Animals
Can sloths hold their breath longer than dolphins?
The short answer
Yes. Sloths can stay underwater for up to about 40 minutes by slowing their heart rate dramatically. Most dolphins manage roughly 10 to 15 minutes. The animal built for water loses to the animal built for hanging in a tree.
Sloths are decent swimmers — better than they are walkers, which is a low bar — and they slow their heart rate substantially when submerged, stretching a single breath much further than their size suggests.
Dolphins are the ones who ought to win this. They are marine mammals with deep-diving physiology and a heart-rate response of their own. What they also have is a much higher metabolism: an active, warm, fast animal burns oxygen quickly.
That is the whole trick, and it is the least surprising thing about a sloth. Doing almost nothing very slowly is an oxygen strategy. It just happens to also work underwater.