A southern sea otter pup was only about one week old when it was found stranded off the coast of central California on Sept 30.
A jogger heard its cry and alerted the Marine Mammal Center, according to the Chicago Tribune.
The female pup weighed only about 1kg, which is seriously underweight for a newborn, Mr Karl Meyer, an animal care coordinator for the sea otter programme at Monterey Bay Aquarium, told the newspaper. He estimated that the pup's mother had abandoned it for at least 16 hours.
After rehabilitation at Monterey Bay, the pup, referred to as Pup 681, now weighs about 3kg. It was transported to Shedd Aquarium in Chicago last week.
The aquarium is one of only a handful of facilities equipped to properly care for otters like Pup 681.
Tim Binder, Shedd's vice-president of animal collections, said a team of animal care experts watches the pup 24 hours a day to make sure she eats, grooms and sleeps.
He said: "It truly takes a village to rehabilitate a young sea otter. Our animal care team is teaching the pup how to be an otter."
Pup 681 has its own pool, where it splashes with long strands of felt normally used in car washes. The long green pieces are intended to replicate kelp. It also chews on plastic key rings and other toys because it is teething, her trainers said.
Pup 681 is learning how to feed and bathe itself. It now takes in instant formula in a bottle and sliced clam pieces.
The pup will stay at Shedd indefinitely, and the aquarium is working to decide how it will choose a name for it. Pup 681 eventually will join four other otters in the Regenstein Sea Otter habitat.