Sunday, June 10, 2001		Nature

SPERM whales are the deepest divers among any air-breathing animal. They go after bigger prizes — giant squids. To reach them, sperm whales must dive very deep. One whale, of South Africa, was credited with a dive that lasted 1.52 hours. When dissected, its stomach was found containing two deep-water sharks, which are found only at the bottom of the sea. The charts showed the place to be 10,476 ft deep.

Why these animals go so deep in search of food is not clear. However, it is thought that they use a food resource that is not exploited by other predators. Squids found in surface water are usually eaten up by a host of fast-swimming fish and sea mammals, like seals, dolphins, sea-lions and other whales, whereas sperm whales dive deep to avoid this competition.

While diving, sperm whales use their powerful tails to cover the distance as fast as possible. They go down straight at speeds of up to 560 ft per minute. Males dive the deepest and longest, while females usually do not go beyond 3,025ft. They return to the surface just as fast, avoiding the ‘bends’ with the aid of specialised respiratory and circulatory mechanisms that ensure that there is no air in the blood when the whale is at a depth.

There is also the suggestion that the spermaceti organ in the forehead has, in addition to being a sound lens, a secondary function. The organ is criss-crossed with a complicated plumbing system of blood vessels, sinuses and nasal passages, and it is filled with a special wax that melts at 84.2°F (29°C) precisely. The whale, it is thought, can melt or solidify the wax at will, thereby changing its density. This has the effect of making its body more or less buoyant, so the whale can control its ability to go down or rise.

It works like this: at the surface, sea water is circulated to cool the wax, which shrinks and becomes more dense, causing the whale to sink. At the end of the dive, body heat generated by the muscles is carried by the blood system into the spermaceti organ, where it melts the wax. The head is then less dense than the water and the animal rises to the surface with minimum effort. No matter how exhausted the animal becomes at the bottom, it is assured a safe journey to the surface — important after a long feeding excursion or a titanic struggle with a giant squid.

When a predator is limited by one gulp of air, speed is necessary to take it to its destination quickly and get it back safely. Several sea-birds, sea mammals and sea-going reptiles have this requirement when diving deep.

Of the flying birds, the loon or the great northern diver has been credited with pursuing prey at 200 ft in Lake Superior and a guillemot was recorded at 250 ft in the cod-rich waters off Newfoundland. Sea-snakes pursue prey, particularly eels which are the right shape for a snake to eat, and may reach depths of 330 ft. They can stay down for up to 5 hours.

The emperor penguins of the Antarctic, the largest of the penguins, is the deepest diving bird. It can stay underwater for up to 18 minutes and reach a depth of about 870ft. The slightly smaller king penguins of sub-Antarctic islands have been tracked down to 787 ft, although only half their dives on a fishing trip might be below 164 ft. In the food-rich waters of the southern ocean, their main food item is the squid.

Hunting birds, with chicks to feed back on land, are out at sea for periods of four to eight days. They must catch about 2.5 kg of food for themselves and a further 3 kg for the chick each day. On every fishing trip, then, a bird must catch between 50-90 squids, weighing 150-200 gm. With an average of 865 dives per excursion, this means that king penguins catch squid on fewer than 10 per cent of their dives.