New details have emerged about Lewis the gorilla, who likes to walk on two legs like a human.
He walks on two legs while carrying food, because he doesn't like to eat it dirty, but zookeepers have discovered there's more to his habit.
If Lewis slips over when it's muddy he'll wipe off his feet and hands with a leaf or a bit of a paper and he prefers to walk on two legs.
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The gorilla has been a star at the Philadelphia Zoo for the past 14 years, but new videos have brought him nationwide attention.
"Gorillas will occasionally stand up for a few seconds or walk a couple of steps," said Michael Stern, a curator at the Philadelphia Zoo.
"But what we see Louis doing is really walking clear across the yard - and that's quite special."
Mr Stern says Louis walks upright for a number of different reasons.
"In the video, he does have his hands full of tomatoes, which is one of his favorite treats," he explained. "I think he just doesn't want to crush them by knuckle-walking, so he stands up and walks.
"He'll also do it more if the yard is muddy. For whatever reason, he does not like the feel of mud on his hands and he just likes to stand up and walks."