The situation was dangerous, says Pam Schaller, a senior aquatic biologist at the California Academy of Sciences.
Feathers keep penguins warm and enable them to identify each other.
"With the appearance of bald patches, [Pierre] was getting picked on by other birds, and he wasn't swimming very often because he was very cold," Schaller tells Robert Siegel.
After medical tests yielded inconclusive results, Pierre was fitted with a neoprene vest, which allowed him to move his wings and flippers.
After six weeks in his new clothes, Pierre's feathers began to re-grow, and he now has a new lease on life.
By penguin standards, Pierre is an elder statesman: In the wild, penguins live about 15 years; in captivity, they live to about 20. Pierre turned 25 in February.
"He is behaving as though he is, once again, the patriarch of the colony," Schaller says.
"Now he's standing tall and strong.