The smallest mammal in the seas is the Commerson's dolphin. An adult weighs 25-35 kg (55-77 lb), 3,500 times lighter than a blue whale, the largest sea mammal. weighs only 50 g (1.7 oz). The tiny pygmy marmoset which lives in South America comes a close second. It weighs up to 75 g (2.6 oz), about as much as a hen's egg.