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Evolution of Brain Structures for Vocal Learning

Parrots, hummingbirds, and songbirds, along with humans, are among a small number of animal groups to display vocal learning. We are studying the brain structures used when learning and producing vocalizations. Similarities in the brains of these three groups may reveal constraints on the evolution of brain structures necessary for vocal learning.

Comparative brain<br /><br />
anatomy in avian vocal learners.

This image shows comparable vocal and auditory brain areas among vocal learning birds and humans. Left hemispheres are shown, as that is the dominant side for human language. Yellow regions andblack arrows indicate proposed posterior vocal pathways; red regions and white arrows indicate proposed anterior vocal pathways; dashed lines show connections between the two vocal pathways; light blue regions indicate auditory regions. For simplification, not all connections are shown. The anatomical boundries drawn for the human brin regions involved in vocal and auditory processing should be interpreted conservatively and for heuristic purposes only. Human brain lesions and brain imaging studies do not allow one to determine functional anatomical boundries with high resolution. Scale bar: ~7 mm. From Jarvis, Ann. N.Y. Acad. Sci 1026: 749-777 (2004).

Vocal learning, the substrate of human language, is a very rare trait. It is known to be present in only 6 groups of animals: 3 groups of birds (parrots, songbirds, and hummingbirds) and 3 groups of mammals (bats, cetaceans[whales/dolphins], and humans). All other groups of animals are thought to produce genetically innate vocalizations. To understand this concept, it is important to distinguish vocal learning from auditory learning. Auditory learning is the ability to make sound associations, such as a dog learning how to respond to the sound “sit”. All vertebrates have auditory learning. Vocal learning is the ability to imitate sounds that you hear, such as a human or a parrot imitating the sound “sit”. Currently only vocal learners have been found to have forebrain regions dedicated to vocal learning and production of these learned vocalizations. Vocal non-learners only have been found to have non-forebrain vocal regions responsible for the production of innate vocalizations.

The objective of this project, a collaboration between Dr. Jarvis’ lab at Duke University and Dr. Mello’s lab at The Rockefeller University, is to determine how the vocal learning behavioral trait and associated brain structures evolved. We utilize vocalizing-driven gene expression to identify vocal brain structures in vocal learning and vocal non-learning species. To date, we have used this approach in 3 vocal learners – songbirds, parrots, and hummingbirds. We have found that they each contain 7 very similar brain structures. If according to the current dominant hypothesis, vocal learning evolved independently in all 6 vocal learning groups within the past 65 million years, then the striking similarities in brain structures of at least the 3 avian groups suggest that there a strong epigenetic constraints on how vocal learning can evolve. We are now determining if this hypothesis is correct, or if there really was a common ancestor with vocal learning, and other groups lost them through evolution. Above, click on the group name to find out about the latest results.