Dolphin vs Shark: Mammal vs Fish Differences Explained

Key differences at a glance

Dolphins and sharks share the same seas and a similar torpedo shape, with a dorsal fin on the back and paired flippers or fins at the front. Yet they are about as distantly related as a human and a goldfish. Dolphins belong to the toothed whales, a group of mammals whose ancestors walked on land. Sharks are cartilaginous fish, a group that was already swimming the oceans hundreds of millions of years before the first mammals appeared; see how long have sharks existed?.

The similar body shape is a product of convergent evolution: water rewards the same streamlined design, whatever the animal. There are about 40 species of oceanic dolphins, including the orca, the largest dolphin (see is an orca a whale or a dolphin?). Sharks are far more diverse, with more than 500 species.

Tail and swimming style

A dolphin swims by moving its whole tail stock up and down, like a human swimmer doing a dolphin kick. Its tail ends in two horizontal flukes, which contain no bone. A shark swims by sweeping its tail from side to side, as most fish do, and its tail fin stands upright. In many sharks, such as the great white, the upper lobe of the tail is longer than the lower one.

This difference is a useful clue at sea. A dolphin breaks the surface to breathe in a smooth rolling arc, so its dorsal fin rises and sinks again. A shark cruising near the surface has no need to come up, and its fin tends to glide along steadily, sometimes with the tip of its tail fin visible behind it. Dorsal fins differ too: dolphin fins usually curve backward like a hook, while many shark fins are more upright and triangular, although this varies between species.

Breathing: blowhole versus gills

Dolphins breathe air. The blowhole on top of the head is a nostril, which opens when the dolphin surfaces and closes under water. A common bottlenose dolphin usually breathes every few minutes, and it must stay partly alert even when resting, so it sleeps with one half of the brain at a time; see how do dolphins sleep?.

Sharks take oxygen from water with gills. Water enters the mouth and leaves through five to seven pairs of gill slits on the sides of the head. Some sharks pump water over their gills while lying still, but others, including the great white, must keep swimming to push water through their mouth. A shark pulled out of the water cannot breathe, while a dolphin can drown if it is trapped under water, for example in a fishing net.

Skeleton, skin and body temperature

A dolphin’s skeleton is made of bone and still contains the bones of a mammal’s front leg, including finger bones, hidden inside its flippers. A shark’s skeleton is made of flexible cartilage, the same material as our ears and nose; see do sharks have bones?.

The skin feels completely different. Dolphin skin is smooth and rubbery, and it is constantly renewed. Shark skin is covered with tiny tooth-like scales called dermal denticles, which make it feel like sandpaper when stroked from tail to head and help reduce drag.

Dolphins are warm-blooded and keep their core at about 36–37 °C (97–99 °F), insulated by a thick layer of blubber. Most sharks are cold-blooded, matching the temperature of the water around them. A few fast predators, such as the great white and mako sharks, can keep their swimming muscles, eyes and brain warmer than the surrounding water.

Reproduction and family life

A dolphin mother gives birth to a single calf after a pregnancy of about a year in the common bottlenose dolphin. The calf is born tail first and drinks its mother’s milk, and it may stay with her for three to six years, learning to hunt and to live in a social group.

Shark reproduction is far more varied. Some sharks, such as many catsharks and horn sharks, lay eggs in tough cases often called mermaid’s purses. Others, including the great white, keep the eggs inside the body until the pups hatch and are born alive, and some, such as hammerheads and bull sharks, nourish their pups through a placenta-like connection. In every case the pups are on their own from birth and receive no parental care.

Teeth, senses and intelligence

Dolphins have one set of simple, cone-shaped teeth for gripping slippery fish and squid, which they swallow whole. Sharks lose and replace their teeth throughout life, rows of new teeth moving forward like a conveyor belt.

Dolphins hunt with sight and echolocation, sending out clicks and listening for echoes, and they communicate with whistles. Sharks have an excellent sense of smell, a lateral line that detects movement in the water, and electroreceptors called ampullae of Lorenzini that sense the weak electric fields of prey hidden in sand.

Dolphins have large brains for their body size and live in complex social groups. They learn from one another, use signature whistles that work like names, and recognize themselves in mirrors. Sharks can learn and remember, but their behavior is much simpler.

Which is bigger, faster and more dangerous?

Size depends on the species. The common bottlenose dolphin, the dolphin most people picture, is usually 2–4 m (6.6–13 ft) long and weighs about 150–650 kg (330–1,430 lb). A great white shark typically reaches 3.4–4.9 m (11–16 ft), and the largest females exceed 6 m (20 ft). The biggest shark, the whale shark, can grow to around 12–18 m (40–60 ft), while the largest dolphin, the orca, reaches about 8–9.8 m (26–32 ft).

Both groups include fast swimmers. Bottlenose dolphins cruise at about 5–11 km/h (3–7 mph) and can sprint much faster for short bursts, while the shortfin mako is generally considered the fastest shark.

For people, neither is a major danger. Wild dolphins very rarely injure humans, although harassed or hand-fed dolphins can bite. Unprovoked shark bites happen around the world each year, but only a small number of species are involved and deaths are rare. If anyone is bitten by a shark or another marine animal, seek emergency medical help immediately by calling 911 in the US, 112 in the EU or 999 in the UK.

How dolphins and sharks interact

Dolphins and sharks often hunt the same schools of fish, and they sometimes feed side by side. Large sharks such as tiger, bull and great white sharks prey on dolphins, especially calves, and many adult dolphins carry bite scars. Dolphins, in turn, may mob and ram a shark to drive it away, and orcas hunt sharks, including great whites, for their oily livers.

Stories of dolphins saving swimmers from sharks are popular, but there is no scientific evidence that dolphins deliberately protect people. To meet one of the best-studied dolphins in more detail, see our profile of the common bottlenose dolphin, or compare it with its closest look-alike in dolphin vs porpoise.