Weddell seals (Leptonychotes weddellii) are the only mammal that dares to swim long distances under sea ice, traveling up to 20 kilometers in hour-long bursts as they scan for air holes and an eventual exit somewhere in the midst of vast Antarctic sheets. There, mothers give birth so that their pups will be safe from leopard seals and killer whales. But how do those pups learn to navigate the risky underwater terrain so quickly? They’re born with big brains, according to a study published online and in an upcoming issue of Marine Mammal Science. Researchers measured 12 carcasses and found that the brains of newborn pups are 70% the size of adult brains—the largest percentage of any mammal. In comparison, the brains of human babies are only 25% the size of adults.