A new study on manatee hearing by Mote Marine Laboratory and collaborators shows that these marine mammals can sense a wide range of pitches despite loud background noise.
The study, published today in the peer-reviewed Journal of Experimental Biology, demonstrates that manatees can hear frequencies produced by boat engines, pointing to new questions about how manatee hearing operates in the wild and why these mammals remain vulnerable to watercraft.
The study tested the hearing abilities of Mote's resident manatees, Buffett and Hugh â€" the world's only manatees trained to participate in behavioral research and husbandry (animal care) procedures.
The project was led by Mote's manatee care, training and research coordinator Joseph Gaspard III in collaboration with a team of scientists who do ongoing sensory studies with Hugh and Buffett: Mote adjunct scientists Dr. Gordon Bauer, professor at New College of Florida, and University of South Florida professor Dr. David Mann, along with University of Florida professor Dr. Roger Reep. Collaborators also included the manatee training staff at Mote Aquarium â€" the public outreach facility where Hugh and Buffett live.
For more than 14 years, Mote research has focused on how manatees use their senses to perceive their environment in an effort to understand the factors that put manatees at risk. Their studies have already shown that manatees' vision is poor, compounded by the turbid and tannic waters where they spend much of their lives.
Florida manatees (Trichechus manatus latirostris) are an endangered species protected by state and federal laws. While conservation and management efforts have played important roles in supporting manatee populations, these marine mammals continue to face many threats, including habitat loss, severe cold stress during winter, entanglement in marine debris and boat strikes.
Can manatees hear boats? And can they hear them above the cacophony of sounds in their natural environment? Sound is absorbed less in water than in air, potentially allowing it to travel farther. It also travels five times faster in water than in air, theoretically warning the animals earlier of an approaching threat, Gaspard said.
Gaspard and his colleagues tested the hearing of Buffett and Hugh using a training program begun through collaboration between New College and Mote. Their results indicate that manatees can hear within the frequency range where boats operate but lead to new questions about why manatees remain at risk.
"Buffett and Hugh are very cooperative and picked up on the elements of the study quickly," said Gaspard, who worked with his animal care and training team to train Hugh and Buffett to participate in the research.
During the study, Hugh and Buffett were trained to swim down to a listening station about 3 feet beneath the water's surface and perform a task when sounds of different frequencies were played. In this case, when Hugh and Buffett heard sounds, they were supposed to touch a yellow paddle. When they heard nothing, they were trained to stay in place.
The team tested their hearing by selecting a particular sound frequency, or pitch, and gradually lowering the volume of the sound until the manatees could no longer hear it. Plotting these "hearing thresholds" on a graph, the team could see that the manatees had good hearing between 8 and 32kHz and could even hear sounds as low as 0.25kHz â€" though the lowest and highest sounds had to be quite loud for the manatees to hear them.
One surprising thing the researchers found was that Buffett appeared to be able to hear ultrasonic frequencies as high as 90.5kHz. "Buffett did the task but refused to continue after the first round at that frequency, so we think the sound itself was aversive or annoying," Gaspard said.
Intrigued by the manatees' apparently sensitive hearing, the team then tested how well the mammals performed when the sound frequencies were also accompanied by background noise.
Playing test tones â€" ranging from 4 to 32kHz â€" against background noise that was centered on the same pitch, the team recorded the difference between the volume of the tone and background noise when the manatee could no longer distinguish the tone. By plotting the critical ratio â€" the level at which the background noise swamped the manatee's hearing â€" against pitch for each animal, the team saw that the manatees struggled to hear lower and higher pitched sounds above background noise.
However, their hearing was much sharper at 8kHz â€" the frequency at which manatees communicate â€" where they could still distinguish tones that were only 18.3 decibels louder than the background.
The findings mean that it appears that manatees should be able to hear approaching motorboats above background noise. But the question remains whether manatees can always focus on these sounds in nature, Gaspard says. "Manatees might be less aware of boat sounds when they are sleeping, eating or performing other activities related to their daily lives that require their full attention,' says Gaspard. "There are also a multitude of environmental factors that come into play. Understanding how animals use their various senses is a complex process. Could their sense of touch also be playing a role here? We are working on that question now."
An abstract of the results is available from the Journal of Experimental Biology at http://jeb.biologists.org/content/215/9/1442.abstract.