Southern right whale, Peninsula de Valdes, Patagonia
Like other right whales, the Southern Right Whale is readily distinguished from other whales by the callosities on its head, a broad back without a dorsal fin, and a long arching mouth that begins above the eye.
The body of the whale is very dark grey or black, occasionally with some white patches on the belly. The right whale's callosities appear white, not due to skin pigmentation, but to large colonies of cyamids or whale lice. The Southern Right Whale is almost indistinguishable from the closely related North Atlantic and the North Pacific Right Whales, with only some minor skull differences. It may have fewer callosities on its head and more on its lower lips than the northern species. The maximum size of an adult female is 18.5 m (61 ft) and approximately 130 tons. The testicles of right whales are likely to be the largest of any animal, each weighing around 500 kg (1,100 lb). This suggests that sperm competition is important in the mating process.