PHOTOS: Gray seal pup born at Brookfield Zoo

BROOKFIELD, Ill. (WLS) -- Brookfield Zoo welcomed one of its newest additions the day after Christmas! A male gray seal pup was born on Dec. 26.

The baby seal, who has not yet been named, is the third pup born to parents Lily and Boone, both 13.

The Chicago Zoological Society, which manages Brookfield Zoo, said he is growing quickly behind the scenes. He weighed 36 pounds at birth and topped 100 pounds over the weekend.

The pup is expected to quadruple his weight by the time he is three weeks old, zoo officials said. That's when he'll transition from his mother's fat-rich milk to other food.

In the wild, gray seal pups must be ready to fend for themselves and learn how to hunt out at sea at this time, officials said. That's why it's important for them to gain lots of weight and grow blubber in a short period.

Gray seals have one of the shortest nursing periods among pinnipeds, which zoo officials defined as a group of marine mammals that have winged or webbed feet.

The baby seal was also born with white fur called lanugo, officials said. This fur will molt in two to four weeks, when he'll grow shorter, stiffer hair, like his parents. Brookfield Zoo guests won't be able to see the pup at his outdoor exhibit, Pinniped Point, until after he's done molting.

This pup is important to the gray seal population in North American institutions accredited by the Association of Zoos and Aquariums. There are currently 25 gray seals at ten institutions, officials said. Five of these animals live at Brookfield Zoo.

Although the species is not threatened in the wild, officials said the information the association can learn from him throughout his life will help make breeding recommendations to increase the sustainability of the population. Gray seals are found in the Western North Atlantic, Eastern North Atlantic and the Baltic Sea.