Underwater to Get Out of the Rain by Trevor Norton

Author:Trevor Norton
Language: eng
Format: epub
Publisher: Da Capo Press
Published: 2010-05-31T16:00:00+00:00

Attempts to return to the sea are nothing new. Whales rediscovered their aquatic roots 35 million years ago. The danger in such a radical transition from a land- bound ancestor is that the intermediate stages may not be well adapted to either milieu and therefore vulnerable to both terrestrial and aquatic predators. On the other hand, this doesn’t seem to inhibit modern penguins or seals that must breed on land where they are clearly less agile than in the water. Indeed, it may be an advantage to be able to pop into the sea when a bear lumbers by and get out when a shark appears.

Marine mammals seem to have come into their own as replacements for the giant marine reptiles such as the ichthyosaurs and plesiosaurs. When these became extinct it left an empty niche that mammals could fill without fear of competition. Some still bear reminders of their ancestry. The fossil record reveals that dolphins were originally derived from small shrew-like animals whose locomotion was a lolloping scamper in which the back flexed vertically. Hence the dolphin’s tail beats up and down not side to side like that of a fish or reptile.

Most land animals can swim when the need arises. The record swim for an elephant is forty-eight kilometres, and they even go snorkelling. Aristotle observed that ‘Just as divers are sometimes provided with instruments through which they can draw air from above the water, and thus remain for a long time under the sea, so also have elephants been furnished by nature with their lengthened nostril, and whenever they have to pass through water they lift the nostril above the surface’. The pressure on the submerged chest of a snorkelling elephant is 20 per cent greater than that of the air in its lungs, enough to burst all the fine blood vessels in the membrane that encloses the pleural cavity, a fluid-filled bag allowing the lungs to inflate uniformly within the irregular cavity of the chest. Bursting these capillaries is potentially fatal. But uniquely among mammals elephants have dispensed with a pleural cavity and instead have layers of connective tissue lacking fragile capillaries. All this padding allows the lungs to expand safely against the chest wall when the elephant goes snorkelling.

The need to breathe air is clearly not a major impediment to living an aquatic life. Of course, it helps if you can hold your breath for well over an hour like the sperm whale and the bigger seals. It was thought that the Fitzroy turtle could hold its breath for three days, until it was caught cheating by breathing through its bum. Diving animals are so adept at holding their breath that they do it all the time even if there is no reason to. A seal dozing on the beach often stops breathing for long periods.

With such prodigious breath-holding abilities it is no wonder the leatherback turtle can descend to 640 metres, the elephant seal and many whales dive down over a kilometre and the sperm whale ventures well over two kilometres beneath the sea.

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