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The whole shape becomes more angled and thinner to swim better, muzzle is angled. Ears smaller for less resistance. Webbed feet become bigger, especially back feet. Back feet are the main ones used for swimming, flippers resemble seals. Coarse fur gets thinner and more sleek for better swimming. Fur gets lighter to soak up less heat. Gains ability to close nose holes for swimming purposes. It became an aquatic mammal for cooling purposes and because there’s more water area and more plant life in the water.