Making Sense of Manatees (2024)

Chris Marshall noticed it when he snorkeled with manatees: even when he remained still and quiet in murky water, they kept a safe distance. It was as if the plant-eating mammals had a sixth sense that kept them posted on his location.

As it turns out, they do.

Marshall, who researched the manatees for his doctorate at the University of Florida, and his major professor recently discovered that manatees use small hairs on their body as antennae to pick up information about water currents, landscape and the presence of other animals.

Such an "underwater distance tactile system" is found in fish, which monitor underwater surroundings through twin lines of sensory pores along their bodies. But it is the first time the system has been found in mammals, say the scientists, whose paper on the research appears in the August issue of Brain, Behavior and Evolution.

"I've been with these animals in very murky conditions, and I've been very quiet and still, and they can always tell where I am," Marshall says. "They are using their small hairs, or vibrissae, to actually feel the pressure waves from my body."

It is far from the first fresh insight into these unique animals to come out of the University of Florida, where an informal manatee research group headed by scientist Roger Reep has earned a reputation as a leader in manatee science. Reep, an associate professor of physiological sciences with the UF College of Veterinary Medicine and the McKnight Brain Institute, has been studying the mammals for nearly two decades. The research has led to advances in the little-understood areas of manatee evolution, brain physiology and behavior.

Manatees are important to evolutionary biologists because they belong to a small and unusual group of herbivorous mammals — underwater counterparts to far more common land-grazing animals.

"What manatees are telling us about is the range of evolutionary potential because they are so weird," he says. "They are what evolutionary biologists refer to as ‘experiments in nature.'"

But the research also has important implications for conservation. Classified as an endangered species, an estimated 2,500 to 3,000 manatees live in Florida waters. Yet scientists continue to struggle with gaps in knowledge about what makes manatees thrive and how to protect them, both from natural threats such as red tides and man-made threats such as boat collisions.

Science Of The Strange

Manatees are large animals, with the biggest adults measuring as long as 13 feet and weighing as much as 3,000 pounds. Despite this heft, they have extremely small brains, with the largest attaining only the size of a small grapefruit. Also, unlike the complex, folded surface of other mammal brains, manatee brains are smooth. These abnormalities led early scientists to speculate that manatees were, to put it kindly, on the dim side of the animal kingdom. Writing in 1902, biologist Elliott Smith said, "The only parallel which can be found for the peculiar cases presented by the manatee and the dugong (a close manatee relative) is that presented in the brains of idiots."

Reep's research is calling that assumption into question. Animal intelligence is a notoriously sticky subject because no single measuring stick can be applied to all species. But Reep's work suggests that manatee brains are small for a good reason. He is also finding they are more complex than suggested by their appearance.

Manatees trace their evolutionary lineage to grass-eating land mammals that lived at least 50 million years ago. Their oldest ancestors were pig-like, four-legged animals that looked, improbably, like a cross between a hippopotamus and an otter, as one scientist has described it. As the animals evolved and entered the water, natural selection began to favor a larger body size because it helped the animal store heat, Reep says. Selection also favored a slow metabolism, which required the manatee to expend less energy moving around and consuming its exclusive diet of plants. Even as the manatee benefited from a large body, it had no natural predators, although sharks are thought to be an infrequent exception, Reep says. Manatees also never chased prey. So while size proved a benefit to the animal in evolution, it had no need for the intricate sensory systems and complex varying behaviors of mammals with bigger brains.

As evolution played out, in other words, the manatee's body grew, but its brain did not. "It's not that its brain is relatively small, it's that its body is relatively large," Reep explains.

To conclude from the manatee's brain size that it is less intelligent than other marine mammals is more human judgment than science — the manatee's brain has proved adequate for this slow and defenseless animal to persist to present times, Reep says. Further, Reep's dissections show that while the brain is indeed smooth on the outside, it is quite complex beneath the exterior. More recent work at other Florida institutions, meanwhile, has shown that manatees can be trained to perform simple tasks.

Part of the difficulty in measuring animal intelligence is that species perceive and interact with the outside world through vastly different channels — bees, for example, see and respond to infrared light completely invisible to humans. To a bee, in other words, a human's movements and decisions would seem strange indeed. Reep's recent research has begun to open a window into the manatee's unique world. One of his discoveries is that the manatee's cerebral cortex has numerous large clusters of nerve cells. Reep believes these clusters are tied directly to a highly developed sense that people can only begin to appreciate: the manatee's antenna-like hairs.

Although manatees' awkward, roly-poly bodies draw the eye, the hairs and "whiskers" around its face may be its more fascinating feature. Thinly distributed around its body, the hairs clearly aren't keeping the animal warm. Its long whiskers, meanwhile, seem more than cosmetic adornments.

