Mammals like otters and seals have the ability to swim to great depths in their search for food.
As the first signs of life left it's watery environment to colonize dry land the race was on in the search for food. After millions of years the increasing competition to survive made some mammals take one of the greatest steps in evolution - they returned to the water. Retaining the fur of their shore bound ancestors' mammals like voles, otters and seals still return to land to breed but yet have the ability to swim to great depths in their search for food. Still breathing air and giving birth to live young, dolphins and whales became the new hunters of the world's oceans. With complex communication, perfect streamlining and great underwater speed these mammals have mastered all the problems that had to be solved to survive in this harsh alien world.