Watch: Philadelphia Zoo Gorilla Louis Walks Upright Like a Human in an Effort to Keep Clean

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. A post on the zoo's website about Louis says he really dislikes getting wet or getting mud on his hands or feet.

"When caught out in a rainstorm, he'll run bipedally across the yard to seek cover, and when he accidentally steps in mud, he'll find a leaf or a paper bag and wipe his hand or foot off until they are clean again," said the zoo's website.

"Bipedally" is the technical term for running on two legs in an upright position. To accommodate Louis's efforts, the zoo added features to the living area where he spends much of his time so that he can stay clean more easily. A fire-hose bridge was put in that leads out to the gorilla yard so that Louis can avoid an area that gets particularly muddy.

But staying clean may not be his only motivation for avoiding going on all fours. According to a caption for a video that the zoo posted to Twitter and Facebook, it's also a more efficient way for him to walk while carrying snacks.

Michael Stern, curator of primates and small mammals at the zoo, told the Associated Press that it's unusual for most gorillas to walk around the way Louis does. The only reason those in the wild might do it is to reach something like food, or to wade into water or swamps without ducking their heads below water.

Watch: Philadelphia Zoo Gorilla Louis Walks Upright Like a Human in an Effort to Keep Clean | U.S.