Animal song is not a well-defined term in scientific literature, and the use of the more broadly defined term vocalizations is in more common use. Song generally consists of several successive vocal sounds incorporating multiple syllables. Some sources distinguish between simpler vocalizations, termed “calls”, reserving the term “song” for more complex productions. Song-like productions have been identified in several groups of animals, including cetaceans (whales and dolphins), avians (birds), anurans (frogs), and humans. Social transmission of song has been found in groups including birds and cetaceans.
Anatomy of sound production
Mammals
Most mammalian species produce sound by passing air from the lungs across the larynx, vibrating the vocal folds. Sound then enters the supralaryngeal vocal tract, which can be adjusted to produce various changes in sound output, providing refinement of vocalizations. Although morphological differences between species affect production of sound, neural control is thought to be more essential factor in producing the variations within human speech and song compared to those of other mammals. Cetacean vocalizations are an exception to this general mechanism. Toothed whales (Odontocetes) pass air through a system of air sacs and muscular phonic lips, which vibrate to produce audible vocalizations, thus serving the function of vocal folds in other mammals. Sound vibrations are conveyed to an organ in the head called the melon, which can be changed in shape to control and direct vocalizations. Unlike in humans and other mammals, toothed whales are able to recycle air used in vocal production, allowing whales to sing without releasing air. Some cetaceans, such as humpback whales, sing continuously for hours.
Anurans
Like mammals, anurans possess a larynx and vocal folds, which are used to create vibrations in sound production. However, frogs also use structures called vocal sacs, elastic membranes in the base of the mouth which inflate during sound production. These sacs provide both amplification and fine-tuning of sounds, and also allow air to be pushed back into the lungs during vocalizations. This allows air used in sound production to be recycled, and is thought to have evolved to increase song efficiency. Increased efficiency of sound production is important, as some frogs may produce calls lasting for several hours during mating seasons. The New River tree frog (Trachycephalus hadroceps), for example, spends hours producing up to 38,000 calls in a single night, which is made possible through the efficient recycling of air by the vocal sac.
Birds
When birds inhale, air is passed from the mouth, through the trachea, which forks into two bronchii connecting to the lungs. The primary vocal organ of birds is called the syrinx, which is located at the fork of the trachea, and is not present in mammals. As air passes through the respiratory tract, the syrinx and the membranes within vibrate to produce sound. Birds are capable of producing continuous song during both inhalation and exhalation, and may sing continuously for several minutes. For example, the skylark (Alauda arvensis) is capable of producing non-stop song for up to one hour. Some birds change their song characteristics during inhalation versus exhalation. The Brewer's sparrow (Spizella breweri) alternates between rapid trilling during exhalation interspersed with lower-rate trills during short inhalations. The two halves of the syrinx connect to separate lungs, and can be controlled independently, allowing some birds to produce two separate notes simultaneously.