We might start by pointing out that there's a controversy about just what rights are and also about whether animals have rights, but let's try to finesse those issues. On one common way of understanding rights, for me to have a right is for people or institutions to be obliged to treat me in a certain way. Whether that's the whole story, it's plausibly at least part of it. But cats, dogs and so on aren't obliged to act in any way; creatures who aren't capable of understanding obligations can't have any obligations.

If we put these two bits together, we get a plausible answer to your question: if there were no humans, then there wouldn't be anyone who had any obligations. (Of course, if there are non-humans who have the right kinds of minds, the story is different.) If there aren't any creatures who could have obligations, then the animals don't have rights.

We can back off this a bit. Let's use the term moral agent for any creature who is of the sort that can have moral obligations. Then even if there weren't any moral agents, it could still be that animals have what we might call "hypothetical rights": if there were any moral agents, they would be obliged to treat the animals in certain ways. But the idea that animals might have rights apart from any questions of how moral agents would be obliged to treat them is hard to fathom.

These are really good questions and there are definitely many empirical issues that should be settled before we can adequately evaluate a proposal such as global vegetarianism, or a ban on animal products by the EU.

First, it is worth noting that not all vegetarians are utilitarians, or even consequentialists, and some may think that animals have rights that should be considered even at the expense of some degree of human suffering. How much human suffering is a hard question for such views.

Second, for the reasons you suggest, those who support global vegetarianism should probably not support the immediate end to all use of animal products. The goal would be, I think, to find alternative ways to feed and clothe ourselves in ways that are consistent with the well-being of animals. This won't happen over night. But we can take steps every day to reduce the pain, suffering, and death we cause to animals. (A nice statement of this "do your best" approach is in Sue Donaldson's Foods that Don't Bite Back, Ottawa: Evergreen Press, 2000, esp. p. 64-5, though I'm not sure it is still in print.) It is true that there are some regions of the world where currently the only way to support humans off the land is to graze animals and eat them. But this leaves many questions: could livestock be bred for such environments that could support a (humane) dairy industry? Are there kinds of crops that could, in fact, be grown there with enough ingenuity? Could the communities move from a meat economy to a different economy over time?

If the issue is global hunger, the longstanding argument has been that fertile parts of the world can easily produce enough vegetarian food to feed us all; the problem is one of distribution. So vegetarians need to work on distribution issues also.

Finally, there are many interesting arguments that suggest that the meat industry (as we know it) is environmentally problematic. It is possible to change the meat industry to be more ecologically sound so this isn't an argument against all meat eating. But it is relevant to the issue of global suffering. Likewise, many health problems would be reduced if we were all vegetarians. Meat-eating causes human suffering too; and this must be weighed in the balance.

I recommend looking at Diet for a New America by John Robbins (this is a bit dated now) and The Omnivore's Dilemma, by Michael Pollan (very recent, and an excellent read).

Sorry, but this is a silly argument. Replace "animal" with "person", and you get an argument in favor of breeding children for slaughter. (Apologies to Jonathan Swift.) But yet, surely, it's better for a person to exist than for it not to exist, right? Actually, that's not so obvious, as we'll shortly see. But if it's not obvious in the case of people, it's certainly not obvious in the case of animals.

The argument purports to show that it's (objectively) better for an animal to exist than for it not to exist by showing that, if it were (objectively) better for it not to exist, then we ought to sterilize all the cows. But this assumes that, if it's not (objectively) better for an animal to exist than for it not to exist, then it must be (objectively) better for it not to exist. But the obvious reply is that there's just no better or worse about it. It's neither (objectively) better for one more cow to exist nor (objectively) worse. But then the argument goes nowhere.

What's fundamentally wrong with the argument, however, is its underlying "utilitarian" premise: that we can judge what's right and wrong by adding up what's valuable and what's not; that's the basic idea behind utilitarianism. And utilitarianism has well-known problems in this area. For example, there seems to be some positive value to each human life. Otherwise, it wouldn't be wrong to kill people. But then it seems as if we ought all be making sure that there are as many people as possible. The mere discomfort people would experience due to the effects of over-population surely can't outweigh the value attaching to a single human life: Otherwise, it would be all right to kill someone and feed him to hungry people, which is absurd.

