Marine mammals are a diverse group of aquatic, primarily ocean-dwelling mammals. Like land-dwelling mammals, they breathe air through lungs, are warm-blooded, produce milk to nurse young and have hair. But they have evolved to live, eat and breed in or close to water – seas, estuaries, lagoons and rivers. They include cetaceans (whales, dolphins, and porpoises), sirenians (manatees and dugongs), pinnipeds (true seals, eared seals and walrus), otters (sea ottera and marine otters) and polar bears.
The order Cetacea contains ninety species, of which five are truly freshwater dolphins. The order is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales, which includes dolphins and porpoises). In India we find 31-33 cetacean species, including one true freshwater dolphin and one sirenian.
Marine mammals evolved from different groups of land dwelling ancestors and converged for life at sea, sharing several adaptive features to deal with extreme ranges of pressure, temperature, light availability and complex bathymetry of the sea surface. They all have a generally large size, hydrodynamic body shapes, modified appendages and various thermoregulatory adaptations. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Different species are, however, adapted to marine life to varying degrees. The most fully adapted are the cetaceans and the sirenians, whereas other groups spend at least some time foraging or giving birth on land. Pinnipeds, marine and sea otters and polar bears are temperate species and are not found in our waters. Three species of otters are found in coastal habitats, estuaries and rivers in India.