Inspiration by Magellanic penguins: reduced swimming effort when under pressure

Some marine mammals may increase their underwater locomotor efficiency by taking down little air for dives and descending passively, although at the point of maximum depth they presumably have to use energy to counteract the downthrust, to stop themselves sinking further. Birds, having considerable quantities of body-associated air, would appear not to have this option. However, measurements of locomotor activity and inspiratory behaviour of free-living, diving penguins has revealed that birds regulated the inspired air volume so that upthrust, primarily derived from depth-related changes in air volume, was minimal and constant at the preferred foraging depth. Although this results in minimized costs of travel, it means total body oxygen stores have to vary with depth, something that helps explain why dive duration is so closely correlated with depth in birds.