25 Dolphin Facts: Clever, Playful Mammals of the Sea

There are around 40 species of dolphin, living in oceans around the world and in a few great rivers.

Dolphins are mammals, not fish. They breathe air through a blowhole on top of their heads and feed their babies milk.

Dolphins belong to the toothed whale family, along with porpoises and sperm whales.

The orca, or killer whale, is actually the largest dolphin. It can grow up to about 8 metres long.

Some of the smallest dolphins are Hector's and Māui dolphins of New Zealand, at only about 1.2 to 1.4 metres long.

Dolphin ancestors once lived on land. One of their closest early relatives, Indohyus, was a small deer-like animal whose fossils were found in Kashmir.

Dolphins find food using echolocation. They send out rapid clicks and listen to the echoes to "see" fish, even in murky water.

Dolphins can hear sounds far higher than humans can, which helps them echolocate.

Dolphins have little or no sense of smell, but their hearing and eyesight are excellent, both above and below the water.

Dolphins call each other by name

Every bottlenose dolphin creates its own unique whistle, called a signature whistle, when it is young. Other dolphins use these whistles to call to particular individuals, much like we use names. When scientists played a dolphin's signature whistle back to it, the dolphin often responded. Studies have also shown that dolphins can remember the whistles of old companions for more than 20 years, which is one of the longest social memories known in animals.

Dolphins sleep with only half of their brain at a time, keeping the other half awake to breathe and watch for danger.

Bottlenose dolphins can recognise themselves in a mirror, a sign of self-awareness seen in only a few animals.

Dolphins have large brains for their body size, second only to humans among animals by some measures.

Some dolphins in Shark Bay, Australia, carry sea sponges on their snouts to protect them while they search the seabed for food. They pass this trick from mother to daughter.

In Florida, some dolphins hunt by swimming in circles to stir up a ring of mud, trapping fish that leap out of the water straight into their mouths.

Dolphins don't chew their food. They use their teeth to grab fish and swallow them whole.

Some dolphins have huge numbers of teeth. A long-beaked common dolphin can have more than 200.

Dolphin calves are usually born tail-first, which helps prevent them from drowning during birth.

Young dolphins stay with their mothers for about three to six years, learning how to hunt and survive.

India's national aquatic animal is a nearly blind river dolphin

The Ganges river dolphin lives in the Ganga, Brahmaputra and other rivers of India, Bangladesh and Nepal. It was declared India's national aquatic animal in 2009. Living in muddy waters, it has tiny eyes and is almost blind, so it relies on echolocation to find fish and often swims on its side. Locally it is called "susu", after the sound it makes when it surfaces to breathe. A recent national survey counted more than 6,000 river dolphins in India.

Chilika Lake in Odisha is home to the world's largest population of Irrawaddy dolphins in a single lagoon.

The Amazon river dolphin, or boto, can be bright pink, and males are often pinker than females.

Many dolphins live in groups called pods, and sometimes several pods join together into "superpods" of more than a thousand dolphins.

Wild bottlenose dolphins can live more than 40 years, and some females have lived past 60.

Not all dolphins are safe. The Yangtze river dolphin of China was declared functionally extinct in 2006, and only around 50 adult Māui dolphins are thought to remain.