"Gorillas will occasionally stand up for a few seconds or walk a couple of steps, but what we see Louis doing is really walking clear across the yard, and that's quite special," said Michael Stern, a curator at the Philadelphia Zoo. He 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, and 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...