You can train them to threaten by bringing your face close to the glass and when it gives an "anti-predator meral spread" threat, quickly withdraw. After a few weeks of this, animals are very prone to threaten when you bring your face close to them - or perhaps just mine do it because I'm so scary. Anyway, this is how I managed to take many of the images that are scattered around the wed (and attached) of G. smithii threatening.
Roy