Grazing sea grass along the subtropical Florida coast, manatees would seem to have an idyllic life; that is until a passing motorboat shatters their peace. But motorboats and other watercraft pose an even greater threat, injuring the peaceable mammals with their propellers, or shattering their ribcages and puncturing their lungs in collisions. Joe Gaspard from the Mote Marine Laboratory and Aquarium, USA, explains that it isn’t clear why the animals are so vulnerable to human activity. Although their vision is known to be extremely restricted in the turbid water, Gaspard points out that 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. However, it wasn’t clear whether manatees have the ability to hear looming watercraft above the cacophony of their natural environment. Teaming up with Gordon Bauer, Roger Reep and David Mann, and a group of trainers from the aquarium, Gaspard decided to test the hearing of two captive manatees, Buffett and Hugh, to find out what they are capable of hearing (p. 1442).
‘Buffett and Hugh are very cooperative and picked up on the elements of the study quickly’, remembers Gaspard, who worked with Kim Dziuk, Adrienne Cardwell and LaToshia Read to train the animals to swim down to a listening station 1 m beneath the surface. Switching on a light to indicate to the animals that a test was about to start, the team then trained the manatees to touch a yellow response paddle in return for a tasty fruit or vegetable snack when they heard a sound. They also trained the manatees to stay in place (in return for another snack) when they heard nothing. Once Hugh and Buffett had got the task in hand, the team tested their hearing by selecting a particular sound frequency (pitch) and gradually lowering the volume of the sound until the manatee 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 32 kHz and could even hear sounds as low as 0.25 kHz – so long as they were quite loud. However, they were even more amazed when Buffett appeared to be able to hear ultrasonic frequencies as high as 90.5 kHz. ‘Buffett did the task but refused to continue once we got to that frequency, so we think it was aversive or annoying’, Gaspard recalls.
Intrigued by the manatees’ apparently sensitive hearing, the team then tested how well the mammals performed when the sounds were accompanied by background noise. Playing test tones – ranging from 4 to 32 kHz – against background noise of 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. Plotting the critical ratio – the level at which the background noise swamped the manatee’s hearing – 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 8 kHz – the frequency at which manatees communicate – where they could still distinguish tones that were only 18.3 dB louder than the background.
So, it appears that manatees should be able to hear approaching motorboats above background noise and Gaspard is curious to find out why the animals do not appear to respond in time. ‘Manatees are good at sleeping and eating and some of these elements may overwhelm their auditory sensitivity’, suggests Gaspard.