Why a Sloth Swims Three Times Faster Than It Walks

A sloth on the forest floor drags itself with its forelimbs at around 0.2 to 0.3 kilometres per hour, hindlegs mostly along for the ride. Its limbs are built as hooks for hanging, not as struts for standing, so terrestrial travel is expensive and slow.

Put the same animal in water and everything improves. Buoyancy removes the load its skeleton is bad at carrying, and those long hooked arms turn into excellent paddles. Sloths swim at roughly three times their walking speed, using a slow breaststroke, head held clear.

Two adaptations make it more than a stunt. Gas from fermenting leaves gives extra buoyancy — an animal full of slowly digesting foliage floats well. And a sloth can drop its heart rate to about a third of resting, stretching a single breath to as long as forty minutes underwater.

This matters in seasonally flooded forest, where the ground disappears for months and the gaps between trees fill with water. Swimming is not an emergency behaviour there; it is the commute.

The lesson generalises. The sloth is not slow in every medium. It is slow in the medium where being slow is safe, and merely unhurried everywhere else.