Feeling really bad about this as I went to give the gerbils some food and realised I could only see one of them. Opened up the cage and could see the other one lying in the sleeping bit, not moving. When I took it out, its head was horrible - looked like it had an eye missing and blood round it. It looked like something had chewed a chunk out of its head.
I'm wondering what could have caused this. The two gerbils have been together for nearly three years and never fought so I doubt it was caused by fighting.
What makes it worse is that I'm not sure when exactly it died. It's a big cage with nooks and crannies and they often hide away. I can't actually remember when I last saw it so it could have been dead a few days.