Double Duty
Have you ever heard of a double agent? It’s
someone who leads a double life as a secret agent, or spy for two different
sides. Seals aren’t spies, but they do live double lives. They’re mammals that
live important parts of their lives on land and other parts entirely in the
water. They breed and care for their babies on land, but spend most of their
lives in the ocean.
Harbor seals spend close to half of their time on land and the other half
in water. They can dive to 1,500 feet (457 m). Sometimes they even sleep
in the water!
©M.Campbell/GLOBIO.org Seals are related to walruses and sea lions, for which they are often confused.
True seals, or earless seals, propel themselves through the water with rear
flippers. They use side flippers to steer in the water and move about on land.
Unlike sea lions, true seals cannot walk on their hind flippers when they’re
on land. True seals look a lot like eared or fur seals but they don’t
have ears you can see.
Seals, walruses and sea lions are all pinnipeds. Pinnipeds are fin-footed
marine mammals that live in the water but also spend some of their time on
land or ice.
©From left-K.Campbell,M.Campbell,G.Ellis/GLOBIO.org Where in the World?
A crabeater seal lies on iceflow in the Antarctic. They spend their
whole lives in an icy environment where they rest, breed, and molt their
skin on the ice.
©G.Ellis/GLOBIO.org Seals can live in oceans as well as freshwater
lakes, such as Lake Baikal in Russia. However, most seals live in and around
Antarctica and the Arctic Circle. Antarctica is so cold that it has no permanent
land mammals, but six species of
seals have found ways to live in the Antarctic Ocean. Many seals live in locations
that are hard for humans to reach, such as Antarctica. That’s why the
populations of seals cannot be counted easily.
Shaped for Speed and Survival
Seals look awkward on land, but they are
graceful in water. Their torpedo shape allows them to swim quickly and efficiently.
This shape also helps them escape predators,
such as sharks, and catch prey,
such as fish.
Seals move so efficiently in the water that they can travel far from land to
find food without using a large amount of energy.
Seals in Many Sizes
Elephant seals are so large that scientists have a hard time figuring out
how much they weigh. The male elephant seal is two to three times larger
than the female.
©M.Campbell/GLOBIO.org The largest seal is the elephant seal. An elephant seal can weigh more than 2220
kilograms, which is about the weight of a minivan. The smallest seal
is the ringed seal, which only weighs 50
kilograms, about the same as a big dog. Most seals are somewhere in between
those extremes. A harbor seal weighs between 70-120
kilograms, about the same as an adult human.
Breeding Season
Injury is a great risk for the female elephant seal during breeding, since
she is much smaller in size. Adult females may mate several times before
returning to the ocean.
©M.Campbell/GLOBIO.org When breeding season
begins, males become more aggressive toward
other males. The males, called bulls,
battle other bulls to see who will be able to mate with the females, called cows.
Most species of seals come out of the water to mate and then return to the
water.
The fertilized egg develops in the cow for nine months. During this time,
the cow eats and builds up fat in her body. Toward the end of the pregnancy,
she returns to the breeding grounds to give birth on land. The reserves of
fat will allow her to feed her pup and go for long periods without eating.
Harp seals are nursed for about 12 days and then abandoned by their mothers.
During this time pups nurse on milk that contains up to 45% fat (compared
to 4% for cow's milk), weighing three times as much as they did when they
were born.
©G.Ellis/GLOBIO.org Seals Going Solo
After the pup is born, it learns to find its mother by smell and call. But
before the pup can find food on its own, the cow abandons it and goes back
to the sea. The pup lives on its own stores of fat while it grows bigger and
learns to fend for itself. Eventually, the pup returns to the sea, but it will
not necessarily see its mother again.
Even as adults, many types of seals, including harbor seals and leopard seals,
live solitary lives.
They interact with other seals of the same species during mating season, but
may spend much of their time alone.
Take a Deep Breath
How long can you hold your breath underwater?
How deep can you dive? Some seals can stay underwater for over an hour. Harbor
seals can dive to more than 500
meters below the surface of the ocean. They’re able to do this because
seals have evolved ways
to conserve oxygen.
Seals as Predators
Leopard seal beheading chinstrap penguin. Leopard seals’ favorite
food is penguin, which comprises 87 percent of their diet.
©Todd Pusser/Minden Pictures Seals are carnivores.
They prey on a variety of fish including krill,
herring, flounder and salmon,
depending on what lives in their habitat.
They also eat squid and mollusks. Generally, seals tear their food into chunks
and swallow the chunks without chewing. Harbor seals use their molars to crush
the shells of mollusks.
Large leopard seals prey on king and emperor penguins. They may lurk under
the water waiting for penguins to dive in, then grasp their prey in their mouths
and shake them to death.
Seals as Prey
Polar bears prey on seals for food. Polar bears wait near an opening
in the ice for seals to come up for air, for hours or even days. Seals
are their primary source of food.
©Patricio Robles Gil/Foto Natura/Minden Pictures Seals are prey for other predators such as
sharks and orca whales. However, human beings are a greater threat to seal populations than
any natural predator. Humans hunt seals for their oil, pelts, and meat. In
some places harbor seals are also killed because they’re blamed for decreases
in the fish population. However, this may not be the case. Overfishing,
ocean pollution,
and destruction of fish habitat may be causing these decreases.
Accidental Endangerment
Hawaiian monk seals are excellent swimmers and divers. The average
monk seal dives 51.2 times a day and can hold its breath for 20 minutes.
©Flip Nicklin/Foto Natura/Minden Pictures One species of monk seal, the Caribbean monk
seal, has been hunted to extinction.
The Hawaiian monk seal is endangered,
and the Mediterranean monk seal is considered critically endangered because
of human disturbances at breeding grounds. Not all these disturbances are on
purpose. Often monk seal cows beach themselves to give birth. Tourists who
don’t realize what’s happening think the cow has become stranded
on the beach. They may chase the cow back into the water, thinking they’re
saving her life. However, this may actually kill both the cow and her pup.
Over time, monk seals have moved to more isolated locations, but there are
few safe places left. Some scientists believe climate
change may also be affecting seals by causing changes in the populations
of animals seals
eat as well as melting the cold, icy habitat where some species live.
We Love Seals
These harbor seals live in the zoo and play with zookeepers. Trained
seals are much easier to take care of in a zoo. Have you seen seals
in a zoo?
©Klaus Berberich Seals are popular in many cultures. They are often featured as entertainment
in zoos and water parks as well as on TV and in movies. Seals are smart, and
can learn to perform many tricks. Because of their great popularity in zoos
and culture, people may not realize how many seal populations are threatened
by hunting and habitat destruction.