Expanding Civil Rights to Encompass Animals?

Something I thought was related to our current discussion on civil rights and civil liberties, PETA has filed a lawsuit against SeaWorld last week for its treatment of 5 killer whales, under the conditions of the 13th Amendment's ban on slavery. Though it's largely viewed as a giant publicity stunt, there might be some contextual material worth thinking about, such as:

Can the text of the 13th Amendment against "slavery and involuntary servitude" be expanded to encompass animal species, or can only humans be under this umbrella? PETA spokesman Jeff Kerr says that the whales are "kidnapped from their homes, kept confined, denied everything that's natural to them and forced to perform tricks for SeaWorld's profit. The males have their sperm collected, the females are artificially inseminated and forced to bear young which are sometimes shipped away." Can this be paralleled to slavery and/or involuntary servitude? On the matter of evidence though, what really happens in the process of obtaining animals for such purposes?

How about the historical context? Some of the arguments against the case range from it being offensive within the context the amendment was established, or that the whales can't really be plaintiffs. African-American slaves back in the antebellum eras were largely seen as either animals or property, hardly as human. A justification in the Dred Scott case was that these African-American slaves were not eligible to be plaintiffs and file lawsuits because they were not U.S. citizens; therefore are animals residing in the U.S. not U.S. citizens? (Can this possible argument draw parallels to arguments about non-citizen voting?)

Skeptics will be surprised to note that the animal rights movement has made some significant headway in the legal sphere, in countries such as Switzerland, New Zealand, and Germany, and in the classrooms of law schools all around the country. :D