How long can you hold your breath for? Californian sea lions can hold their breath for up to 10 min during foraging dives. But how do they do it? To investigate, Birgitte McDonald from the Scripps Institution of Oceanography, USA, decided to measure oxygen saturation in the venous blood. She explains that, at rest, venous blood is about 78% saturated with oxygen after supplying the body with oxygen, but wondered how much it decreased during dives (p. 3332).

With the help of her postdoctoral supervisor, Paul Ponganis, McDonald captured seven lactating sea lions off the coast of California and inserted an electrode into the vena cava vein of anaesthetised sea lions, enabling her to measure partial pressure of oxygen (a measurement of oxygen levels). Equipped with this partial pressure of oxygen data logger and a time depth recorder, McDonald released the sea lions back into wild.

After recovering the equipment, McDonald converted all her measurements into percentage oxygen saturation. McDonald saw marked differences in oxygen depletion patterns between shallow and deep dives. During shallow dives lasting less than 3 min, oxygen saturation levels went down to anywhere between 10 and 80%. After a shallow dive, the sea lions didn't always replenish their blood oxygen stores back to 78% and in some cases dived down with only 15% saturation, which may explain why some appeared to have very low saturation levels.

McDonald found that at the beginning of a deep dive, the sea lions increased their venous oxygen saturation more than for a shallow dive, in some cases reaching up to 95% saturation. She explains that it is possible that arterio-venous shunts exist that allow saturated arterial blood, fresh from the lungs, to bypass tissues and oxygenate the venous blood. By shunting this arterial blood they would maximise their blood oxygen storage before a dive. During the dive there was near-complete oxygen depletion, and after resurfacing, unlike after a shallow dive, the sea lions remained at the surface for longer periods to replenish their oxygen stores. In conclusion, McDonald's data suggest that sea lions know when they're going to dive deep or remain shallow and alter their oxygen management strategies accordingly.

B. I.
P. J.
Insights from venous oxygen profiles; oxygen utilization and management in diving California sea lions
J. Exp. Biol.