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. 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. Even further complicating the original binary classification is evidence from recent studies that suggests that there is greater variability in a non-learner's ability to modify vocalizations based on experience than previously thought. Findings in suboscine passerine birds, non-human primates, mice, and goats, has led to the proposal of the vocal learning continuum hypothesis by Erich Jarvis and Gustavo Arriaga. Based on the apparent variations seen in various studies, the continuum hypothesis reclassifies species into non-learner, limited vocal learner, moderate vocal learning, complex vocal learner and high vocal learner categories where higher tiers have fewer species. Under this system, previously identified non-human vocal learners like songbirds are considered complex learners while humans fall under the “high” category; non-human primates, mice, and goats, which are traditionally classified as non-learners, are considered limited vocal learners under this system. The most extensively studied model organisms of vocal learning are found in birds, namely songbirds, parrots, and hummingbirds. The degree of vocal learning in each specific species varies. While many parrots and certain songbirds like canaries can imitate and spontaneously combine learned sounds during all periods of their life, other songbirds and hummingbirds are limited to a certain songs learned during their critical period. The first evidence for audio-vocal learning in a non-human mammal was produced by Karl-Heinz Esser in 1994. Hand-reared infant lesser spear-nosed bats (Phyllostomos discolor) were able to adapt their isolation calls to an external reference signal. Isolation calls in a control group that had no reference signal did not show the same adaptation. Further evidence for vocal learning in bats appeared in 1998 when Janette Wenrick Boughman studied female greater spear-nosed bats (Phyllostomus hastatus). These bats live in unrelated groups and use group contact calls that differ among social groups. Each social group has a single call, which differs in frequency and temporal characteristics. When individual bats were introduced to a new social group, the group call began to morph, taking on new frequency and temporal characteristics, and over time, calls of transfer and resident bats in the same group more closely resembled their new modified call than their old calls. Male humpback whales (Megaptera novaeangliae) sing as a form of sexual display while migrating to and from their breeding grounds. All males in a population produce the same song which can change over time, indicating vocal learning and cultural transmission, a characteristic shared by some bird populations. Songs become increasingly dissimilar over distance and populations in different oceans have dissimilar songs. Whale songs recorded along the east coast of Australia in 1996 showed introduction of a novel song by two foreign whales who had migrated from the west Australian coast to the east Australian coast. In just two years, all members of the population had switched songs. This new song was nearly identical to ones sung by migrating humpback whales on the west Australian Coast, and the two new singers who introduced the song are hypothesized to have introduced the new 'foreign' song to the population on the east Australian coast.