April 13, 2012
Manatee Hearing Is Sharp Enough To Hear Boat Motors
Manatees are often referred to as “seacows” due to their peaceful existence, lazily grazing on sea grass.
Though harmless, these manatees are often in danger of being injured by motorboats and other forms of watercraft. These vessels can crash into the unsuspecting manatees, leaving gashes and scars and sometimes even broken rib cages. Though much is known about the seacows, it isn´t yet known why these quiet animals are so vulnerable to human activity.Joe Gaspard works with the Mote Marine Laboratory and Aquarium in Florida, which has been studying the manatees and how they use their senses to perceive their environment. These studies, according to Gaspard, will help the researchers better understand the factors which put manatees at such a high risk. For example, previous Mote research has shown the manatees have poor vision. This, in cooperation with the dark and cloudy waters where they live is one factor in the manatees vulnerability to accidental human interaction. Their research now is focused on whether or not the seacows can even hear the boats and watercraft overhead.
As sound is less absorbed in the water than in air, it travels further and more quickly underwater than above it. This data alone should provide the manatees with ample warning and escape time.
Working with a group of trainers from the aquarium, Gaspard tested the hearing of Buffett and Hugh, two Mote resident manatees.
Their discoveries, published in the Journal of Experimental Biology, suggests the manatees can in fact hear approaching boats, leaving the team with more questions about the risks of the manatees.
In order to test the manatees, Gaspard and his team of trainers taught the animals to swim down to a listening station 1 meter below the surface. There, the manatees were trained to tap a yellow response paddle to earn a treat when they heard a sound. Not to be put out if they didn´t hear anything, the manatees were also given another treat if they stayed in place when they didn´t hear a sound.
"Buffett and Hugh are very cooperative and picked up on the elements of the study quickly," said Gaspard.
Once Buffet and Hugh had the practice down, Gaspard and team began testing noises at different frequencies, gradually reducing the sound until the manatees could no longer hear the sound. The team then plotted these frequencies on a graph to determine where the manatees´ hearing threshold was. According to their data, the manatees heard very well between the frequencies of 8 and 32 kHz and even heard sounds as low as 0.25 kHz as long as they were loud enough.
In fact, Buffet surprised the team when he responded to ultrasonic frequencies as high as 90.25 kHz.
The team then tested the seacows´ ability to hear the same frequencies amidst background noise centered on the same pitch. The team then plotted the resulting data in the same way they had done for the first test and found the manatees struggled to distinguish the higher and lower pitched tones behind the background noise. However, the manatees heard quite well around 8 kHz, the frequency at which they communicate.
So, while manatees can in fact hear sounds underwater, it´s still left to be seen why they do not react when boats come near.
“Manatees might be less aware of these 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.”