How Manatees Find Their Way
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Manatees spend most of their time in rather murky water, feeding on underwater grass beds. Recent research has begun to yield clues about how manatees navigate in such an environment. (Photo © Walker Stanberry)
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By Dr. Roger Reep
Manatees are often photographed in clear water springs. However, they actually spend most of their time in rather murky water, feeding on underwater grass beds that are distributed in patches. The coastal environment frequently consists of islands, inlets, and other topographic features that create spatial complexity. Recent research has begun to yield clues about how manatees navigate in such an environment.
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Recent studies at Mote Marine Laboratory and Aquarium, led by trainer Joe Gaspard (above), are focused on what may be the most sophisticated sensory system of manatees -- tactile hair. (Photo courtesy Roger Reep)
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Defining the Perceptual World of Manatees
Behavioral training and testing of captive Florida manatees has helped us to better understand the perceptual world of the manatee. Their vision is poor, but their hearing and tactile sensitivity are excellent. The basic design of these experiments is to train a manatee to respond one way if it detects a stimulus (for example, a sound of a specific frequency), and another way if it does not detect anything. Food rewards are given for correct choices. By performing many trials over many months and even years, the sensory-perceptual abilities of manatees may gradually be defined. At Lowry Park Zoo in Tampa, Ed Gerstein and colleagues worked with Stormy and Dundee to define the hearing spectrum (audiogram) for manatees. They found that manatees hear best at 15-20 kHz, which is at the upper end of the human hearing range. At Mote Marine Laboratory and Aquarium in Sarasota, Gordon Bauer, David Mann, and colleagues have worked with Hugh and Buffet to examine visual, auditory, and tactile abilities. They confirmed the findings of the Gerstein study and, led by Debi Colbert, they performed a detailed analysis of the ability of manatees to localize sounds of various frequencies. The Bauer group also has found that manatees hear very well in noisy environments, better than ringed seals or bottlenose dolphins. These findings on sound localization and noise relate to how well manatees detect and respond to sounds from boat motors. In fact, these studies on captive manatees complement several studies of the behavior of wild manatees in response to approaching boats, illustrating the value of both kinds of data. On the one hand, captive studies allow the researcher to control many aspects of the environment, so that a variety of stimuli are presented in a similar setting. On the other hand, field studies do not allow for the same level of repeatability, but do assess what happens in the environment where most manatees live. Finally, the Bauer group found that manatee visual acuity is very poor, being somewhat worse than a legally blind human, comparable to a cow, and significantly worse than dogs and dolphins.
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Manatees possess tactile hair over their entire body. In most mammals, this specialized type of hair is known as whiskers. (Top photo: courtesy USGS, Sirenia Project; Bottom: Chatsworth the cat by Connie Graham)
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The most recent studies of the Mote group, led by trainer Joe Gaspard, are focused on what may be the most sophisticated sensory system of manatees. Manatees and their relatives the dugongs possess tactile hair over the entire body. In most mammals, this specialized type of hair is present only on the muzzle, and is known as whiskers. In many mammalian species, the whiskers are used for tactile exploration and orientation, particularly in darkness. The movement of each tactile hair is coded by sensory receptors and the associated nerve fibers that innervate each hair follicle. This information is then transmitted to the brain. Collectively, the array of whiskers is able to convey information about the shapes and textures of surfaces and objects that are encountered.
Tactile Hair: A Sophisticated Sensory System
Tactile hairs are much more complex than normal hair. Studies by Diana Sarko revealed that each manatee tactile hair follicle contains several types of sensory receptors and an abundance of nerve fibers. Diana’s work also defined the regions of the brain that process this tactile information. When one considers that there are about 2,000 tactile hairs on the face of a manatee and approximately 3,000 on the rest of the body, this represents a lot of tactile information being sent to the brain! In fact, over 200,000 nerve fibers convey the signals from the tactile hairs to the brain.
All manatee hairs are not the same. Chris Marshall showed that some of the stout hairs on the lip region of the face (perioral bristles) are used to grasp plants during feeding. Anyone who has observed manatees underwater and has seen them mouth an anchor line or floating vegetation knows what this looks like. What they may not have realized is that manatees (and dugongs) are the only animals known to use hair in this prehensile way. The 600 hairs on the oral disk, that part of the face between the lips and the nostrils, are used in tactile investigation, by touching or sweeping this array of hairs across the object or surface being investigated. Thus, the hairs of the face are used in direct tactile contact. The work at Mote has shown that the facial hairs are as sensitive as a human fingertip, and more sensitive than the tip of an elephant trunk.
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See and hear an audio slide show
The Mind of the Manatee, narrated by Dr. Reep, at The New York Times web site
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The 3,000 tactile hairs on the post-facial body of the manatee comprise an array that appears to detect water movement, much like the lateral line system in fish, rather than being used in direct contact like the hairs of the face. For animals in the underwater environment, this capability is useful for the localization of water currents, tidal flows, and approaching animals. In addition, as an animal moves, it creates a bow wave that reflects off large features in the environment like a sandbar or shoreline. Therefore, if manatees are able to detect this reflected wave they can obtain information about the topography of their surroundings.
Joe Gaspard’s current work at Mote is examining the ability of Hugh and Buffet to detect water movements of various frequencies and amplitudes, generated by a vibrating underwater sphere about the size of a raquet ball. Even at this early stage in his research, Joe has found that manatee whiskers are more sensitive than those of harbor seals. This is significant, because harbor seals use their whiskers to follow hydrodynamic trails generated by swimming fish, in order to capture and eat them. Joe is also examining the ability of manatees to localize the source of such water movements.
We still have a lot to learn about how manatees navigate their environment. Living in water and being large and relatively slow-moving seems to have been conducive to the evolution of unique sensory abilities involving hair, and now we are finding that the manatee’s tactile system is incredibly sensitive.
Dr. Reep is a Professor in the Department of Physiological Sciences at the University of Florida College of Veterinary Medicine and a member of the SMC Board of Directors.
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| Manatees (and dugongs) are the only animals known to use hair in a prehensile way to grasp plants during feeding. (Photo © David R. Schrichte.) |
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