Humans aren’t the only ones who lose their hearing as they grow older. Scientists report that wild Indo-Pacific humpback dolphins (Sousa chinensis), which can live 40-plus years, also have trouble picking up sounds as they age. Other researchers had noted that the hearing of captive bottlenose dolphins and a captive false killer whale declines over time, but this had never been observed in the wild. In the new study, a team learned that a male humpback dolphin about 40 years old had been rescued after stranding itself in a river near Foshan, China. The scientists took the opportunity to test the animal’s hearing after its health stabilized. Using electrodes attached with suction cups to the dolphin’s head and back, the scientists produced audiograms (barcodelike images) that showed the animal’s auditory response to a range of tone pips, like those used in the Greenwich Time Signal, varying their frequency and loudness. By comparing the audiograms, the researchers were able to determine the softest noise the cetacean could detect. They also recorded the dolphin’s echolocation clicks, which the creatures use to navigate, hunt, and communicate, as it swam freely in a pool. In 2011, the researchers had collected similar data from a 13-year-old male humpback dolphin that had also been rescued after being stranded. Compared with the younger dolphin, the 40-year-old had trouble hearing high-frequency sounds; he also made echolocation clicks that were of a lower frequency than those of the 13-year-old, suggesting that the older dolphin had shifted his clicks into a range that he was able to hear, the team reports today in The Journal of Experimental Biology. Perhaps the dolphin’s hearing had been affected by the shipping and boating noises in his habitat; but more likely, the researchers say, his auditory sensitivity had declined for the same reason it does in humans, and likely most mammals: He had lived a long life.
*Correction, 1 November, 3:10 p.m.: Researchers collected data from the 13-year-old male humpback dolphin in 2011, not 2007, as was previously reported.