Can Manatees Hear Boat Propellers?

Some news from my old stomping grounds at Mote Marine Lab in Sarasota, Florida. Back in undergrad I worked there with the manatees Hugh and Buffet to test their visual acuity (its bad), now the marine observatory is attempting to test the hearing of manatees. This is essential, as the most likely cause of death for a manatee is an "meeting" with the propeller of a speed boat. Can they hear the approach of a boat? And, if so, how loud or far away can they hear it?


When a buzzer sounds, the speed bump-shaped mammal slowly flips his 1,300 pounds and aims a whiskered snout toward one of eight loudspeakers lowered into the water. Nosing the correct speaker earns him treats.

Researchers contend that if the plant-eating beasts seem slow-witted, it is because they faced no threats to their survival before the advent of boat propellers.

"They're not under any selection pressure to evolve the rapid-type behavior we've associated with hawks, a predator, or antelopes, a prey. They look like very contented animals that don't have very much to do all day," said Roger Reep, a neuroscientist at the University of Florida's College of Veterinary Medicine.

The experiments under way at the independent Mote Marine Laboratory, could help scientists protect Florida's manatees, an endangered species, from propellers and other dangers.

At least 75 manatees have been killed this year in collisions with watercraft, according to the Florida Fish and Wildlife Conservation Commission.

Despite recent findings that suggest the animals hear well enough to avoid boats, researchers are not sure why manatees keep getting hit. They could be surfacing to breathe while sleeping, or they may have grown too accustomed to the sound of boats.

At the Mote laboratory, the buzzer experiments are hearing tests in which the tones gradually grow shorter and softer. The researchers want to know: At what distance could Hugh hear a boat's propeller churning in the water? Could Hugh determine where the sound is coming from?

Scientists have long assumed brains with many folds -- such as those belonging to dolphins and humans -- are a sign of intelligence. But Reep argues the cause behind those brain folds is unknown, and smooth-brained manatees don't seem to be missing anything important.

"The brain looks just as complex internally as any other mammalian brain," said Reep, co-author with Bob Bonde, a Florida biologist with the U.S. Geological Survey, of a book on manatee physiology.

Hugh and his half-brother Buffett, both born in captivity, have spent many days since 1998 showing researchers what they can learn. The manatees have been trained to respond to whistles and stop at underwater targets.

Scientists want to determine how well manatees distinguish colors and objects, and how sensitive their hearing and sense of touch are.

Hugh and Buffett are rewarded with handfuls of sliced apples, carrots and beets for correct test responses. The manatees pull the food into their mouths with lips dexterous enough to untie knots.

In the vision tests [authors note: this is what I worked on], Buffett and Hugh do not see particularly well out of their tiny eyes, though it appears they do see color, an unusual trait for marine mammals. But even by manatee standards, Hugh seems to be visually impaired.

In manatee brains, the regions that process sight are quite small compared to auditory and tactile regions, according to Reep and Bonde.

Researchers have discovered that manatee muzzles and bodies are covered with sensitive hairs that may help them detect underwater objects and current changes.


More like this

They just act stoned. As Dean Wormer would tell you, "Fat, drunk, and stupid is no way to go through life." We can rule out that manatees are drunk -- although alcohol run off from Panama City Beach may bump the Gulf of Mexico up to 10 proof, that's hardly enough liquor to get a half ton beast…
As a newly-minted ScienceBlogger, now I get to chime-in on all the fun "Ask A ScienceBlogger" questions. This week's question is: "Assuming that time and money were not obstacles, what area of scientific research, outside of your own discipline, would you most like to explore? Why?" As an undergrad…
I grew up in Florida, in central Florida to be exact. As a kid we went to Blue Springs and other manatee havens on field trips, to observe these gentle and curious animals. They are huge, and as they tend to inhabit shallow areas of the Florida coastal waterways its easy to see them in the clear…
As alerted by other ScienceBloggers, I have recently learned that the US Fish & Wildlife Service is proposing to the White House that Florida's manatee be removed from the "endangered" list, downgrading the marine mammal to merely "threatened." The result would be a relaxation of boating…

When I was in elementary school, we "adopted" a manatee, and studied about them and the dangers that they face. What a mess. They're a pretty curious animals, too. Since they don't face any natural predators, I wonder if they hear the noise from the boats, but don't recognize that the noise is something dangerous until it's too late?

They're a pretty sad case. Besides the threat that boats and poachers represent, there's also the problem of the manatees' increased reliance on artificial heat sources during the cold months. When they shut down or move the power plants, it looks like many of the animals that rely on that heat end up dying from cold stress when they don't move to warmer waters.