Study finds turtles among animals vulnerable to hearing loss
New research shows that turtles can experience temporary hearing loss from excess underwater noise. This phenomenon, previously observed in other marine animals such as dolphins and fish, was not widely understood for reptiles and highlights another potential risk to aquatic turtles. This loud noise, called underwater noise pollution, can be caused by passing ships and offshore construction.
These preliminary results were part of a study led by the Woods Hole Oceanographic Institution that is being presented at the 2022 Ocean Science Meeting, which was held online from February 24, 2022 to March 4, 2022.
“Our study is the first to support that these animals are vulnerable to underwater hearing loss after exposure to loud noise,” said Andria Salas, WHOI postdoctoral researcher and co-author of the study. “We hypothesized that turtles experience hearing loss when exposed to sufficiently intense sounds as observed in other animals, but no data were collected specifically on turtles.”
Aquatic turtles are predicted to rely on their sense of underwater hearing for environmental awareness, such as navigation or detecting possible predators, and some species have been shown to use acoustic communication underwater. Previous studies have focused on the effects of excessive noise on a range of animals, from squid to fish to whales, and in fresh and salt water environments. But less work has been done on reptiles, such as turtles, according to Salas.
The results of this study provide the first evidence of underwater noise-induced hearing loss in turtle species and suggest that turtles may be more sensitive to sound than previously thought.
Salas and his collaborators, including OMSI associate scientist Aran Mooney, were surprised at how much the turtles’ hearing was affected by the relatively low noise level. Noise exposure induces what is called a temporary threshold shift (TTS), which is the resultant decrease in the animal’s hearing sensitivity due to noise. The absence of TTS studies on turtle species has led to a lack of data for endangered sea turtles and aquatic turtles more generally.
“If this happened in the wild, turtles would be less able to detect sounds in their environment on these time scales, including sounds used to communicate or warn them of approaching predators,” Salas said. “More than half of turtle and tortoise species are threatened, and noise pollution is an additional stressor to consider as we work to protect these animals.”
“It was surprising that we found that noise could induce underwater hearing loss in turtles, and then it was surprising that this hearing loss was at much lower levels than estimated, so lots of surprises all around,” said Mooney said. “Furthermore, the turtles remained fairly calm (or showed no behavioral response) despite the noise being loud enough to induce temporary hearing loss.
Notably, this temporary hearing loss is a normal physiological phenomenon in animals. We now see it at all levels (mammals, birds, fish and reptiles). Importantly, in this case, it may be a predictor of larger and more deleterious sound impacts, such as permanent hearing loss or hearing damage.”
To execute the study, the team conducted experiments on two species of non-threatened freshwater turtles. They used a minimally invasive device, inserted just under the skin above a turtle’s ear, to detect tiny neurological voltages created by the turtles’ auditory system when they hear sounds. The method measures hearing quickly, in just minutes, and is similar to how hearing is measured noninvasively in human infants. Before exposing the turtles to loud white noise (similar to the sound of the static radio), they first determined the turtles’ lower threshold of underwater hearing and the tones (frequencies) they heard best. .
After exposing the turtles to the noise and then removing them from the noise, the researchers continued to measure the turtles’ hearing for about an hour to see how they recovered their short-term underwater hearing, then checked two days later. to see if the recovery was complete. While turtles have always recovered their hearing, hearing loss can last anywhere from around 20 minutes to over an hour. However, sometimes hearing had not recovered by the end of the hour of testing, indicating that they needed more time to fully recover from the noise exposure. A turtle experienced hearing loss for several days.