Brydie Storer, of Balclutha Primary School, asks :-

How can whales stay under water for so long?

Alex Davies, of Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

All whale species are impressive divers, from Sperm and Bottlenose whales that can stay underwater for up to 90 and 120 minutes respectively, to the Common dolphin at 3 minutes. You will need to do a lot of training if you want to dive for as long as 3 minutes. Even more impressively, sperm whales hunt giant squid as deep as 3200 meters. They have to solve the special problems created by the pressure at these depths, in ways we have yet to understand.

Neither do we understand all about how whales can hold their breath for such a long time. We do know that whales take a deep breath at the surface, and manage to exchange lots of oxygen very efficiently with each breath. Whales breathe in before diving, while seals breathe out. This doesn't seem to matter much: the exchange is during breathing, not while holding the breath. In diving animals, the lungs are not proportionately larger, in fact the longer the species of whale can dive, the smaller are the lungs.

We also know that whales have a huge store of oxygen in their body apart from the lungs. There have twice as much oxygenated blood relative to their size as land mammals, mostly kept in massive veins in the chest and abdomen. In this blood there is a higher concentration of red blood cells, the part of the blood with the red pigment hemoglobin that stores the oxygen. The oxygen carrying pigment in the muscles is also highly concentrated, making them very dark in colour. But even with all this storage, there is still not enough oxygen to last for such a long dive.

When all animals dive, the heart rate slows. The heart of a diving dolphin beats at 40 per minute, but this rises to about 90 per minute while breathing. The body also has a way of supplying only essential organs, especially the brain and the heart wall and not the intestines and the kidneys, during a dive. This is the diving response. If you dive frequently, you will train this response so that you can soon stay under for much longer. Diving animals also tolerate toxic products that arise from burning body fuels without oxygen and without the ability to clear the wastes through the lungs. Whales accumulate large amounts of lactic acid and carbon dioxide during a dive with little apparent effect on their tissues.

Whales have intriguing structures on the inside wall of the chest near the backbone, and elsewhere. They consist of contorted spirals of tiny blood vessels that people noticed a long time ago and called "wonderful nets". They are best developed in diving animals, but their function is as mysterious as the world in which these amazing creatures live.