But the right conclusion here isn't that we should all breed ourselves as often as possible. It's that there is something fundamentally wrong with utilitarianism. The reason it's wrong to kill people isn't that there is some value to each human life, value that outweighs any degree of human suffering. It's wrong to kill people because people have certain rights, for example, the right not to be killed. Fundamentally, it's wrong to kill people not because it's bad, in some objective sense, that that person should die. (Maybe it is, but that's not why it's wrong to kill them.) Rather, it's wrong to kill Fred because it is bad for Fred or bad from Fred's point of view that Fred should die, and Fred's point of view is deserving of a certain degree of respect.

The central arguments for vegetarianism are arguments based upon the idea that animals too have certain rights. One can argue that animals do not have such rights, or one can argue that, even if they do, it's still permissible to kill them for food. But you can't argue against this kind of view in the way illustrated here. To counter such arguments with broadly utilitarian reasoning is to miss their point.

I should add, in closing, that there are people who still want to defend utilitarianism in one or another form. But such people have their own ways of evading the let's-all-make-babies argument, and the moves that work there are likely to work in response to this argument, too.

It's a fine question, isn't it. Short, sweet, and deeply provocative. In the interests of full disclosure, however, I should, at the outset, let you know that I don't think we should eat beef--in part because of the sort of reasons this question elicits. That being said, I don't think that the claim the question seems to advance is by itself decisive--namely that it's human's superior intelligence that provides grounds for eating beef. After all, if minimal intelligence itself justified eating an organism, then humans with minimal intelligence (including the aged, those with brain injuries, infants and fetuses, the mentallly retarded, public officials, etc.) would be candidates for consumption, and various computers would have moral standing. But establishing moral standing isn't simply a matter of determining intelligence. Rather, I'd say that what principally (not exclusively) marks an entity as one not to be consumed is its sharing or its capacity to share (or have shared) in certain projects and forms of life, especially individual projects and forms of life, we find both valuable and respectable.

The projects and forms of life I refer to commonly depend upon a certain standard of intelligence, but they also depend upon certain emotional and sentimental capacities. So, the very fact that we could present or attempt to present arguments on our behalf to the aliens (that is, to engage in the forms of life that include crafting arguments, making moral defenses, arguing for the value and standing of something) might do it. The aliens might also be made to understand and appreciate our the projects that we develop across time like our sciences, industries, and nations. But we might also succeed in getting the aliens to value and respect and understand of those human forms of life that compose our arts, literatures, friendships, love affairs, and families, etc. In addition, and perhaps crucially, we should also show the aliens that these projects and forms of life matter to us, that we care about them, that we value them, that it's a matter of profound importance to us whether they continue to exist or not, that we have an interest in their existence and therefore our own. The 'replicants' in Blade Runner establish their own moral claim to existence, I think, in much this way. (A typical computer, by contrast, might be able to argue and even produce art; but since it wouldn't matter to the computer whether or not we switched it off and recycled its parts, doing so wouldn't be much of a moral issue.)

If, however, the aliens have no capacity to sympathize with us, to feel for themselves that those projects and forms of life are valuable, to care that we care about such things, then even if our lives can be adequately explained to the aliens and even if our intelligence were on par with theirs, we're done for. For myself, with regard to beef, I have found that the discoveries of animal behaviorists, anatomists, etc., expose certain forms of life among cattle with which I can sympathize and about which I have come to care--in particular, their social relationships, their ways of fearing, suffering, desiring, and enjoying, as well as these things mattering to them. And so, for the most part, I refuse beef as I would hope the aliens would refuse us.

I think it is reasonable to suppose that in order to have the character defect of being cruel (and thus, in order to act not just as a cruel thing would act, but actually to act cruelly) the being in question has to be able to do something like deliberate about the moral or ethical value of different courses of action. A hawk might be able to do something like deciding whether to attack this squirrel, but I strongly doubt that hawks can consider whether or not attacking squirrels is for the best.

Now, if you tortured a mouse, I would fault you as cruel, because I regard you as capable of deliberating about whether torturing mice is for the best. But when your cat does it, the cat acts in the way a cruel thing acts--you, for example, if you did that--but the cat does not act cruelly and is not cruel, in my view.

