The world is a perilous place for the endangered manatee. While the mammals are at risk from natural threats, human activity also poses a great danger to manatee numbers. Debborah Colbert, from the Association of Zoos and Aquariums,explains that many manatees die and are seriously injured in collisions with boats every year. However, little is known about how manatees perceive their environment. Whether they can localise sounds, and specifically whether they can tell which direction a boat is approaching from, are crucial factors in the development of manatee protection programmes. Colbert and her colleagues decided to test whether the mammals can pinpoint sound sources(p. 2105).
Working at the Mote Marine Laboratory and Aquarium in Florida, Colbert was able to work with two male manatees, Hugh and Buffett, when she initiated a research programme to find out more about these enigmatic creatures. Born in captivity, the young males had already surprised people who had thought that manatees were `not intelligent enough to train' says Colbert. Both young males had been trained to participate in a series of sensory studies, so Colbert,Joseph Gaspard 3rd, Gordon Bauer and Roger Reep patiently trained the animals to swim to a specific stationing platform in their enclosure where they could listen to sounds played from one of four speakers arranged around their heads.
Knowing that the manatees' hearing was most sensitive to sounds ranging from 10 to 20 kHz, while the animals' calls range from 2.5 to 6 kHz, Colbert and David Mann designed three sounds ranging from 0.2 to 20 kHz, 6 to 20 kHz and 0.2 to 2 kHz to play to the animals. The team also selected two single frequency (tonal) sounds at 4 kHz and 16 kHz to test how the manatees responded to less complex sounds. Having trained the manatees to swim to the speaker that they thought the sound came from, the team then played the broadband sounds, of 0.2, 0.5, 1 to 3 s, from each speaker at random while monitoring the animals' responses.
Yet again the manatees impressed their supporters. Buffett successfully identified the source of the broadband sounds with almost 90% accuracy, while Hugh did slightly less well. The team was also surprised that the manatees were able to locate the sources of both the 4 kHz and 16 kHz tones, although they only tested the animals with the longest of the two tonal notes, as the manatees had shown signs of frustration when they heard these sounds.
So how are the animals able to localise sounds? Colbert explains that many terrestrial animals use the time difference between a sound arriving at their two ears to find the source. However, this time difference is probably extremely short in aquatic animals, as sounds travel five times faster in water than in air. Animals also use the intensity difference as the sound arrives at each ear, which is more pronounced in high-pitched noises, to pinpoint the source. Colbert suspects that the manatees use combinations of these and other cues to help them localise sounds, as they were able to locate the sources of high- and low-pitched sounds equally well.
Crucially, the animals can probably hear approaching speed boats and tell which direction they are coming from, which is an essential piece of information for conservation organisations as they battle to save this gentle giant.