high-frequency sounds, with a cut-off about 30–40 kHz lower than for the younger dolphin.
The team next went on to record the dolphin's echolocation clicks using a hydrophone. Overall, compared with the younger dolphin, the older dolphin's peak and centre click frequencies were about 16 kHz lower. This shift suggests that the older dolphin shifted its echolocation clicks towards frequencies that it is able to hear. Both the decreased sensitivity to high-frequency calls and the compensation in vocalisations strongly suggest that wild dolphins also suffer from hearing loss.