The Steller's sea cow was the closest relative to the dugong and grew to eight metres long and weighed more than 6000 kilograms. That's three times the length and fifteen times the weight of a dugong. The species fed on kelp and became so well adapted to shallow waters that it could no longer dive, making it easy prey for hunters. The Steller's sea cow was the first marine mammal recorded as becoming extinct in aproximately 1770 by hunters in America.