A well known study by Seyfarth et al. investigated the referential nature of the alarm calls of vervet monkeys. These monkeys have three set alarm calls, with each call directly mapping on to one of the following referents: a leopard, an eagle, or a snake. Each call is used to warn other monkeys about the presence of one of these three predators in their immediate environmental surroundings. The main idea is that the alarm call contains lexical information that can be used to represent the referent that is being referred to. Essentially, the entire communication system used by monkeys is made up of the L system such that only these lexical-based calls are needed to effectively communicate. This is similar to the L component found in human language in which content words are used to refer to a referent in the real world, containing the relevant lexical information. The L component in human language is, however, a much more complex variant of the L component found in vervet monkey communication systems: humans use many more than just 3 word-forms to communicate. While vervet monkeys are capable of communicating solely with the L component, humans are not, as communication with just content words does not output well-formed grammatical sentences. It is for this reason that the L component is combined with the E component responsible for syntactic structure in order to output human language.