Neuroscience for Kids - Manatee Senses

Manatee Senses

By Ellen Kuwana
Neuroscience for Kids Staff Writer
April 28, 2003
Our five senses help us navigate our world. Manatees have an extra sense that humans do not -- a system of hairs around their mouth and on their body that helps them sense in the shallow, murky waters where they live. These hairs not only feel for the aquatic plants that make up the manatees' diet, the whiskers can actually grasp sea grasses! This combined sensory and motor use of the whiskers seems to be unique to manatees.

The Hairy Details
Dr. Roger L. Reep and his colleagues at the University of Florida have been studying these amazing manatee hairs. They found that these gentle animals had approximately 1500 hairs on each side of their body (dorsal and ventral, or topside and underside) and that each hair was spaced far enough apart so as to be able to move around without touching another hair. Each hair is connected to 20-50 axons. This means that the each hair conveys information to many neurons. (By comparison, each hair on your arm is connected to approximately five neurons.) This suggests that the hair system must be very important to the manatees. The scientists theorize that the manatees use the hairs to get information about water currents and other animals in the water. This sensory system would be quite useful in gathering information in the dark waters where the animals have limited vision.

A Mystery Solved
Why, despite laws requiring boats to slow down in Florida waters where manatees live, are manatees still killed by speedboats? Manatee deaths reached record highs in 2000 and 2001. Although manatees can swim up to 6.4 meters per second, most of the time the animals swim slowly. Why do they collide so often with boats? Why don't they swim away from the boats?
To investigate this paradox, scientists analyzed captive manatees' sense of hearing. They found that manatees have good hearing abilities at high frequencies, but not at low frequencies. Manatees hear in a range of 400-46,000 hertz. The peak of sensitivity in this range appears to be 16,000-18,000 hertz. Unfortunately for the manatee, slow-moving boats create low frequency sounds. Most boats produce sounds around or below 1,000 hertz. These low-frequency sounds are difficult for the manatees to locate. Manatees have fused cervical vertebrae, which limits their neck mobility. This contributes to their difficulty in telling where sounds are coming from. So the laws requiring boats to slow down, which were meant to protect manatees, may actually make it more difficult for manatees to avoid colliding with boat engines...the manatees do not hear the boat propellers until it's too late!

Did you know?

The manatee is Florida's official marine mammal.
Manatees are also called "sea cows."
The West Indian manatee has been on the earth for 50 million years.
The Florida manatee (subspecies of the West Indian manatee) can live for 60 years, reaching a length of 4 meters and a weight of 1300 kilograms.
Florida manatee brains are about the size of a Florida grapefruit (350g) which is unusually small for their body size.

(Sources: Gerstein, E.R., Manatees, Bioacoustics and Boats, American Scientist, vol. 90, 2002)

References:

Photographs on this page are courtesy of Jim P. Reid and James A. Powell (manatee/diver photo), U.S. Fish and Wildlife Service.
The Manatee Brain from the Mammalian Brain Collection
Manatee Mortality Rates from the Florida Marine Research Institute

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