Family DELPHINIDAE (Marine Dolphins)

This is the largest family of cetaceans, with more than 30 species as members. Some taxonomists split it into up to three different families. Fifteen species are recorded from around India. In size the dolphin family ranges from the Black Dolphin Cephalorhynchus eutropia of Chile, which is about 1.6 m in length, to the Killer Whale Orcinus orca, which grows to 9.5 m. The geographical range of some delphinids is highly restricted. Peale’s Dolphin Lagenorhynchus australis, for instance, is found only near the coast of southern Chile and Argentina. The Killer Whale, on the other hand, can be seen in literally any marine region. The habitat preferences of delphinids are varied too. Some are purely deep-ocean species, whilst a few may be found in rivers and tidal mangrove areas. Some are strictly tropical and others are found in cold water only.

The delphinids all have conical teeth, and a streamlined fishlike shape. A number of them have a prominent beak, and most, a melon. The rear margin of the tail is notched in the middle. The marine dolphins are built for fast and efficient swimming. Speed is needed to catch fish and squid, the food of most of the family. In fact, the typical dolphin’s shape has been incorporated into the design of ships and submarines. A few species such as the Killer Whale and the False Killer Whale Pseudorca crassidens are well known for taking warm-blooded prey.

Even the skin of a dolphin contributes to the swimming efficiency of the animal. It is ribbed with narrow ridges like the lines of a fingerprint, and it ripples in areas where turbulence occurs, reducing the drag exerted by the water. Unlike human skin, a dolphin lacks a thick layer of dead cells above the living tissue. Oil is released at the surface as the dolphin passes through water, lubricating its motion.

At slower speeds, dolphins ‘cruise’. They swim in the water, coming to the surface occasionally to breathe. When they move faster, they “run”. They come up frequently to breathe, and leap out of the water. Above a certain speed, this saves energy.

Dolphins are well known for their habit of riding the bow waves of boats and ships. They position themselves on the forward slope of the wave, and are propelled by the force of the water. They stop swimming then. Dolphins ride the bow waves of large waves also, and young dolphins bow ride their mothers. One Common Dolphin Delphinus delphis is known to have travelled like this for 113 kilometres before stopping.

A dolphin calf has to dive underneath its mother to feed, as the mother’s two milk ducts are located beneath grooves near her tail. The calf wraps its tongue around a teat and the mother then sends a stream of milk into its mouth.