He's eating quite eagerly and drinking well; he's using the box. He's sleeping a lot--even for a cat!--and doesn't seem to be running around or playing as much as before. He's obsessed with 'The Bird'; for those of you who don't know it, this is a cat toy consisting of a three-foot long flexible aluminum (aluminium for our British friends) rod with a heavy cord attached to one end, and a tuft of feathers on the end of the cord.
When he was younger, Zane would get on his hind legs and hop around like a small, fat, grey kangaroo. Now he will bat at it with his paws, but won't chase it around. When he catches it and starts mouthing the feathers, I take it away and give him a kitty treat. He's now going through jags of wanting to play with it, then loosing interest, then 5 min. later demanding I get it out again. Sometimes he'll wake me up at night demanding I get it out for him. The vet suggests that this might be a manifestation of the feline equivalent of Altzheimer's, but he seems to be mentally OK otherwise.