Marine mammals are animals that spend their entire lives or a very large part of their lives in the sea. They can live entirely in the water or they might live on land and find their food in the sea. This group of animals is very diverse, including for example, dolphins, seals, whales, elephant seals, and polar bears!

In order to study them better, scientists have divided them into 5 main categories:

• Cetaceans, (eg. Dolphins, whales),

• Pinnipeds, (eg. Seals, walruses),

• Sirenians, (eg. Manatees and dugongs),

• Sea otters

• Polar bears

There are approximately 120 species of marine mammals in the world.
14 species of marine mammals can be found in Greek seas. Of these, 1 species is a pinniped and 13 are cetaceans.

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BASIC MAMMAL CHARACTERISTICS
Mammals are animals with the following characteristics:

They breathe air using lungs.
They are warm-blooded and therefore always maintain a stable body temperature (36-38οC), regardless of the temperature of their environment.
They have hair.
Of course, these characteristics alone do not make an animal a mammal. For example, birds and amphibians have lungs, and birds are warm-blooded. Some fish and amphibians also give birth to live young. And some marine mammals, like cetaceans, lost their fur in order to move faster in the water.
So then why do we call some animals mammals? What distinguishes them from the other animals?

The only characteristic that all mammals share is that mothers nurse their young with milk. No other category of animal on earth nurses its young with milk. This is the key characteristic that, in combination with the above features, distinguishes mammals from other classes of animals.

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CETACEANS

Cetaceans are the only mammals that live entirely in the water. The class Cetacean includes dolphins, sperm whales, beaked whales, porpoises, and whales. Some species of cetacean are the largest mammals ever to live on our planet.

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The name cetacean comes from the Greek word “κήτος,” which means large sea animal or sea monster.

They have a streamlined shape, elongated body, and some other characteristics that they share that allow them to live an aquatic life, without ever having to come ashore. On their front ends they have fins that are adapted for swimming.

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They are divided into two large families:

• Cetaceans with teeth, or odontocetes, such as dolphins, sperm whales, harbour porpoises and beaked whales; and

• cetaceans with baleen, in other words, whales who have baleen instead of teeth. These animals are called mysticetes.

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In all the world there are 83 species of cetaceans and 13 of them live in Greek seas. Of those, 8 are in Greek seas year-round, while the remaining 5 are occasional visitors.

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Cetaceans have changed greatly in size along their long evolutionary path. They range in length from 1.25 to 33.5 meters and in weight from 23 to 136.000 kilograms. Included in this group is the blue whale, the largest animal ever to live on Earth!

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PINNIPEDS

Pinnipeds are mammals that spend most of their lives in the water but also come out on the shore. Their name is complex and comes from the Greek words πτερύγιο (flipper) and πόδι (foot).

 

Pinnipeds include seals, walruses, sea lions, and elephant seals.

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As described by their name, pinnipeds have swimming flippers instead of limbs for the needs of their aquatic lives.

Most pinnipeds prefer cold climates and are most often found in Polar Regions. There are, however, pinnipeds that are adapted to life in warmer waters.

The only pinniped that lives in the Mediterranean Sea and especially Greek seas and coasts is the Mediterranean monk seal. It is universally considered to be one of the marine mammals most threatened with immediate extinction.

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In the entire world there are 33 species of pinnipeds. They are divided into 3 families based on morphological characteristics:

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• True seals. Included in this family are seals that lack external ears but have ear openings (eg. monk seals, elephant seals).

• Walruses. Walruses are the only pinnipeds with tusks.

• Seals with ears (otariids). Included in this family are sea lions and fur seals, who have ear flaps outside their ears.

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All pinnipeds have a streamlined shape, eyes that see very well in water, whiskers, a thick layer of fat called blubber, and 4 flippers instead of legs.

Pinnipeds are called “the acrobats of the sea” because, although clumsy and sluggish on land, in the water they are very graceful and are agile swimmers.
One of their characteristics is that the front and rear flippers have 5 fingers, a remnant from their land-living ancestors. They also retain the nails that were used by their ancestors used on land.
They can be found in many different sizes, shapes, and colors. Their length may range from 1-6 meters.
The largest pinniped, the elephant seal, can weigh up to 5.500 kilograms.

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SIRENIANS

Sirenians are marine mammals often called sea cows. They are the only vegetarian marine mammal.

 

The order Sirenia includes dugongs and manatees.

 

The name sirenia, comes from the sirens of ancient Greek mythology.

