The Brookfield Zoo in Illinois welcomed a baby gray seal in late December, and the pup is already aiming to quadruple his weight by the time he is three weeks old.The pup, born on December 26, is spending time with mom, Lily, behind the scenes nursing and learning to swim.The baby seal was born at 36 pounds, and the fat-rich milk he receives from his mother allows him to put on a lot of weight in a short time, the zoo said. The seals are born in the winter in the wild and will put on several pounds in a day with the blubber acting as an insulator.Gray seals are born with long, white fur called lanugo, which they lose in two to four weeks when it is replaced with shorter, stiffer hair similar to that of adults. Credit: Facebook/Brookfield Zoo via Storyful