Understand how the Bengalese finch acquired its virtuoso singing, and you can learn a lot about the forces that freed our own tongues
WHEN you watch Kazuo Okanoya on stage, bobbing up and down, chirping, you know he is passionate about his work. His lab at the University of Tokyo is alive with the sound of the birds that inspire his performance – row upon row of cages full of Bengalese finches. You can see why he is so taken by them. They are beautiful and good-natured, and they sing like a dream.
Okanoya was brought up in rural Japan surrounded by farm animals as well as his own menagerie of pet hamsters, turtles, hermit crabs, chipmunks and finches. "As a child, I loved animals more than humans," he says. That he ended up studying birds is hardly surprising. But what he has discovered certainly is. He set out to explore how singing cements the intense bond between pairs of Bengalese finches and underpins their devoted parenting. Instead, his experiments might have implications for one of evolution's most enduring mysteries: the emergence of human language.
The Bengalese finch's mystery hinges on its ancestor, the white-rumped munia. It is a rather drab and reclusive bird, found across South-East Asia, with a somewhat repetitive and simple song. But around 250 years ago, breeders started to transform it into a beauty, with a subtle plumage of chocolate to platinum feathers. As a side effect, the Bengalese finch also acquired a remarkably complex song. So how did a bird that was bred for its plumage come to sing intricate melodies composed of many phrases?
It is by answering this question that Okanoya has found some clues about the evolutionary forces that loosened our own ancestors' tongues. In the past, it seemed obvious that language emerged through the slow process of natural selection, driven by the benefits of better communication. His work on the Bengalese finch, however, suggests that speech could have come about spontaneously through an entirely different process.
Our understanding of language has long been hampered by the fact that there are very few creatures with anything approaching human vocal talents – and none among our primate relatives. Nature contains just six groups of animals with complex, learned vocalisation: songbirds, hummingbirds, parrots, cetaceans, bats and humans. Our language is far more advanced than the communication of any other animal. Nevertheless, the Bengalese finch's song can be thought to possess a simple syntax. It uses a variety of melodious chunks of sound in a creative way, akin to how we arrange words and phrases to produce sentences. That makes it a cut above most other animals. In addition, its less-articulate forebear, the munia, is still alive, which allows Okanoya to compare the two in a bid to work out how the vocalisation became so elaborate.
His first step was to quantify the difference in complexity between the songs of munias and Bengalese finches. To do this, Okanoya and his collaborators divided the total number of different notes produced by each during a 2-minute performance, by the number of different note-to-note transitions over the same period. This gives a measure of "linearity" – essentially, how predictable the next sequence of the song is, given what came before. The munia's linearity score, at 0.6 out of a possible 1, was almost twice as high as the finch's, indicating a simpler repertoire.
The art of imitation
Munias might nevertheless be able to produce sophisticated tunes, given the right tutors. Like most songbirds, only the males sing and they learn from their fathers and other nearby males, reproducing the sorts of songs they hear around them. So Okanoya and his colleague Miki Takahasi raised munia chicks with Bengalese finch foster parents. The result? The munias learned finch songs with 80 per cent accuracy, compared with the 99 per cent they achieve on the songs of their own species. By contrast, finches reared by munias reproduced their foster parents' songs with 90 per cent accuracy, just as they do for their own (Ethology, vol 116, p 396). In other words, the Bengalese finch's genes don't dictate the specific sequence of the song, leaving more room for learning and improvisation, compared with munias. "The relative looseness of the Bengalese finch's learning compared with the munia's is a key to the emergence of complexity," says Okanoya.
That ability for virtuoso singing is probably the result of many different changes that came about through domestication. Some of the changes seem to be in the finch brain's nucleus interfacialis, one of its song-control centres (see "Not so bird-brained"), which are larger in finches than in munias. When Okanoya surgically removed this region from Bengalese finches, they started singing like munias. He has also discovered that the brains of the Bengalese finches express more of a gene for glutamate receptors, found at the synapses between neurons. These receptors are thought to be vital for learning and memory.
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