I've always had my theories on sugar gliders barking, but I've never seen it backed up in any papers regarding them, especially in the wild. There is not much material on sugar glider behaviour in their natural habitat.
Sugar gliders live in very, very large colonies in the wild. Up to around fifteen?
The fact of the matter is that sugar gliders bark for many different reasons, it seems.
Other gliders seem to listen rather closely when another starts to bark, especially if it's one of their own cagemates. I think it's just a basic communication noise? Sometimes it seems like a warning signal to others, in my opinion, just based on how closely other gliders will pay attention, but they also don't sometimes too.
Mine typically stop barking if I open the door and check on them, but occasionally not, too. Some have said it's a loneliness thing, and a way to attract attention, which is probably not entirely incorrect, especially with captive bred pet sugar gliders. Mine usually do it if they're still hungry, but that wouldn't be accurate wild glider behaviour ..
Barking is too much of a generic noise to really pinpoint what it is in gliders, I think. There's a lot of 'variations' of barking, too. Sometimes a TON of gliders will bark at the same time. No idea what's that about. Locating eachother?