Manatees can hear 10 times better than humans underwater, a new study suggests. The discovery leaves researchers wondering why so many get hit by boats.

Very little is known about manatees' hearing ability, so researchers tested the range of sound frequencies the creatures could hear using a technique similar to what doctors use to check hearing abilities of newborn humans.

It turns out that manatees, sometimes described as ugly sea cows, are able to follow sounds changing at rates of up to 1,400 cycles per second, with peak responses at 150 and 600.

"This is remarkably ten times faster than that of humans but only half that of dolphins," said study leader David Mann of the University of South Florida.

There are an estimated 3,500 manatees in Florida waters. Despite their excellent hearing, between 70 and 80 are killed by boats each year. Scientists have wondered if they are good at locating where a sound is coming from.

People locate the origin of sounds based in part on how quickly a sound is heard by each ear. If our brain determines that a sound reached the right ear first, even by only a tiny fraction of a second, the sound's origin must be to our right.

But put us underwater and we can't figure out where a sound is coming from. Sound travels about five times faster underwater and our brain can't tell which ear picks it up first.

Manatees, however, appear to have the ability to sort things out.

"As far as their auditory systems go, indications are that they can hear where boats are coming from," Mann told LiveScience. "A boat's engine is actually a broadband sound that should be easy to locate."

So why do they get hit so often? Maybe manatees do avoid most boats, and we note only the few times one is hit. Or it could be that manatees are just bad at dodging boats, either moving too slowly or inadvertently putting themselves in the way of an oncoming boat.

"It's hard to do unless you're really good at it," Mann pointed out. "Dolphins get hit by boats. Humans get hit by cars."

One common misconception is that boat propellers do the most damage to manatees, but it's actually high-speed collisions with boat hulls, which cause the beasts' lungs to collapse, that are most fatal.

The research was detailed in a recent issue of the Journal of Comparative Physiology.

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