Vocal learning

Vocal learning is the ability to modify acoustic and syntactic sounds, acquire new sounds via imitation, and produce vocalizations. “Vocalizations” in this case refers only to sounds generated by the vocal organ (mammalian larynx or avian syrinx) as opposed to by the lips, teeth, and tongue, which require substantially less motor control. A rare trait, vocal learning is a critical substrate for spoken language and has only been detected in eight animal groups despite the wide array of vocalizing species; these include humans, bats, cetaceans, pinnipeds (seals and sea lions), elephants, and three distantly related bird groups including songbirds, parrots, and hummingbirds. Vocal learning is distinct from auditory learning, or the ability to form memories of sounds heard, a relatively common trait which is present in all vertebrates tested. For example, dogs can be trained to understand the word "sit" even though the human word is not in its innate auditory repertoire (auditory learning). However, the dog cannot imitate and produce the word "sit" itself as vocal learners can.

Historically, species have been classified into the binary categories of vocal learner or vocal non-learner based on their ability to produce novel vocalizations or imitate other species, with evidence from social isolation, deafening studies, and cross-fostering experiments. However, vocal learners exhibit a great deal of plasticity or variation between species, resulting in a spectrum of ability. The vocalizations of songbirds and whales have a syntactic-like organization similar to that of humans but are limited to Finite-State Grammars (FSGs), where they can generate strings of sequences with limited structural complexity. Humans, on the other hand, show deeper hierarchical relationships, such as the nesting of phrases within others, and demonstrate compositional syntax, where changes in syntactic organization generate new meanings, both of which are beyond the capabilities of other vocal learning groups Vocal learning phenotype also differ within groups and closely related species will not display the same abilities. Within avian vocal learners, for example, zebra finch songs only contain strictly linear transitions that go through different syllables in a motif from beginning to end, yet mockingbird and nightingale songs show element repetition within a range of legal repetitions, non-adjacent relationships between distant song elements, and forward and backward branching in song element transitions.Parrots are even more complex as they can imitate the speech of heterospecifics like humans and synchronize their movements to a rhythmic beat.

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