Funny that this thread should pop up the morning after I dream about my rats dying. I have a recurring dream where I take in a whole lot of abandoned/homeless rats and keep them with my two, but this time I came home to find the rat cage on fire with all of the rats panicking and dying inside. I could only save a couple, and was so traumatised that my legs buckled and I couldn't hold myself upright in my dream.
My last two rats faded away at half the age they should have lived to. They were sisters- the first to die got a pituitary tumour and couldn't walk, eat, drink or even stand without falling over. For the last few days I kept her in a pouch in my shirt, and eventually took her to be put down.
The thing with rats is that they're so sociable, and if one cagemate dies, the other/s need to see the body to understand that they're gone. If they don't, the remaining rats can spend weeks searching for the missing one. So I showed the dead rat's body to her sister, and man- I honestly never thought a rat's face could fall. The remaining rat became depressed and faded away not long after.
Before anyone says I'm being melodramatic about rats mourning, do your research.