Whales, dolphins and porpoises have no hind limbs, just one “vestigial” bone that remains hidden inside of the body.

The red circle shows the remains of the hind limb in cetaceans
© Azcolvin429/Wikipedia Commons

 

A humpback whale showing off its fluke  © Wikimedia Commons

A humpback whale showing off its fluke
© Wikimedia Commons

Instead of hind limbs, cetaceans have a big paddle at the end of their bodies, called a fluke. This fluke is not supported by muscle or bones but is made of tough connective tissue, like your ears. Like a human diver (or an Olympic butterfly swimmer), cetaceans use their fluke in an up-and-down movement to swim. When a whale is close to the surface of the water, this movement creates smooth round patterns on the surface of the water, often called “footprints” or “flukeprints”. You can see the same effect in a bathtub or pool if you put your hands underwater and move them up and down close to the surface.

 

This photo of Blue Dolphin Marine Tours is courtesy of TripAdvisor

The front limbs of cetaceans have evolved into paddles (called flippers, or pectoral fins) specialized for balance and steering. The flippers are supported by bones, like our arms and hands. But since we need to use our hands and arms for lots of different things (not just swimming), they are a very different size and shape to a dolphin’s flipper.

Most cetaceans have a dorsal fin that sits on top of their body. It‘s made of connective tissue and not supported by bones. Scientists believe it helps them to balance, make sharp turns and regulate their body temperature. Interestingly, most polar whale species (beluga, narwhals, right whale and bowhead whale), plus gray whales don’t have a dorsal fin.

Dorsal fin © P.G.H. Evans

Dorsal fin © P.G.H. Evans

Cetaceans don’t have fur, but they still need to keep warm. They are insulated by a thick layer of fat called blubber, which can be up to 70cm (2ft) thick in bowhead whales. Blubber can also be broken down to provide energy when food is scarce. Because fat is lighter than water, the blubber also helps the whales to stay afloat.

 

You can test the effect of blubber with some friends, using your own hand. Want to know how? Click here .

 

Blowhole of a harbor porpoise  © A. Lind-Hansen

Blowhole of a harbor porpoise
© A. Lind-Hansen

The nostrils or “blowholes” of cetaceans can be found on the top of the head instead of at the front. This makes it easy for them to breathe without breaking their swimming motion, as they can just bring the top oftheir head to the water’s surface. Unlike land mammals, marine mammals are voluntary breathers. This means that they have to decide when to breathe, they won’t just do it automatically like we do. Their blowhole stays closed until the whale or dolphin actively opens it to breathe. This is an advantage for water-living animals, because it means they don’t have to think about keeping their nostrils shut to prevent water getting into their lungs.

Underwater the blowhole of the harbor porpoise is closed  © A. Lind-Hansen

Underwater the blowhole of the harbor porpoise is closed
© A. Lind-Hansen

Many cetaceans have a snout or beak (scientists call this the rostrum) that gives them a more streamlined shape. The bones that make up the beak are equivalent to those that you have in your upper jaw.  
Toothed whales have a special organ at the top of their head, the melon. It’s an oil-filled sac that is connected to the vocal cords of the animals. The melon is an acoustical lens for focusing sound waves. Toothed whales rely on it when they hunt for their prey. They use sound like bats to “see” their surroundings. Being able to focus the sound waves is critical for cetaceans, just like being able to focus your eyes on a certain object is important for you.
In contrast to toothed whales, baleen whales don’t have a melon because they don’t hunt fish using echolocation (so they don’t need to locate their prey), but filter water to catch their prey. To do this they use their baleen like a sieve. Each whale has hundreds of baleen plates in his or her mouth that overlap a little to keep the prey trapped inside. You can try this activity to get an idea of how a baleen works.

A whale skeleton with baleen hanging from the upper jawbones  © Thesupermat/Wikimedia Commons

A whale skeleton with baleen hanging from the upper jawbones
© Thesupermat/Wikimedia Commons