Can you view them in the second, more recent outdoor enclosure they built, or is it still largely offshow?
In my opinion i think that bark would be better flooring than hay but i suppose its good enrichment for the gorillas.
Hay( or straw) is more interesting for them to be able to sort though. Bark mulch may look more attractive but if it dries out it then becomes very dusty. Also some types of bark/woodchips are hard/sharp on the Gorillas' sensitive feet too and they do not like walking on it. So the straw/hay is probably preferable on both counts.
The second enclosure is still offshow, but you can just-about view it by looking through a small gap in the wooden screening.
