Sirenians exhibit pachyostosis, a condition in which the ribs and other long bones are solid and contain little or no bone marrow. They have among the densest bones in the animal kingdom, which may be used as ballast, counteracting the buoyancy effect of their blubber and help keep sirenians suspended slightly below the water's surface. Manatees do not possess blubber, per se, but rather have thick skin, and, consequently, are sensitive to temperature changes. Likewise, they often migrate to warmer waters whenever the water temperature dips below . The lungs of sirenians are unlobed; they, along with the diaphragm, extend the entire length of the vertebral column, which help them control their buoyancy and reduce tipping in the water.