Marine mammal

From Freepedia

A marine mammal is a mammal that is primarily ocean-dwelling or depends on the ocean for its food. Mammals originally evolved on land, but later marine mammals evolved to live back in the ocean.

There are five groups of marine mammals:

1. Order Sirenia: the manatee, dugong, and sea cow
2. Order Carnivora, family Ursidae: the polar bear
3. Order Carnivora, infrafamily Pinnipedia: the seal, sea lion, and walrus
4. Order Carnivora, family Mustelidae: the otter
5. Order Cetacea: the whale, dolphin, and porpoise

Since different groups of marine mammals originate from different ancestors, this is a case of convergent evolution.

Since mammals originally evolved on land, their spines are optimized for running, allowing for up-and-down but only little sideways motion. Therefore, marine mammals typically swim by moving their spine up and down. By contrast, fish normally swim by moving their spine sideways.

All mammals have hair but marine mammals, except polar bears and otters, have lost most of their hair, thus decreasing water resistance. Instead, for heat insulation they rely on a layer of fat.

The polar bear spends a large proportion of its time in a marine environment, albeit a frozen one. When it does swim in the open sea; it is extremely proficient and has been shown to cover 74 km in a day. For these reasons, some scientists regard it as a marine mammal.