According to keeper Phil Ridges
Ambam's father Bitam used to display the same behaviour if he had handfuls of food to carry. Ambam also has a full sister, Tamba, and a half sister at Howletts, who also sometimes stand and walk in the same way. All gorillas can do it to some extent but we haven't got any who do it like Ambam and he is quite a celebrity at the park.Channel 4 News has reported that Ambam has become an internet celebrity.
We think he might use it to get a height advantage to look over the wall when keepers come to feed him and standing up can also help him in looking for food generally in his enclosure as it gives him a better vantage point. Ambam can also carry a lot more food if he stands and uses both hands and walking on two feet also means he doesn't get his hands wet when it is raining!
Gorillas have short legs.