How manatees use solid bones and digestive gas to hover

If you watch a manatee for five minutes, you will notice something spooky. It does not swim like a dolphin or paddle like a seal. It hangs suspended in the water column like an airship, drifting up and down without flicking a single fin. Most marine mammals rely on thick layers of blubber to stay buoyant and active muscle power to dive. The manatee does something much stranger. It controls its depth with dense stone-like bones and a massive, constantly shifting internal balloon of gas.

A manatee is an herbivore built like a bowling ball. Plants do not offer much fuel, so a full-grown adult has to pack away around a hundred pounds of seagrass and water vegetation every single day. Processing that mountain of roughage takes a digestive tract that stretches more than one hundred and thirty feet long. While billions of microbes ferment all those greens, they generate absurd amounts of methane. In most animals, that kind of gas would just be a nuisance. For the manatee, it is an adjustable floatation vest.

Buoyancy is a tricky balancing act underwater. If you are full of gas, you shoot straight to the surface and cannot reach the grass on the river floor. That is where their skeleton comes in. Most mammals have hollow bones filled with marrow to keep them light and agile. Manatees went the opposite direction. Their ribs and long bones are completely solid, lacking marrow cavities entirely. Scientists call this pachyosteosclerosis. Their ribs look less like typical mammalian bones and more like heavy lead diving weights strung along the chest.

These heavy bones pull the manatee down, while the giant fermented gas pockets in their guts push them up. Their lungs help manage the trim. Instead of sitting high in the chest like ours, a manatee's lungs are flattened, running nearly the entire length of their back right alongside their spine. By contracting abdominal muscles and shifting gas pockets along the digestive canal, the animal can tweak its center of gravity on the fly without making a splash.

When a manatee wants to sink down to graze, it can contract its gut, compress the internal gas, or simply let a burst of methane rip out the back end. Constipated manatees actually struggle to dive. Biologists who rescue sick manatees regularly find individuals bobbing helplessly at the surface, their tails stuck high in the air because trapped gas will not let them level out. Once wildlife vets feed them mineral oil to pass the blockage, the gas clears, and they glide right back down to the riverbed.