Speciesm [Ed: Or speciesism] is a commonly invoked argument against the eating of animals. The suffering of animals is said to be the same as that of humans and since you wouldn't make a human suffer, you wouldn't make an animal suffer.
However, I don't see anything wrong with speciesm.
Just because an entity is sentient does not mean that harm done to it is wrong.
If I code a computer program to respond to my pressing of the "A" key with a response of "ow", am I harming it when I press the "A" key?
If the program is complicated enough to have routines that mimic the human response to pain, would we say it was suffering, much less that its "suffering" was morally equivalent to that of humans? (side note: if we don't, then we are guilty of biocentrism - why should artificial life be discriminated against?)
If the suffering of animals is morally equivalent to that of humans, then we are morally obliged to kill (or permanently incapacitate/incarcerate) those who cause animals to suffer, just as we would kill (or call the police, who would set in chain a process that could kill or permanently incapacitate/incarcerate) a murderer if we could (otherwise we would be condoning murder). Factory farms are worse than Auschwitz as many more sentient beings suffer.
Knowing that the Guinea Worm is going extinct due to an active campaign by humans, I should also regard this as a case of genocide and strongly condemn this - would it be okay to make humans go extinct?
Yet, the logic has to be extended further. If we knew a human was suffering in the wilderness we would try to save him. We mount expensive rescue missions to save humans who are trapped in the wilderness so they don't die.
With reference to Peter Singer's argument about "The Life You Can Save", we must extend this further to animals. You are a bad person if you don't donate everything above what you need for a reasonable life to the cause of animal welfare. Knowing that animals are suffering and dying in the wild, we need to save them.
Further, we need to prevent animals from harming other animals. If a cat would hunt and kill ten mice, we are obliged to kill the cat - we are saving ten lives at the cost of one. It doesn't matter that the cat does not know any better - if a criminally insane human shooter is on a shooting spree, we would kill him to stop him from killing more people, even if he weren't cognizant of his actions.
I doubt any animal rights activists - even those who, like the Animal Liberation Front, use bombs to fight for animal welfare, would go so far.
So we can see that even the most ardent vegan does not hold the suffering of animals to be morally equivalent to that of humans.
This does not mean that animals do not deserve any moral consideration at all, but we can see that it is surely less than that we give to humans.
Eating meat, for example, gives many great pleasure and a perpetual vegan diet would immiserate many. So even under utilitarianism meat eating can be justified. Recall too that as per JS Mill, it is better to be a human being dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied. Similarly, we could say that the pleasure of the meat-eating human being outweighs the suffering of the slaughtered pig .
Meanwhile, one peculiar claim is that we should look at biomass to uphold rights. So a tapeworm in a human gut can morally be killed because the human, being bigger, has rights to an improved quality of life that outweigh the tapeworm's right to life. Yet, invoking mass as a determinant of moral worth seems even more arbitrary than speciesm. Why should bigger animals have more of a right to survive than smaller animals?
If a starving lion (~250kg) is faced with a human baby (~2kg), does it have a right to eat it? Humans aside, lions prey on animals many times their size. Is it wrong for a lion of 100kg to hunt and kill a 290kg wildebeest and should we be coercing it to eat smaller prey?
Also does this mean humans can eat chickens and fish, since they are lighter than us?
Addendum: Actually utilitarian vegetarians should just kill themselves.
No matter what you eat or do, animals will die as long as you live.
Even organic farms kill mice (not to mention many other animals).
Since this is utilitarianism, intent is irrelevant - we need only look at the outcome.
So if you go by the principle that animal suffering (and life) is the same as human suffering (and life), you should kill yourself so hundreds of mice won't die for your food to be produced.