They are large, slow-moving creatures that are adapted to an aquatic lifestyle. Like cetaceans, they live completely in the water.

Their front limbs are modified into flippers, while the hind limbs have completely disappeared–a flat tail has replaced them.

Because of their dietary habits, they live in shallow water.

They wander slowly through beds of sea grass and graze through waters with sandy bottoms, digging for roots with their thick upper lip. They can dive for approximately 3 minutes at a time in search of food.

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They prefer warm waters. They once lived in the Mediterranean, and sirenian fossils have been found in Crete.

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All of the sirenians are threatened with extinction. Man previously hunted them for their meat, skin, and blubber. Hunting them is now banned, and they are considered as protected species.
Currently their main threats include water pollution that destroys their habitat and collisions with boats that injure or kill them.

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SEA OTTERS

The otter is a mammal belonging to the same family as weasels and ferrets. It is an aquatic animal that lives mainly in rivers and lakes.
Two species of otter, however, have adapted to life at sea and are considered marine mammals. Sea otters are the smallest marine mammals.
Sea otters do not exist in Greece. There are, however, many otters that live in the rivers and lakes of mainland Greece and on some Greek islands.
Sea otters are found in North America, Russia, and on the coasts of Chile and Peru.
They have very thick fur, the thickest fur of all the animal kingdom. And, though they can walk on land, they live mostly in the ocean.
They love to float on their backs in the sea. In this position they sleep, rest, eat, and carry their young, holding them tightly in their arms. They often curl their bodies around large stalks of seaweed so sea currents don’t pull them away.
They eat fish and marine invertebrates (like crab, sea urchin, clams, and mussels). When they eat, they use rocks to open clam and mussel shells.
Until the beginning of the 20th century, they were heavily hunted by man for their fur. Their greatest threat today is pollution in the sea.

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POLAR BEARS

Polar bears are classified as marine mammals.

They are animals that live only in the Arctic Ocean, and they are adapted for life at sea.

Their favorite habitat is on top of sea ice, where they mate, hunt for food, and raise their young.

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They are very good swimmers. They can swim for a long time in the freezing sea. Their body has a streamlined shape in the water. Between their toes there are membranes that help them swim. They also have a layer of fat under their skin that keeps them warm.

Their diet is almost exclusively from the sea. They feed mainly on seals, but they will also eat walruses, whales, reindeer, sea birds, seaweed, and eggs.

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Polar bears hibernate from October until April. Each bear digs itself a large hole in the snow, leaving a narrow tunnel entrance. During hibernation, a female polar bear gives birth in her sleep, typically to two cubs. The young are blind when they are born, weight about one kilogram, and have very fine fur. Inside the snow cave, the mother nurses her little ones with fat-rich milk. By the time they leave the snow cave, the young will weight roughly 15 kilograms and will have thick white fur. Cubs remain with their mother for around 2 years.

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The greatest threat, which polar bears face, is climate change that causes the melting of Arctic ice, their habitat.

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EVOLUTION

Cetaceans evolved over a period of 55 million years and are believed to be descendents of a common ancestor that lived on land and resembled a wolf with hooves. This ancestral animal, known as Pakisetus, walked on four legs and was a carnivore. It looked for its food along the coast and evidence suggest it ate fish. It gradually began to swim in deeper waters, either in search of food or to avoid predators.

Through the process of evolution and over a long period, the descendants of Pakiseus began to adapt to the aquatic environment.
They became large and elongated animals and their front limbs, which had five fingers, became fins for swimming. The hind limbs atrophied and disappeared completely.

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Pinnipeds lived on land before they became marine mammals.

Evidence of this, are found in fossil skeletons of prehistoric ancestors, aged between 20 and 24 million years. The fossil remains have shown that the pinniped ancestor had legs instead of flippers and its body resembled the present-day otter. It had a long tail, a seal head, dog-like teeth, and webbed feet. This indicates that the animal was not only able to walk on land but was also a capable swimmer.

Due to the lack of food on land, the ancestral pinniped, gradually began to adapt to new aquatic conditions. Its body began to acquire a progressively hydrodynamic shape, while the five fingers were joined by skin and turned into flippers. And so we arrive at the pinniped in its current form, which retains from its ancestor, fur and a rudimentary tail, between its rear flippers.

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ADAPTATIONS

An adaptation is any change in the body or behaviour of an organism that is essential for its survival in a given environment. A species adapts very slowly to changes in its environment, over a period of countless generations. Organisms that have best adapted to their environment are more likely to survive and have offspring than those who have failed to adapt appropriately.

