Why don’t seals drown?

HUMAN FREEDIVERS CAN DESCEND to significant depths on just one lungful of air.

- Sheena Harvey

To do this they massively overbreathe beforehand to clear their systems of as much carbon dioxide (CO₂) as possible. This is the gas that triggers the normal urge to breathe when we hold our breath. But the technique doesn't always work. Freedivers can black out underwater and must have teams of scuba divers on standby to rush them back to the surface.

Seals can dive to great depths for long periods of time, chasing their fishy prey. So why do they never seem to end up unconscious? This question intrigued marine ecologist Chris McKnight and his colleagues at the University of St Andrews.