Marine mammals are voluntary air breathers which means they have to make a conscious effort to rise to the surface to breathe. If they are unable to do this due to entanglement in a net or for any other reason, they drown. This is why their nostrils or 'blow holes' are located on the top of their heads, as it enables them to surface to breathe while keeping most of their body underwater. Here, an Indian Ocean Blue whale exhales at the surface after a 12 minute dive off Mirissa, Sri Lanka.
- Philip Hoare is the author of 'Leviathan or, The Whale' (Fourth Estate), winner of the 2009 BBC Samuel Johnson Prize for Non-Fiction