A Sloth Can Put Its Heart on Slow Motion
The unusual advantage is not a special lung or an enormous store of oxygen. Sloths can sharply slow their heartbeat, reducing how quickly their bodies use the oxygen already in their blood and tissues. That gives them more time underwater before they need to breathe again.
This is an extension of a broader sloth trait: an exceptionally slow metabolism. Their bodies run on a low-energy budget, matching a leafy diet that provides relatively little fuel. When submerged, conserving oxygen becomes especially useful. A slower heart circulates blood less rapidly, helping stretch the available supply rather than spending it at the usual pace.
A Familiar Diving Response, Taken Further
Many mammals respond to submersion by slowing their heart rate, a reflex that helps conserve oxygen. Sloths can make this response pronounced: their heart rate may fall to about a third of its usual rate. The result is that they can remain underwater far longer than their unhurried appearance might suggest—often reported as up to about 40 minutes.
Dolphins are built for diving too, with adaptations that let them store and manage oxygen during dives. But they generally surface to breathe much sooner. The comparison is striking because sloths are land mammals, not aquatic specialists: their advantage comes from slowing the body's demand, not from becoming better swimmers.
For a sloth crossing water, the payoff is practical. It can swim between patches of forest while spending oxygen slowly, turning an otherwise risky stretch into a manageable passage.
