Echolocating animals rely on their hearing to navigate and hunt. Bat's hearing is finely tuned to their high-pitched squeaks but what about echolocation toothed whales' (odontocetes) hearing? How sensitive is it and what can they hear precisely? Aude Pacini from the University of Hawaii says, ‘We don't have a lot of information about the hearing abilities of whales, so whenever we get the opportunity to test a new species, wherever it is in the world we go to it.’ So, when Pacini's supervisor, Paul Nachtigall, heard about a young long-finned pilot whale called Nazaré at Lisbon Zoo that had been saved after stranding as a baby, he struck up a collaboration with the Zoo's chief scientist, Arlete Sogorb, and her assistant, Sonia Matias, with the hope of measuring the whale's hearing ().
Months before Nachtigall, Laura Kloepper and Pacini got on a flight to Lisbon, Sogorb's team in Portugal trained Nazaré to familiarise him with the equipment that the team would use to measure his hearing. Arriving in Europe, the team were joined by Meike Linnenschmidt from the University of southern Denmark, ready to test Nazaré's hearing.
Stationing the pilot whale at the side of his enclosure, the team attached a suction cup with an electrical sensor just behind his blowhole to measure Nazare's brain activity when he heard the sounds, and two other electrodes on the animal's back and dorsal fin to keep track of all of the other electrical activity in the whale's body. Playing beeping sounds at various intensities and pitches ranging from 4 kHz up to a high pitched 100 kHz, the team gradually built up a picture of the Nazaré's hearing and were surprised to find that the whale hardly responded to frequencies above 50 kHz.
‘For toothed whales 100–150 kHz is usually the high-frequency cut-off,’ says Pacini. ‘Even though it was a new species and it was a stranded animal, I would have expected more sensitive hearing.’
Wondering what could account for Nazaré's relatively limited hearing range, the team asked for Nazaré's medical records and found that he was prescribed antibiotics during his recovery – some of which are known to damage the high-frequency hearing of humans and could explain Nazaré's relatively limited hearing range.
Alternatively, Nazaré's hearing could be at the weaker end of the pilot whale hearing spectrum or it could even be perfectly normal for pilot whales. Pacini explains that false killer whales have a similar hearing range to Nazaré.
‘The next step is really important: what do other animals of the same species hear? Is Nazaré an exception or representative?’ says Pacini, who is keen to measure the hearing abilities of other long-finned pilot whales soon.
Pacini adds that our current understanding of odontocete hearing is relatively limited and it is important to learn more about whale hearing to design the best conservation strategies to protect the delicate hearing of these sensitive echolocating animals and possibly even prevent other pilot whales from stranding like Nazaré.
