Animal language are forms of non-human animal communication that show similarities to human language. Animal communication may be considered complex enough to be called a form of language if: the inventory of signs is large, the signs are relatively arbitrary, and the animals produce them with a degree of volition (as opposed to conditioned instincts). Animal communication can also be evidenced through the use of lexigrams (as used by chimpanzees and bonobos) in addition to signs.