Schoko didn't seem to well recently, not being able to hold food and staying mostly on the bottom level of the cage. She also kept falling over a lot for no apparent reason, and I got a bit worried that she was on her way to university, as they say.
Initially, with the helpful hand of the internet, I suspected hind limb degeneration, which apparently is quite common (in older male rats, something I missed whilst panic-reading). When I took her to the vet, it turned out though that it's probably neurological, something which is more common in older female rats. So, looks like she's on her way to developing a non-operable tumor in her pituitary gland. The good news is, that she's not in any pain, and, with the help of steroid injections, will probably live another few months.
We'd always hoped that the rats would live long enough for the baby to get to know them. Doesn't look too likely now, as by the time the kid's old enough to remember them, they'll probably both have gone. It's kinda sad to have to deal with pet deaths before they even happen.