any sound produced through the action of an animal's respiratory system and used in communication. Vocal sound, which is virtually limited to frogs, crocodilians and geckos, birds, and mammals, is sometimes the dominant form of communication. In many birds and nonhuman primates the adult repertoire comprises a number of different calls, used to indicate territoriality, aggression, alarm, fright, contentment, hunger, the presence of food, or the need for companionship. Bird song (birdsong) (q.v.), the most intensively studied of animal vocalizations, consists primarily of territorial and mating calls.