Marine mammals can hold their breath for extended periods underwater; the question is, how do they know when it's time to resurface?

The seals’ ability to detect the amount of oxygen in their blood may help them make diving decisions and avoid drowning.

As seals dive more frequently—and for longer—than humans do, the range of carbon dioxide in their bodies varies more widely.

Dr Chris McKnight has been researching how seals can hold their breath as part of his work at the University of St Andrews.