Good advice from Farside. Don't feed them, just make sure they have water. I personally always check any "pets" in the aft cargo if I have notification and top up water etc. Not many do I guess, but 15+ hours is not a strain. The AVI notification alerts the tech crew to put the heaters on and they (the pets) do really well back there by and by.
It really isn't that grim. A tad traumatic in turbulence I guess, but I have moved a number of dogs and parrots this way and it has worked out OK. The parrots are usually really pissed when they come out of the "box", the dogs are just pleased to see you again. It is a question of relative intelligence I guess. (and you can take that any way you like - lol)
MG