Intelligence has nothing to do with reason. Animal can know the meaning of some symbols because it has good memory, but cannot grasp any logical relations between the symbols, therefore it cannot learn any language. It can't understand the meaning of sentences, and ethical principles or laws always are sentences, imperatives. Some people, children for example, can't do that as well but they still have rights (shouldn't be killed) because of their potential of reason, just as every person when it is asleep or drunk. And animals can be killed by it's human owner and it is nothing morally worse then sheep killed by a wolf.