For the manatees who call Florida’s coastal tributaries home, speeding boaters are like charging bulls in an underwater china shop. University of Florida researchers have discovered that despite the placid sea cows’ huge size, their bones are actually as brittle as some porcelain plates. That may make them even more vulnerable than anyone thought to suffering life-threatening injuries in a collision.
Boat strikes are the leading cause of manatee deaths in Florida, but until now scientists haven’t understood the mechanics of what happens to the endangered marine mammals when these deadly accidents occur. The surprising finding could ultimately change public policy for the management of Florida’s waterways, said Roger Reep, a professor in the UF College of Veterinary Medicine’s physiological sciences department.
“When you pick up a manatee rib, it’s much denser than a cow bone or a human bone,” Reep said. “Most people would think these ribs would be really strong, as they’re so heavy. But in fact they behave like a ceramic material. We feel this information will contribute significantly to our understanding of manatee-boat interactions, and will be critical in establishing boat speed zones adequate to minimize the chance of fatal impacts.”
Manatee bones have no marrow cavity, which is why their bones are so dense. That density makes manatee bones fragile and more likely to break than most other types, with fractures occurring more or less along straight lines as opposed to being dispersed within the bone, Reep said. The typical manatee rib weighs about 2 pounds and has a higher mineral content than other types of bone, researchers also found — up to 70 percent compared with 65 percent. While the difference seems small, it apparently translates into large changes in mechanical properties, they said.
Additional findings from the ongoing project, which mingles veterinary physiology and engineering expertise in a first-ever effort to describe the biomechanics of impact injuries, will be published in an upcoming issue of the Journal of Biomechanics. UF scientists also will discuss the study April 9 at the UF-sponsored Marine Mammal Medicine conference in Gainesville.
Using an air gun to hurl a 2-by-4-inch board toward a manatee bone target, and strain gauges to measure load at the moment of impact, the researchers are able to reconstruct the way various forces are distributed through the bone.
“You can actually measure the amount of energy that was propagated through the bone just by looking at the geometry. What we’re doing is getting an idea of the amount of energy it takes to break a bone,” said Reep, who has teamed with Jack Mecholsky, the study’s other principal investigator and a professor and associate chairman of the department of materials science and engineering at UF’s College of Engineering. They are working with UF graduate student Kari Clifton on the project, who began the study as part of her dissertation research in 1998 with funding from UF’s Marine Mammal Medicine Program.
The force applied by a boat to a manatee during impact depends primarily on boat speed, but also on variables such as the size of the boat, researchers said.
“One thing we’re not sure about yet is how much of the force of the boat actually reaches the ribs, since manatees don’t get hit directly on the ribs, but rather on the soft tissue covering the ribs,” Reep said. “This is an unanswered question.”
Manatees, listed as an endangered species by the federal government since 1967, are large, slow-swimming, gentle mammals that are entirely aquatic. Human activities are the major threat to their survival through boat-related injuries and deaths, habitat loss or degradation, and in some countries, hunting, according to the U.S. Geological Survey’s Sirenia Project.
Only about 3,000 manatees remain in the wild. Most are concentrated in Florida, but can be found in summer months as far west as Texas and as far north as Virginia. West Indian manatees can also be found in the coastal and inland waterways of Central America and on the northern coast of South America.
Officials have documented 5,329 manatee deaths in Florida from 1974 to 2004, of which 1,164 were attributed to watercraft collision, according to the Florida Fish and Wildlife Conservation Commission.
The U.S. Geological Survey’s 2003 population model predicted that if the manatee mortality rate from boating accidents continues to increase at the rate observed since 1992, the situation in the Atlantic and Southwest regions is dire, with no chance of the manatee population recovering within the next century.
“Most concerning is the fact that watercraft collisions are the leading cause of death of adult, reproductive-age manatees,” said Patti Thompson, director of science and conservation for the Maitland, Fla. based Save the Manatee Club. “Reducing adult manatee mortality is the most effective method to increasing the manatee’s recovery rate, and the reduction of watercraft-related mortality is the most productive and reliable means to reduce these deaths.”
Thompson said the UF research is significant because it could eventually lead to better boat management in the environment.
“It’s a surprising outcome of UF’s research that their bones are much more fragile than anyone expected,” Thompson said.
