"Male hooded seals have a fleshy sac above the nostrils that they can inflate. It grows as the animal gets older, and looks a lot like a hood over the nose—thus the name ""hooded seal."" The seals can also inflate the strip of flesh that separates the nostrils. Blown up, it looks like a red balloon. Males use these structures to attract females, or perhaps to threaten other males, or both. They mate with several females, and protect each female and her offspring from other males while the pups are nursing. Nursing lasts only four days, and in that time, the pup doubles its birth weight, gaining 7 kg per day, mostly in blubber, by feeding on milk that is twice as rich in fat as whipping cream. After four days, the mother leaves her pup, who lives on the stored fat for several weeks, until it is old enough to swim and catch fish to eat."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account