Facts about dolphins

Currently there are 49 dolphin and porpoise species which are grouped into six families: the oceanic dolphin family is by far the largest with 38 members; the porpoise family has seven members; and there are four river dolphin families, each containing just one species. However, classification is not an exact science and as more information and discoveries come to light, deliberations will continue and some dolphin species are likely to be further split into more than one species and/or subspecies.

Names and nick names can certainly be confusing; there are eight dolphin names that feature the word ‘whale’, including pilot whales, killer whales, false killer whales and melon-headed whales. Two species have whale and dolphin in their name; the northern right whale dolphin and the southern right whale dolphin – no wonder both the public and scientists get confused!

Dolphins live in the world’s seas and oceans and in some rivers too. Some dolphin species prefer to live in coastal areas, others like shallow water but prefer to live away from the coast close to patches of shallower water which are located further out to sea.

Orcas are the only dolphins which live in the Arctic and Antarctic. Their large size means that they have more protection against the harsh cold of the freezing seas. Most dolphins prefer tropical and temperate waters as they are warm-blooded mammals and so it is easier for them to regulate their body temperature in these environments.

River dolphins such as the Amazon River dolphin (boto) and South Asian river dolphins live their lives only in fresh water rivers and lakes, a long way from the ocean; they are sometimes known as the ‘true river dolphins’. There is another group of so-called river dolphins; these are fresh water populations of marine dolphin species some of whom permanently live in fresh water rivers, these include the tucuxi (or sotalia), the Guiana dolphin, Irrawaddy dolphin and the finless porpoise.

Porpoises are smaller than dolphins; they are less than 2.5m (8ft) long. They are also characteristically chunkier than dolphins and have a small head, little or no beak, and a small triangular dorsal fin (except the finless porpoises!). Porpoises and dolphins also have a different teeth shape. Porpoise teeth are spade-shaped whilst dolphins are conical.

Dolphins vary from fish in a number of ways. Both dolphins and fish have adapted to live their whole lives in the water, both have streamlined bodies and fins. But, dolphins are mammals and so they need regularly visit the surface to breathe air to survive, otherwise they would drown. Fish on the other hand extract oxygen from the water and do not need to surface.

Dolphins evolved from land mammals whose legs were underneath their bodies. As a result, dolphins tails move up and down as they swim, whereas a fish’s tail moves from side to side.

Dolphins are warm-blooded and have blubber to keep them warm. Fish are cold-blooded and are able to adjust to the different temperatures of the water without the need of body fat to keep them warm.

Most species of fish are also known to lay eggs and can lay hundreds of eggs in a single year. Dolphins give birth to a single baby once every 1 – 6 years and then feed their babies milk through their nipples.

The biggest member of the dolphin family is the orca (killer whale). The larger males grow up to 9.8m (32ft 2in) and weigh up to 10,000kg (22,046lbs). Their black, towering dorsal fins grow up to 2m (6ft 7in) and are unique among all whales and dolphins. At birth, orcas are 2.1 to 2.6m (6ft 11in - 8ft 6in).

A group of dolphins is called a pod. Dolphins are social mammals that interact with one another, swim together, protect each other, and hunt for food as a team. Pod life plays a very important role in protecting dolphins from predators such as sharks. Most pods contain anywhere from 2 – 30 dolphins depending on the species and the situation, however there are occasions when pods gather with other pods to form superpods of 100 or even a few thousand dolphins!

Superpods usually only last for short periods of time, and may form during times of mating or when prey is particularly abundant in an area. Once they finish participating in the activity, the dolphins will return to their smaller pods with their family and/or friends until they meet up with other dolphins for the next gathering.

The baiji or Yangtze River dolphin, once found in the Yangtze River in China, is sadly now believed to be extinct. The baiji holds the unfortunate award for being the first whale or dolphin to be driven to extinction by humans; the baiji was sacrificed to population pressures and industrialisation of China.

Entanglement in fishing equipment and habitat destruction resulted in the serious decline of the baiji from the mid-1900s and the building of the Three Gorges Dam lead to further loss of habitat. Wild baiji sightings consisted only of individuals killed in fishing gear. In 2006 an international expedition searched for surviving baijis for six weeks. The boat survey covered the entire known range and devastatingly failed to record a single baiji; the scientists were forced to conclude that the baiji is probably extinct.