Yeah, I'd go for lipid and protein content.
Chocolate always has some kind of lipid and cakes and the like have protein in the shape of egg whites and often have lipids also.
As a kid we had a cat who would beg for peanuts and crisps and I put that down to the salt and fat. Oh, yes, another thing cats love is salt and I'm sure I read their food is much saltier than dog food which is why you can feed a cat dog food (plus other stuff for nutrients) but shouldn't feed a dog cat food.
Oh, as for Rats I think they're evolving so that they can eat almost anything including (relatively) large amounts of cyanide and arsenic.
(Me, I'm not... in any way intoxicated, Sat 24 Dec 2011, 16:50, Reply)