The Steller's sea cow (Hydrodamalis gigas) is an extinct herbivorous marine mammal. It was the largest member of the order Sirenia, which includes its closest living relative, the dugong (Dugong dugon), and the manatees (Trichechus spp.). It reached up to 30 feet in length, making it among the largest mammals other than whales to have existed in the Holocene epoch. Within 27 years of discovery by Europeans, the slow-moving and easily captured Steller's sea cow was hunted to extinction.