I noticed when I was cleaning them out last night that Pin - the grey one - was less lively than usual, and when I picked him up he was slightly cool to the touch. Having had gerbils in the past I was rather expecting a corpse in the morning; since he was still hanging in there but obviously poorly, I registered with the vet in the next street and made an appointment after work.
Pin was so out of it that I was able to take him out in the waiting-room and hold him in my hands, which he'd never normally allow. The couple sitting next to me swapped places as the woman was frightened he was a rat. Tsk.
We were seen by a lovely Australian vet who was terribly kind to us both. She explained that it was very hard to tell what might be wrong with such a small creature - which I well knew - and said "I just want you to be prepared for the fact that the little guy probably isn't going to make it", at which point it all got a bit too Rolf Harris and I started blubbing.
I have had enough gerbils to know that when one is sitting with its fur looking all ruffled and its eyes half-shut, breathing rapidly, it's very probably not worth paying £20 for an injection of fluids and a course of antibiotics.
But I did, of course.
Now I'm just waiting.