An 18-year-old silverback gorilla at the Philadelphia Zoo likes to stay nice and clean. To do so, he walks on his hind legs so that only his feet have to touch the ground—which has given visitors to the zoo, as well as those watching him from around the world online, quite a shock.While it’s not totally unusual for a gorilla to walk upright in the manner of their human cousins, Louis does it more frequently than most.