Seals live a schizophrenic lifestyle as both land and sea animals. Antarctic seals have two homes – one in the surrounding ocean and the other on a cool patch of ice or prime beach-front property.
Seals are remarkably adapted to ocean living. These aquatic mammals have powerful sleek bodies that are encased in blubber and taper down into a tail. Their thick no-neck physiques and loosely interlocked vertebrae make them strong and flexible enough to surf the waves and navigate ice and rocky shores.
Seals have slits for nostrils that naturally close under water – and they shut even tighter with increased water pressure. This feature works better than those attractive nose clips we humans wear in diving class. And speaking of diving, seals can hold their breath for a very long time… up to two hours for elephant seals. Because of a custom-designed mouth and larynx, they can even eat while underwater without sucking sea water!
Ever notice how big a seal’s eyes are? That’s another underwater adaptation. Seals have flattened corneas and pupils that can open wide to let in light while swimming. Unlike land animals, a seal’s eyes consist only of rods (sensory cells) that work great in low light, plus they don’t have cones (other sensory cells) to detect color. In water, a seal’s eye lens sends an image directly to the back of the eyeball. Land mammals use their lens for focusing only. Though seals have retinas like land animals do, they don’t have the curved eye surface to refract light and project an image onto the retina at the back of the eyeball.
Some types of seals have even more specialized visual adaptations, depending on what they eat. Seals also have long whiskers with many nerve endings that are sensitive to the movement of prey and help them navigate murky waters.
Blubber helps insulate seals in polar conditions. True seals rely on blubber more than fur seals do because true seals live a more aquatic life. Fur seals depend more on their special under-fur that is waterproof and helps regulate their body temperature.
Ever see a completely hairless seal? Probably not, since even a little fur helps keep protect them from the cold and wet. All seals molt to replace their old fur with new fur, though they don’t lose all their fur at once.
To find food, seals must be master divers – especially the true seals like the Weddells. Seals have developed special features to keep them from getting the bends.
Most mammals have 13 pairs of ribs, but seals have two extra pairs so there is more room for their slightly larger lungs. During a deep dive, the pressure of other organs collapses the seal’s diaphragm against its lungs to force out any air.
Seals don’t take a huge breath like humans do before jumping in, but they do hyperventilate before a dive. They store the oxygen in their blood and muscles and expel the air. Seals have more blood than land animals of a similar size, plus more hemoglobin to carry oxygen. That means a seal can carry a lot more oxygen for its body weight.
Seals have other special diving adaptations, such as a reduced heart rate (from 60-70 bpm to 15 bpm) during a long dive. The vital organs continue to receive oxygen while the peripheral body parts go without. If a seal runs out of O2, it then converts glucose to lactic acid through a process called glycosis. Weddells and other true seals even have extra-big spleens to store red blood cells that are released later during a dive.
Back on shore, seals enjoy a dive recovery time that’s around twice as long as their actual dive time. During recovery, the seal’s heart rate returns to normal and its body gets rid of the lactic acid.
Like all mammals, seals need water, but you rarely catch them at the company drinking fountain. Some get their fill by eating fish with low salt content. For those with diets higher in salt, they rely on their kidneys, which act like natural desalinization machines! They can extract fresh water from salt water as well as urinate high concentrations of salt.
Seals spend most of their lives in the water, but they also depend on land and ice for breeding and birthing. They “haul out” of the water onto the ice at certain times of the day for their terrestrial activities – which often include lounging and sleeping and occasional barking, bellowing and biting.
Fur seals have big, burly shoulders that support equally strong front flippers. They need these to paddle in the water, but they also use these flippers to stride across the land or ice.
They can even engage their tails, which are really hind flippers, like legs by hoisting them forward under their bodies in a running motion.
[TRIP TRIVIA: No petting the wildlife! Conscientious expedition leaders have been known to take trekking poles to scout the shores along South Georgia Island. These heroics are intended to protect passengers from territorial and fierce adolescent fur seal gangs. These ruffians view tourists as invaders, and they can quickly charge unsuspecting bystanders. Caution: A fur seal bite is full of bacteria and can become badly infected.]
True (earless) seals aren’t quite as adept at the running part, since their tails are more adapted to swimming. As in water, they undulate their hindquarters on land. They also hump their body up with their flippers to cover ground surprisingly quickly. Ice-dwelling true seals have longer claws that help them grip slippery surfaces.
In response to the cold Antarctic temperatures, a seal’s blood vessels constrict and cut off the warm blood sent to skin that touches the ice surface. That means a seal’s skin gets very cold (close to freezing). This fridge-friendly feature means that the seal’s blubber can insulate the animal’s internal organs without fighting to keep the exposed skin warm. All the energy is used to protect the seal’s critical parts and pieces, like its heart and brain. A seal’s core body temperature is around 38 degrees C (100 degrees F).
Seals also use Antarctica’s solar energy to heat up… which can be a bad thing on warm days! They can quickly overheat when moving from the cold ocean to Antarctica’s solar panel of ice and snow.
To keep from over-heating, seals have a built-in cooler in the form of an alternative blood flow system. In simple terms, mammals use arteries to take blood from the heart to arterioles and the capillary bed. Blood then travels through venules to veins that return the blood to the lungs, where it’s re-oxygenated.
Seals can skip the capillary bed entirely. They can dilate special blood vessels that are near the surface of the skin and bypass the capillary bed, which lets warm blood reach the surface quickly to disperse heat into the environment. That same process also lets seals return cooled blood to their internal body for more heat extraction… and back to the surface for more cooling, and so on.
Seals use other tricks to keep cool, such as covering up with damp sand.
So far, there are no accounts of cowboy hats or parasols… but you never know what’s next in their bag of intriguing adaptations to the polar environment.