Though certain giant whales and otters are exceptions, a preponderance of the marine mammals lie just above the 500 kilogram bodyweight mark (that's 1,102 pounds). That's true regardless of the body size of their terrestrial counterparts. Dogs, giraffes, and moles all belong to similar evolutionary groups as seals, whales, and manatees, yet the aquatic versions vary a lot less in size. That suggested to the researchers that this was not just a matter of water allowing mammals to get bigger. They seemed to be drawn to a particular weight—500 kg.