Most of a manatee's bones lack marrow cavities and so are unusually dense, possibly serving as ballast. The aquatic plants they feed on are low-grade forage, which produces a lot of gas in their intestines. Without dense bones, manatees probably would be too buoyant to maneuver easily underwater. Adults eat the equivalent of 10 percent of their body weight daily. Many of the grasses they consume are abrasive and the silt the animals stir up when bottom-feeding is even more so.
Few animals have molars strong enough to withstand all the grinding.
Throughout the animal's lifetime, new teeth keep erupting at the back of its jaw and move forward as the worn-down teeth in front fall out. A puzzling feature of sirenian physiology is a surprisingly small brain. Roger Reep, a biologist at the University of Florida's College of Veterinary Medicine who, with Tom O'Shea, has studied the matter, suggests that is not the right question. The pressures of natural selection have probably made large, bulky bodies advantageous to manatees over time. That does not mean a manatee leads a dull-witted life, just a relatively uncomplicated one.