Jarvis says scientists seeking to understand human vocal learning would traditionally study the process in an animal more closely related to people, like a chimpanzee. Jarvis notes, however, that nonhuman primates lack the ability to imitate and learn sounds.
Scientists believe that vocal learning evolved independently in three kinds of birds (songbirds, parrots, and hummingbirds), humans, bats, and marine mammals (like whales, dolphins, and porpoises).
Since the trait evolved independently, some scientists have argued it is not possible to compare vocal learning in one animal to vocal learning in another.
Jarvis says modern scientific techniques like brain scans and gene cloning, however, suggest the pathways for vocal learning in bird brains may be similar to those in human brains.
A good analogy, Jarvis notes, is wings, which evolved independently on birds and bats. On both creatures, wings are located in similar body locations and share many characteristics.
"It suggests there are physical constraints on the organisms' interaction with the environment," Jarvis said. "If you are going to evolve wings, nature says it is done this way. Same thing with vocal learning."
Dogs bark, cats meow, and bears growl. The ability for these animals to make each of their signature sounds is hardwired from birth. But scientists say none of these animals can learn to bark, meow, or growl in a new way. They are not vocal learners.
By contrast, songbirds, humans, whales, dolphins, and bats are born with the ability to make certain sounds. As individuals age and mature, they learn how to modify those sounds to make new sounds, Jarvis said.
"Humans are born with an innate set of phonemes"the basic building blocks of phonetic sound used in language, like the g in "goal""which we then modify to strings of phonemes, to make words, and strings of words, to make sentences," Jarvis said.
Like humans, songbirds modify calls (the birds' equivalents of phonemes, or bits of sound) and put them into a sequence to make songs. Cetaceanswhales, dolphins, and porpoisesare thought to do the same with their innate sounds.
According to Jarvis, the process of vocal learning is one of passive imitation. Children listen to adults and with partial success imitate what they hear. As children mature, they go through a stage of "crystallization," in which their imitations become accurate.
Vocal learning for humans gets harder after puberty, which is why educators say it is best to learn a second language while in grade school. Yet humans never completely lose the ability to learn new vocalizations, Jarvis says.
Of the more than 4,000 different songbirds, some, like canaries, can learn new songs their entire lives. Others, like zebra finches, find it impossible to imitate a new song after their puberty-like phase in life. These species are called closed-ended vocal learners, said Jarvis.
By studying the differences among songbirds, hummingbirds, and parrots, Jarvis and his colleagues gain more insight into the process of vocal learning and ultimately the process of how the brain, in general, generates behavior.
In 2002 a research team led by Anthony Monaco at the University of Oxford, England, found that the FoxP2 gene is associated with language production in humans. That finding led Jarvis and colleagues to look for the gene in other vocal-learning animals.
The researchers found the FoxP2 gene in a range of vocal-learning birds, including finches, canaries, and hummingbirds. They also found it in the nonvocal-learning ring dove and crocodile, the closest living relative to birds.
The researchers then compared the FoxP2 gene in songbirds to the FoxP2 gene in humans. The scientists found that two isolated mutations in the human gene were not present in the birds, suggesting that the mutations are not required for vocal learning.
"The third question we askedmutation or not[was], Is there something about the FoxP2 gene in vocal learners that is unique to vocal learners?" Jarvis said. "The answer is yes. There are unique patterns of expression in FoxP2 in the brains of vocal learners."
Jarvis and colleagues found that levels of the gene expression increase right before animals learn to imitate new sounds. The gene expression, meanwhile, falls off when animals are not learning. This suggests that FoxP2 is associated with a behavioral plasticity that makes vocal learning possible.
Schmidt, the University of Pennsylvania neurobiologist, said the paper pushes the idea that "neural circuits, whether in humans or birds, may need to adopt common strategies in order to become functionally capable for vocal learning."
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