Sounding the Voices of Manatees
Florida manatees—those big gangly gray elephants of the sea—do not have vocal cords. Yet they are able to make their high-pitched squeaks and squawks to communicate, a Xavier biology professor has found, using a modified structure of tissue folds that distort the air moving through the upper respiratory area and frontal cavities in the head.
Professor Charles Grossman and three Xavier biology and physics researchers published the results of the study into how manatees make vocal sounds in the current issue of the Online Journal of Biological Sciences.
The initial research project began in 2001 when Grossman, a physiologist and immunologist, became interested in searching for ways to protect manatees from injury by boat propellers. The project was featured in Xavier magazine in 2003 and also in 2009.
Florida manatees are an endangered species of mammals that forage for grass and weeds in the shallow waters of the Gulf of Mexico and into spring-fed rivers along the coast during the winter months. Because they are hard to see when they float near the surface, manatees are easily struck by speeding boats and Jet Skis, and they and their calves often die from their injuries.
To help find a way to protect these gentle giants from approaching boats, Grossman began studying how they respond to sounds in the water. He put together a research team that began working with captive manatees, including the two manatees in the Cincinnati Zoo’s Manatee Springs exhibit. But they found that the manatees were attracted to the sounds they introduced into the tank rather than repelled by them. Grossman and the team were hoping to identify a sound that manatees would try to avoid and replicate that for boats to reduce the number of manatee-boat collisions.
When that failed, the team began researching how the animals make their sounds. Past anatomical studies had concluded that manatees have no vocal cords, but no one understood exactly how they vocalized. In 2003, the team began making silicone models of their animals’ respiratory systems. They studied the sounds made when air is forced through the models and compared them to actual sound recordings made of living manatees.
They also studied the larynxes of 10 manatees that had died in Florida. The Florida Fish and Wildlife Commission provided the larynxes of the manatees to Grossman at Xavier. The research paper published in the Online Journal concludes that even though manatees don’t have vocal cords, they do have a modified structure that allows them to make their squeaky sounds.
“It’s a thin strip of tissue they use to make their squeaks—long, short, intense, all squeaks,” Grossman said. “It’s pure research. We tried to learn about how they react to boat sounds to keep them away from boats, but we couldn’t learn anything. We do know now what frequencies they use. That’s the whole point of any of this stuff. The more you know, the more chance you have of utilizing it for the greater good.”
The paper documents the probable cause of death of each manatee they studied from 2009 to 2011. Five were from “chronic watercraft” injury, one was from ingesting fish line and the others died of cold-weather stress or unknown causes. Grossman said the manatee population in Florida is stable this year at more than 3,500, though hundreds are killed each year by boats and other human-related causes.