The elephant seal, belonging to the genus Mirounga, holds the distinction of being the largest true seal species in the world. Its immense body size places it among the largest carnivorans on the planet, second only to some of the great whales in overall marine mammal mass. This genus is split into two distinct species, the Northern and the Southern elephant seal, both of which exhibit extraordinary dimensions.
The Two Giants: Northern Versus Southern Species
The two species of elephant seals are geographically separated and display notable differences in size. The Northern Elephant Seal, Mirounga angustirostris, primarily inhabits the Pacific coast, ranging from Baja California up to British Columbia, often breeding on offshore islands. In contrast, the Southern Elephant Seal, Mirounga leonina, lives in the frigid waters of the sub-Antarctic and Southern Ocean, with breeding colonies established on islands like South Georgia and Macquarie Island. Southern elephant seals are generally larger than their Northern counterparts, making the Southern bull the largest seal in existence.
Extreme Sexual Dimorphism in Elephant Seals
The sheer size of elephant seals is driven by a biological phenomenon known as extreme sexual dimorphism, where the adult male, or bull, is dramatically larger than the adult female, or cow. The size disparity in this genus is considered one of the most pronounced among all mammals. A mature bull can weigh five to six times more than a cow, and some Southern bulls have been documented to be up to ten times heavier than the average female. This massive size difference is a direct result of intense male-male competition during the breeding season. Bulls must physically fight for control over large groups of females, known as harems, a process that selects for the largest, most dominant males. The bull’s immense bulk serves as a weapon and a shield in these violent, high-stakes confrontations, driving the evolutionary pressure for ever-increasing body size. The male also develops a large, inflatable proboscis, which is another secondary sexual characteristic used to produce loud roars that signal dominance and warn off rivals.
Record Measurements and Average Sizes
An average adult male Southern Elephant Seal measures between 4.2 to 5.8 meters (14 to 19 feet) in length and typically weighs 1,500 to 3,700 kilograms (3,300 to 8,200 pounds). The largest reliably estimated specimen, a bull killed in 1913 in South Georgia, measured approximately 6.85 meters (22.5 feet) long and was estimated to weigh 5,000 kilograms (11,000 pounds).
Southern females are considerably smaller, averaging 2.6 to 3 meters (8.5 to 9.8 feet) long and weighing 350 to 900 kilograms (770 to 1,980 pounds). The Northern Elephant Seal is somewhat more modest, with adult males reaching lengths of 4.3 to 4.8 meters (14 to 16 feet) and weights up to 2,500 kilograms (5,500 pounds). Northern females are generally between 2.5 and 4 meters (8.2 to 13 feet) long, weighing 400 to 800 kilograms (900 to 1,800 pounds).
These animals experience significant weight fluctuations, as both sexes fast during the breeding season on land, living entirely off their blubber reserves. Males lose a substantial portion of their body mass, sometimes up to a third of their weight, while they compete for breeding access and fast for weeks. Once they return to the sea, they regain this weight by foraging on deep-sea prey like squid and fish.