There are at least three different kinds of argument in favor of vegetarianism, and each of the arguments have slightly different implications for what is OK.

One argument is concerned with human health (so is more prudential than moral). The idea is that eating dead animals is not healthy for humans, or at least a balanced vegetarian diet is more healthy. This view is not really compatible with eating roadkill, but would be compatible with eating meat if there was insufficient vegetarian food to keep one healthy.

Another argument is concerned with the environment. The idea is that factory farming wastes precious resources (like water) and is inefficient in producing the nutrients humans need. (For details, see "Environmental vegetarianism" in Wikipedia.) This argument also doesn't preclude killing or eating animals where the practices used to raise them are environmentally sound (but it can be developed into a case for a qualified veganism, given the parallel concerns about the environmental impact of the dairy industry).

The "animal rights" argument is concerned with the morality of killing, or causing unnecessary pain to, an animal. Singer, as Prof. Lipton has mentioned, is primarily concerned with pain and suffering. But many vegetarians are opposed even to painless killing of animals for food, sport, or fashion. One need not be religious to believe that if something is sentient then it is something of value and/or it has interests that we ought to respect. To destroy something of value unnecessarily is wrong. To act in a way that unnecessarily violates the interests of another is wrong. These are familiar enough ideas, and some vegetarians just make the added assumption that animals are sentient. As you point out, these ideas don't yield absolute prohibitions against ever eating meat, but they do suggest that routine meat-eating (hunting and leather-wearing) is wrong.

The most compelling reason not to eat meat is not because it involves killing animals but because it so often involves causing animals to suffer, especially in factory farming. From this point of view there might be nothing wrong with eating free range chicken (because their lives are not miserable) or shrimp (if they are incapable of feeling pain). But as Peter Singer argues, causing unecessary suffering seems wrong. If some factory farming were the only way for some people to get enough iron, then it might be a tough call. But I doubt it is, and if I am wrong about this the argument still applies to many of us who do have easy alternatives. And causing unecessary suffering seems wrong even if the perpetrators are acting perfectly naturally.

Other things being equal, perhaps we might. But of course they're not equal. Our social morality -- the morality we live by -- is 'speciesist' in the sense that human beings -- whatever their mental or physical capacities -- are considered to be due special protection. If we were to seek to remove that protection, chances are that it would probably degrade our ethical sensitivities to the point that things went overall worse for non-human animals than they do at present. What we should be asking, rather than your question, is: Why should we continue testing drugs and cosmetics on non-humans to the extent we do, when we wouldn't dream of carrying out such testing on human beings with similar capacities?

I just answered another question related to this one. Please see my answer to that one. In gist, (1) which animals did you have in mind--"animals" is too generic for me to get much traction on an ethical issue. And (2) why do you think it is more appropriate to run experiments on mentally challenged or disabled human beings? Are (some species of) animals somhow more deserving of our moral respect? Why? Are (some species of) animals more ethically important than (some) human beings? What makes you think that?

I can think of a great variety of what seem to me to be ethically significant issues that need to be visited here. One is the question of consent, which I discussed in my answer to the other question. Others include: ability to experience pain, cognitive capacities (such as memory, self-awareness), normal life-expectancy of the creature, normal "quality of life" expectancy of the creature (and how much experimentation would lead to deviation from such expectancies).

I tend to think that we do a great deal of projection of our own capacities onto other species, and I also tend to think that our doing so leads us to misjudge ethical issues and their degree of applicability to other species. I certainly do NOT think that it is OK to torture cats and dogs for kicks--but I do think that global judgments about what is and is not ethically appropriate involving animals is likely to be far more complex than the kind of simplicity your question assumes.

I think the PETA people will think I have a very blind moral eye, but I am inclined to think that your question makes the issue far more simple than it is. For one thing, I think there are morally significant differences between different species of non-human animals. I wouldn't think of causing gratuitous suffering or death to a wild primate, for example, but gladly crush mosquitoes to death whenever given the chance. For another (and related to the first, in fact) I think the very idea of whether animals do or do not consent and how this notion may apply to them is hardly obvious, and perhaps simply otiose. If animals (or some species) do not and cannot give consent or refuse it, then it seems to me this is not a useful more indicator for them.