Can an Animal Be a "Slave"?

Animals are now considered slaves since they are being held against their will. According to PETA, an animal rights organization, the thirteenth amendment is being violated and they use whales to support their argument. They began a lawsuit against Sea World for keeping and forcing killer whales to perform. They also state that this amendment never says it refers only to humans and therefore animals such as the whale have rights as well. Some people believe this organization is only ridiculing itself by doing this. They argue that it is not only the thirteenth amendment that specifies slaves have to be human. Basically the whole constitution does not refer only to humans therefore if it were also be referring to non-humans that would mean animals should also be accounted for theft, murder, and other crimes. This is only one of the objections to PETA's lawsuit yet there are many that do not support their beliefs.