International Day of the Seal

What the day is

International Day of the Seal is observed on March 1. The record carries no description, and the animal's physiology is the subject worth the space.

Origin and background

A seal diving is not simply a good breath-holder. The dive response reorganizes the whole body. Heart rate drops sharply, in deep divers to a small fraction of the surface rate. Blood vessels serving the skin, gut and muscles constrict, so circulation concentrates on the heart and brain. The muscles are left to work without fresh blood, running on oxygen bound into myoglobin, which seals carry at concentrations far above land mammals and which is what makes seal muscle almost black.

They also exhale before diving rather than filling their lungs. Lung air would compress at depth and, worse, would dissolve into the blood under pressure and cause decompression problems on the way up. The oxygen is carried in blood and muscle instead, which is the safer store. Elephant seals take this furthest, diving for well over an hour and spending most of their lives underwater.

The whiskers are a second remarkable system. Seal vibrissae detect the hydrodynamic trail left behind a swimming fish, and experiments with blindfolded seals have shown them following a trail made seconds earlier, which allows hunting in darkness or murky water.

The conservation picture is mixed in an encouraging way. Several heavily hunted species have recovered strongly under protection, with northern elephant seals rebuilding from a few dozen animals, though that near-extinction left them with very little genetic diversity. Entanglement in fishing gear and disturbance at haul-out sites are the main current pressures.

How to take part

• Keep well back from hauled-out seals, especially pups. Disturbance costs them energy they need.
• Never assume a pup alone on a beach is abandoned. The mother is usually feeding.
• Report entangled animals to a marine rescue organization rather than intervening.
• Read about the whisker experiments, which are genuinely surprising.
• Keep dogs on leads near haul-out sites.