Yesterday, in The Predator Problem Revisited, I argued that animal rights concerns does not necessarily imply vegetarianism or the view that it is wrong to kill animals.
However, as Chris pointed out in a comment to that posting, animal rights concern is broader than this. Therefore, let me take a few moments to state some broad morally relevant facts about animals.
First, animals operate on a system of beliefs and desires. The best way to explain a wide range of animal behavior is in terms of statements about what the animal wants (sex, food, avoidance of pain, comfort) and what the animal believes to be true about the world. Somebody who owns a pet makes perfectly good sense when he says that the pet wants to go outside, or that the pet "thinks that I still have his toy."
Desires are reasons for action. This is no less true in non-human animals as it is in humans.
Animals' desires are reasons-for-action for realizing a state in which humans have particular desires and aversion. That is to say, animals have reasons-for-action for causing humans to have desires that fulfill the desires of animals, and for inhibiting in humans those desires that thwart the desires of animals.
What animals lack, and humans have, is an advanced capacity to realize complex relationships between states of affairs and desires and, thus, the ability to make complex plans that will help to fulfill those desires. Consequently, animals cannot think to promote desires in humans that they have reason to promote, or to inhibit desires in humans that they have reasons to inhibit.
Those reasons for action still exist, even if animals lack the capacity to act on them in particular ways.
However, humans (and animals) seek to act so as to fulfill their own desires, and act so as to fulfill their desires given their beliefs. So, humans are going to act to realize states that promote their own desires, which means that humans are going to act so as to promote in others to desires that fulfill and/or prevent the thwarting of their own desires.
Yet, humans do have a reason to promote in others an aversion to thwarting the desires of those who cannot act on their own behalf. This is true because each of us, at times, cannot act on our own behalf (while unconscious or otherwise disabled) or care about somebody who cannot act on their own behalf (infants, pets, friends who might fall unconscious or are similarly disabled).
These "reasons for action that exist" for promoting those desires in others are "reasons for action that exist" for promoting desires that fulfill the desires of animals, and inhibiting in others those desires that thwart the desires of others. What another human cannot do to an animal that is incapable of defending its own interests is something he or she cannot do a human that is incapable of defending its own interests, and leaves all of us more secure.
These, then, represent a body of morally relevant facts concerning the status of animals. What follows from these facts gets complex. However, this at least provides a foundation from which further conversations can be launched.