For example, an animal with little fur in a very cold environment has less chance to survive and reproduce than an animal with thick fur.

Cetaceans and pinnipeds have specific adaptations to their body and behaviour that have helped them to survive and, in many cases, thrive in an aquatic environment.

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Blubber

Cetaceans and pinnipeds have a thick layer of fat under their skin, called blubber. This layer of fat has many uses:

• It functions as insulation to keep the animals warm.

• It serves as food storage. When food is scarce, the fat can provide the necessary energy.

• It works like a lifejacket. Because fat is lighter than water, these large animals with huge amounts of fat can easily float in the water.

• It helps shape the animal so that it is tapered and rounded, without angles, so that it is more hydrodynamic.

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Fur

Pinnipeds have a layer of fat and thick fur to protect them from the cold. The fat, which is called blubber, protects them from the cold sea and the fur from the cold on land. When the young are born, they do not have blubber under their skin, so they have longer hair to keep them warm in the first months of life living mostly on land. Gradually, the fatty milk from their mothers helps the young build a layer of fat under the skin, so they can “replace” the long fur for a coat more suitable for life at sea.

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Cetaceans are the only mammals that don’t have any hair (except from some rudimentary whiskers and/or eyebrows that some whale species have). It is speculated that their ancestors, who lived on land, had fur that gradually disappeared to reduce the body’s water resistance.

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Teeth

Pinnipeds and odontocetes have teeth just like us; however they do not chew their food. The teeth are used to grab and bite their prey and then they swallow the whole of it.

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Breathing

Marine mammals have lungs and therefore need to come up to the sea surface to breathe frequently.
In cetaceans, the “nostrils” are located on top of the head and are called blowholes. This makes it easier for cetaceans both to move through the water and to breathe at the surface, as only the top of the head must be above water.
Toothed cetaceans, called odontocetes (eg. Dolphins, sperm whales) have only one visible blowhole (that inside it separates in two nostrils), while baleen whales, called mysticetes, have two prominent blowholes.

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When a cetacean comes to the surface, it forcefully exhales air from its lungs. In cold climates, when the animal exhales, both air and water vapor from the lungs create a characteristic mist called a blow.

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Pinnipeds have two nostrils located above the mouth.

Since cetaceans and pinnipeds spend most of their time in the water, an important consideration is how long they can hold their breath underwater. Some animals can hold their breath for 3-5 minutes. Others, such as the sperm whale, Cuvier’s beaked whale, and elephant seal, can hold their breath underwater for more than one hour!

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Since cetaceans and pinnipeds spend most of their time in the water, an important consideration is how long they can hold their breath underwater. Some animals can hold their breath for 3-5 minutes. Others, such as the sperm whale, Cuvier’s beaked whale, and elephant seal, can hold their breath underwater for more than one hour!

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Sight

In some cetaceans, vision is impaired. Their eyes are comparatively small. They are located on the sides of the head and are not bothered by salt water, as often happens to us when we swim in the sea with our eyes open. Dolphins make the difference because they have excellent vision both in and out of water!

In pinnipeds, the eyes are large and perfectly adapted to seeing underwater, even in conditions where light is scarce. On land, however, their vision is blurry. The eyes continuously secrete a special mucous to protect them from salt water. When the seals are ashore, this mucous makes their eyes appear wet, as though they are constantly crying!

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Hearing

Cetaceans have very good hearing. They have two small, inconspicuous pores in their ears that close when they dive, protecting the ear from the increased pressure of the deep sea.

All of the pinnipeds have excellent hearing both in and out of the water. Their ears are more visible than those of cetaceans. True seals have small ear openings, while otarides have external ear flaps.

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Whiskers

Pinnipeds have very sensitive whiskers. They use them to detect and select their food when there is poor visibility on the seabed. Studies have shown that the whisker sensitivity of some seals, when selecting food, is comparable to the sense of touch in terrestrial animals.

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Cetaceans, mostly the newborns and the young ones, have only some residual whiskers (very little hair).

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Echolocation

Cetaceans have the ability to orient themselves and perceive their environment by emitting ultrasounds that are reflected back to them like an echo. This phenomenon is known as echolocation. When the sound returns to the ear, the brain creates a sonic image that shows whether something is prey or something that must be avoided.

Odontocetes can use echolocation to determine size, shape, distance, speed, and direction of potential prey and other objects.

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Other animals, besides odontocetes, who use this technique are bats.
The other marine mammals do not use echolocation.

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