Apollo and Bomassa wrestle in the weeds,baring their teeth. They are fighting, but they don't mean it. They're having fun while they are getting stronger and developing their self-protection skills. Here you see Apollo encircling his big brother's head with his arm. Later when I got a close-up view, I could see that Bomassa had a little bit of a scratch on his head, complete with fresh blood.
We can expect more and more of the same, as the gorilla boys get older. Here are some photos of considerably older Mountain Gorillas, still juveniles, knocking around with each other in Volcanoes National Park in Rwanda. Apollo and Bomassa are a different species, they are Western Lowland Gorillas. But this mock fighting among the youth in gorilla troops seems to be natural to all species of gorilla. It's just part of growing up.