So, this past week I read an article (http://www.huffingtonpost.com/2009/03/25/vegan-pet-food-is-it-ok-t_n_178880.html) about people who, being vegans themselves, would like for their cats to be vegans too. I can see the logic of this: if you love animals, and don't believe it's right to slaughter them for meat, then it's a bit of a disconnect to think it's alright for them to be slaughtered for petfood (for non-vegans, imagine it's a horse being killed for you to eat, vs. that same horse going to the knackers for your pet to eat, to get something of the same emotional impact). The problem is, like bears we're natural omnivores, evolved to eat almost anything we can get our hands on (one of the great secrets to our success as a species), whereas cats, we're told, are pure carnivores.
Except they're not. Anyone who's a cat owner can testify that while they like meat v. much, especially in the form of small bugs they catch themselves, a great many housecats will gladly help themselves to human food v. much not designed for feline consumption if they happen to find it lying about and unguarded. I remember reading years ago that the main difference between house cats and their wild ancestors is that domestic cats have intestines that are twice as long, enabling them to live off pretty much whatever's available when they have to -- in short, over the past few thousand years they've come part-way along the path from carnivore to omnivore, like us.
Even more to the point, if you give your cats catfood, they're probably omnivores already: most dry catfood is mainly grains (wheat, rice, corn) rather than meat. For example, the Ingredients list on one of the kinds we feed our cats, Hills Science Diet light, lists its ingredients in order from largest proportion to smallest thusly: "brewers rice" (grain), chicken by-product meal (ground-up animal material), corn gluten meal (grain), powdered cellulose (fiber?), ground whole grain corn (grain), chicken liver flavor (note that this is not actual chicken livers but only "flavor"), animal fat (meat), soybean mill run (basically ground hulls, hence I assume fiber), and a lot of additives.
So while that's certainly nothing a vegetarian would want to eat, it's nonetheless far from being what most of us would call "meat", being more grain than animal parts. That being the case, it wouldn't be too far a leap to replace the meat-byproducts with meat-flavored grain. We're almost to that point now; our local health food store, Minkler's in Renton (right across from the Renton airport), carries a vegetarian dog food, though so far as I cd tell they don't have a vegetarian cat food to match it, at least not in stock last time I was there.
So, we'll see. I suspect we'll all gradually shift to cat foods that are more and more vegetable-based, with a bit of meat flavoring to satisfy the carnivore within. And if most cats do eventually switch to a non-meat diet, it'll be because their owners have done the same. We'll see.