Reep's research is revealing the role of these features. Late in the 1990s, he and colleagues published a series of papers highlighting how manatees use the whisker-like bristles on either side of their mouths as living cutlery to grasp and take in food. Manatees can also control their lips independently of one another and use them pre-hensiley, like giraffes. Contrary to what might be expected, these two features make the manatee a remarkably dainty diner. A close-up video Reep made of a manatee eating water hyacinth shows the manatee eating choice parts of the plant on one side of its mouth while simultaneously rejecting unwanted parts with the other, all at a pace of about two bites per second.

hom*osassa Springs Wildlife State Park serves as a rehabilitation and refuge center for injured West Indian manatees. Many other manatees prefer to spend the winter months around these constant-temperature springs of the hom*osassa River, making it an ideal place to study the creatures in their natural habitat.

Dainty Eater, And Other Surprises

Although manatees' awkward, roly-poly bodies draw the eye, the hairs and "whiskers" around its face may be its more fascinating feature. Thinly distributed around its body, the hairs clearly aren't keeping the animal warm. Its long whiskers, meanwhile, seem more than cosmetic adornments.

Reep's research is revealing the role of these features. Late in the 1990s, he and colleagues published a series of papers highlighting how manatees use the whisker-like bristles on either side of their mouths as living cutlery to grasp and take in food. Manatees can also control their lips independently of one another and use them pre-hensiley, like giraffes. Contrary to what might be expected, these two features make the manatee a remarkably dainty diner.

A close-up video Reep made of a manatee eating water hyacinth shows the manatee eating choice parts of the plant on one side of its mouth while simultaneously rejecting unwanted parts with the other, all at a pace of about two bites per second.

"Manatees have a lot more fine motor control with their mouths than you might expect," Reep says. The manatees' sensory body hairs are also unique.

Although many residents and visitors to Florida journey to the state's clear springs to see them, manatees spend most or their lives in water stained by tannins or clouded with sediment, says Marshall, the former doctoral student in Reep's lab who is now an assistant professor of marine biology at Texas A&M University. Researchers had long puzzled over how the animals, which have relatively poor vision, find their way in these conditions, he says. Also puzzling to scientists was 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 says.

Roger Reep

Marshall and Reep 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. They 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 say. Margaret Stoll, a biological scientist at UF, participated with Reep and Marshall in the latest research.

Marshall and Reep 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. They 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 say. 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 says. "So it's really an integral part of the sensory system of the animal."

People may feel a breeze or light touch with their body hair. The manatees' perceptions are apparently much more acute, allowing them to detect objects at greater distances and with greater specificity.

Bones And Boats

The manatee's unusual sensory system may help it navigate or find food, but it never evolved to deal with one of the animal's biggest modern threats: boats. When active, manatees ascend to the surface to get air about once every five minutes, which these days often puts them in the path of one or more of Florida's 900,000 boats. Each year, collisions with boats account for about 25 percent of roughly 300 manatee deaths. By the end of May this year, such collisions had already caused 54 manatee deaths, according to the Florida Marine Research Institute in St. Petersburg.

Chris Marshall

Marshall says manatees' antenna system appears to be no help because the animals receive information about boats too slowly for them to respond when the boats are traveling fast. The results of another Reep research project, however, may prove more useful to manatee protection. With an eye on the speed restrictions the state imposes on selected waterways to try to protect manatees, he and veterinary medicine doctoral student Kari Clifton are looking into the properties of manatee bone.

As with so many of the manatee's other attributes, the structure of its bone is unusual, Reep says. The rib bones in particular are extremely dense and heavy. It is thought that these heavy ribs provide ballast for the manatee like a weight belt for a scuba diver — ballast needed in part because the manatee's diet results in a lot of gas, Reep says. It might seem that such bone would be strong, but it is in fact quite brittle. The bulk of deaths due to boat collisions do not result from propeller cuts but rather from trauma and broken bones caused by the collision itself, Reep says.

Reep and Clifton are subjecting manatee rib bones to stress tests to find out their "energy of fracture," or just how strong they are. Reep hopes this will lead to a better understanding of when boats pose the most danger to manatees — and thus lead to more useful and effective regulations for boating in areas where manatees live.
"This will give us a scientific answer to the question of where the danger level is," Reep says. "Is there a safe, slow speed?"

Reep's research has other implications for conservation. For example, learning about the antenna system may indirectly help managers save manatees by enabling them to identify and preserve the habitat the animals need most.

"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 says. "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."

Roger Reep

Associate Professor, Department of Physiological Sciences

(352) 392-4700 x3859

reep@ufbi.ufl.edu