Sloths can hold their breath longer than dolphins
Sloths are famously slow on land, but they are surprisingly accomplished swimmers. Even more shocking is their lung capacity: a sloth can hold its breath underwater for up to 40 minutes. They achieve this feat by actively slowing their metabolism and reducing their heart rate to less than a third of its normal pace. By comparison, dolphins must surface for air roughly every ten minutes.
An Unexpected Swimmer
On land, sloths appear remarkably clumsy. Their long claws and low muscle mass make walking on all fours an awkward drag across the forest floor, where they move at an average speed of mere feet per minute. Because their anatomy is specialized almost entirely for hanging suspended beneath canopy branches, any terrestrial journey is energetically expensive and leaves them exceptionally vulnerable to ground predators.
Drop a sloth into a tropical river, however, and its movement transforms. Sloths use their long front limbs like broad oars to perform an efficient dog paddle, cutting through water at roughly three to four times their terrestrial pace. In the flooded forests and sprawling river basins of Central and South America, swimming is not an emergency response to an accidental fall; it is a regular mode of travel that enables sloths to navigate between fragmented forest patches and cross wide bodies of water.
The Physiology of the 40-Minute Breath-Hold
The most extraordinary dimension of a sloth's swimming capability is its endurance beneath the surface. A sloth can remain submerged for up to 40 minutes at a time without coming up for air. By contrast, many marine mammals that spend their entire lives in the sea, such as coastal bottlenose dolphins, generally surface every few minutes to breathe, rarely extending dives beyond ten to fifteen minutes under ordinary conditions.
This aquatic breath-holding ability relies on an extreme form of bradycardia, where the animal deliberately decelerates its heart rate. When fully submerged, a sloth can reduce its pulse to less than a third of its baseline rhythm. By suppressing cardiac output and restricting blood flow primarily to essential organs like the brain and heart, the animal dramatically reduces oxygen consumption, stretching a single lungful of air over a remarkable duration.