The sea mammals must rise to the surface every now and then to breathe. The warm blood makes the newly born delicate and therefore in need of care. All these mammals have a large brain and are also more or less highly intelligent and this is particularly so in the case of the dolphins, the sperm-whales and the killerwhales. The "Calypso" crew recorded the sounds made by whales, dolphins, seals, sea cows and killer whales. These mammals have become easy prey for the whalemen, particularly the whales and seals are at the mercy of man.