[Socialist Worker Online]

A DIFFICULT test of one's moral character is to always look beyond difference and prejudice, and recognize the value of another. It is always easier to show kindness to one is who similar and to one who is like you, but the real test is to stand up to the rest of society and stick up for those who relate to you the least--those who you understand minimally.

This is true not with just humans, but with all. No matter how challenging it may be or how ridiculous others may like to make it appear, we must bring ourselves to a point where we can widen our circle of compassion to encompass all who are deserving of it, regardless of their biological species.

Paul D'Amato (whose work I have tremendous respect for) makes the argument in his article "Socialism and 'animal rights'" that no nonhuman animal is worthy of rights on the sole basis that they don't have the capacity to comprehend the meaning of rights, or solely because they are unable to fight for their own rights themselves.
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Most importantly, though, animals have the capacity to feel pain and suffer in the same ways we do. If they suffer, then why is their suffering justifiable, and ours isn't? Why does their suffering hold less value than ours? Violence is violence; exploitation is exploitation, regardless of the species.
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If this is true on an individual basis, then there is strong inclination for one to believe this is also true on a societal basis. As the Jewish writer Theodor Adorno once said "Auschwitz begins wherever someone looks at a slaughterhouse and thinks: they're only animals." A society that exploits animals is a society that is doomed to exploit other humans. For animal liberation is indeed human liberation.

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full story:
http://socialistworker.org/2009/11/18/what-we-share-with-animals