Deep dives push marine mammals to their physical limits. During long periods underwater, animals can switch to 'anaerobic' (oxygen-less) metabolism in organs outside the heart and brain, leading to a buildup of lactic acid. Although many diving species have evolved ways to reduce the risk of 'the bends', nitrogen bubbles can still form in their bloodstream. Scientists have long believed these animals recover mainly while floating or swimming at the ocean's surface between dives.

A new study published in Frontiers in Physiology suggests that recovery may continue long after fur seals leave the water. Researchers discovered that Cape and Australian fur seals experience dramatic increases in heart rate several hours after returning to land, possibly as part of a delayed recovery process from the intense demands of foraging at sea.

"Here we show in Cape and Australian fur seals that there is a positive relationship between their heart rate at sea during foraging and their heart rate on land during rest.…