Such an "underwater distance tactile system" is found in fish, which monitor underwater surroundings through sensory pores set along their bodies in twin lateral lines. But the results represent the first time such a system has been documented in mammals, the scientists say.

"In the underwater environment, if you don't have echolocation, and most of the time you're in a situation where the water is not all that clear, then another option is to use the tactile sense," said Roger Reep, the paper's lead author and UF associate professor of physiological sciences affiliated with the UF College of Veterinary Medicine and the Evelyn F. and William L. McKnight Brain Institute.

"Fish use their lateral line to detect movement and objects in the environment, and we're arguing that manatees are doing something similar with tactile hairs."

Although many Florida residents see manatees in Florida's clear springs, they spend most or their lives in water stained by tannins or clouded with sediment, said Marshall, now an assistant professor of marine biology at Texas A&M University.

Researchers have long puzzled over how manatees, which have relatively poor vision, find their way in these conditions, he said. Also puzzling to scientists: manatees' proclivity for taking advantage of water flow. For example, manatees often swim from an estuary into a river just as the tide starts coming in, Marshall said.

In research beginning in the early 1990s, the UF scientists focused first on manatees' unusual facial hairs, known as vibrissae.

That research, which has appeared in Marine Mammal Science and other journals, showed that manatees use the vibrissae both as tools to grasp plants to eat and as sensory organs. Although only the long hairs near their mouths are used for grasping, the sensory hairs are distributed all over their faces, which is very unusual for mammals, Reep said.

"If you look at cats and dogs and most other mammals, their whiskers are largely limited to what you might call the moustache area," Reep said. "Manatees have these hairs on nine distinct regions of their faces."

The findings prompted the scientists to probe the role of hairs on the rest of the manatee body -- hairs that are so sparsely distributed their function clearly is not to keep the animal warm, Reep said.

The scientists examined the anatomy surrounding individual hairs by dissecting carcasses of manatees that had been killed in boat collisions and stored at the Florida Marine Research Institute's necropsy laboratory in St. Petersburg.

Sure enough, the scientists found that each hair on the body is a tactile hair, with a specialized follicle and dense nerve connections. If these are anything like tactile hairs in other animals, they are surrounded by motion detectors called mechanoreceptors, with nerves connecting to the brain, the researchers said. Margaret Stoll, a biological scientist at UF, participated with Reep and Marshall in the latest research.

"When a hair is deflected, the mechanoreceptors on that side get squeezed, and they send a signal through a network of nerves to the brain," Marshall said. "So it's really an integral part of the sensory system of the animal."

A major killer of manatees is collisions with boats. Marshall said the animals' unique antenna system is no help in these circumstances, because it is only sensitive to the environment near the animal, meaning if boats are traveling too quickly the animals will receive the information too late to respond.

However, he said, learning about the antenna system may indirectly help managers save manatees, which are protected by the Endangered Species Act.

"No one knows how these animals dig down into the substrate to get at the roots they eat, and my idea is they're doing it with these vibrissae around their mouths," Marshall said. "So this study is allowing us to understand the natural history of the animal, and by understanding the natural history we can better manage and protect it." - By Aaron Hoover

[Contact: Roger Reep, Chris Marshall, Aaron Hoover]

08-Mar-2002