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Human language incorporates meaning at three different levels: the individual word or morpheme (lexical semantics), the entire utterance (propositional semantics, based on syntactic rules of combination), and the situation of a discourse within a real-word context (pragmatics). Primate alarm call systems exhibit a stable association with real-world situations, but it is not clear that a primate alarm call "means" or "refers to" a certain predator type so much as it is a response to a particular type of impending danger. Language-trained apes and other animals are able to learn larger repertoires of symbols with stable associations to real-world referents in whatever language-like system they have been trained in. Claims have been made that language-trained apes can learn category labels (e.g., tool, fruit) and that African grey parrots can learn properties (e.g., color, size, material) and relational concepts (e.g., bigger, same, different). However, there is no evidence of animals in either their natural or human environments ever having produced meaningful utterances or discourses, though claims have been made for novel meaningful two-sign combinations in chimps, gorillas, and orangutans trained to use individual signs of human sign language. There is evidence from one controlled study that a bonobo was able to successfully carry out decontextualized commands given in English at the same level of accuracy as a 2-year-old child (i.e. about 2/3 of the time).