Songbirds are helping scientists decipher the foundations of human speech. But new work on bats may provide missing pieces of the puzzle.

Several times a week, neuroscientists at the University of California, Berkeley, shuttle a furry brown bat named Cooper down the hall to visit a computer that offers sips of fresh smoothie in exchange for conversation. Once inside a customized sound chamber, Cooper lets out a high-pitched trill and cranes his neck eagerly toward a nozzle that dispenses his sweet treat.

In recent years, a handful of vocal learning researchers have turned their attention to bats’ social vocalizations, including the Egyptian fruit bat’s repertoire of screeches, trills, and other cries. Image courtesy of © Steve Gettle/Minden Pictures.

For the past year, Michael Yartsev’s team has been training Cooper and a handful of other bats to “chat” with the computer. It’s the first step in a project aimed at cracking the mysteries of how human speech works, a component of which is vocal learning, or the ability to imitate and create new sounds. Humans share this ability with a select group of animals, including whales, seals, dolphins, elephants, parrots, hummingbirds, and songbirds (1). Increasing evidence suggests that some bats are vocal learners too (2).

That’s welcome news for researchers who use songbirds as the animal model of choice to understand vocal learning. These birds are revealing much about the neurobiology and genetics of vocal learning. Even so, 600 million years of evolution and radically different brain architectures separate birds from humans. As a new mammalian model, the bat could bring researchers even closer to the answers they seek. Many hope that bats like Cooper will help bridge the evolutionary gap between birds and humans.

“We have a lot of catching up to do to the bird work, but there’s also the excitement that comes with being a really young field,” says Sonja Vernes, a neurogeneticist at the Max Planck Institute for Psycholinguistics in Nijmegen, The Netherlands.

Bird Brains Vocal learning, in general, refers to the production of novel sounds by learning or imitation. But what counts as “novel” is debatable. Some scientists say that it includes the ability to modify innate sounds in response to social experience. Others limit the definition to the creation of entirely new sounds, as when a baby speaks its first words. By this measure, birds provide an especially clear and dramatic example of vocal learning. Around the 1950s, scientists began observing striking parallels between avian and human vocal learning. Birds don’t learn to sing properly—and babies don’t learn to speak normally—without early exposure to adult vocalizations, a requirement not shared by most other animals. Young male songbirds listen to and form a memory of an adult tutor’s song (usually the father’s), then gradually shape their immature “babbling” sounds to match it through trial-and-error practice, similar to how babies learn (3). In recent years, modern molecular and electrophysiological techniques have provided growing evidence that these behavioral similarities reflect some common biology. This is surprising, because a bird’s brain looks quite different from a mammal’s. In particular, birds lack the six-layered cerebral cortex that encases mammalian brains. Especially enlarged in humans, the cortex is associated with “higher” functions, such as learning and cognition. The absence of a layered cortex led to the early belief that the avian brain consisted entirely of the more “primitive” basal ganglia, a collection of subcortical brain areas involved in motor planning and coordination. Birds also lacked the canonical cortical-basal ganglia circuits believed to facilitate complex, learned movements in mammals. “This made people think birds couldn’t learn anything—that they were just dumb and automatic, acting purely on instinct,” recalls neuroscientist Sarah M. N. Woolley at Columbia University in New York. But by the early 2000s, a different picture began to emerge, helped by new techniques for tracing connections between neurons and labeling different cell types and signaling molecules. Researchers showed that in songbirds, neurons dedicated to song are organized in clusters—called nuclei—that are roughly analogous to different layers of the mammalian cortex. Not every songbird nucleus has been precisely matched to a layer of the mammalian cortex, but some nuclei share key features with specific layers. In 2004, an international group of researchers, called the Avian Brain Nomenclature Consortium, renamed many parts of the bird brain to highlight these proposed analogies (4). Since then, additional work has confirmed and deepened many of these comparisons. Just as each mammalian cortical layer has a unique pattern of gene expression, researchers have found similar patterns in nuclei involved in birdsong. The findings suggest that certain cell types are shared across the two systems, but are configured into clumps in birds and layers in mammals (5). “The macrostructure throws you off, but if you look at the cell types that are there and which types are connected to which other types… they’re very similar,” says Woolley. For example, in the avian brain region most similar to the mammalian auditory cortex, connections between certain groups of avian neurons resemble the wiring patterns between the analogous brain regions in mammals (6). In both birds and mammals, these groups of neurons show similar firing patterns (with some firing more rapidly and others less so), and respond in the same order, suggesting that information is processed in much the same manner (7). A juvenile zebra finch learns song from his adult tutor. Image courtesy of Sarah M. N. Woolley.

Of Genes and Songs Given these similarities, Woolley and other avian neuroscientists see potential for uncovering the circuits and computations that give rise to vocal learning in humans. But others take a more conservative view. Evolutionary and cognitive biologist Tecumseh Fitch at the University of Vienna in Austria argues that in some cases, anatomical differences between the bird and human brain are too great for the two organs to be properly compared. Instead, he believes that songbirds may be more useful for studying the genes involved in vocal learning. “Once we get to the genetic level, we have a whole other foundation we share with birds,” says Fitch. “Many of the same genes are playing the same roles. It’s what some people are calling a ‘deep homology.’” One such gene is FOXP2, the first gene discovered to cause a language disorder in humans. In 2001, researchers reported that about half the members of a British family carry a FOXP2 mutation that results in impaired grammar and language comprehension. Affected family members also struggle to coordinate sequences of oral and facial movements to produce speech (8). FOXP2 encodes a protein that regulates the expression of a multitude of other genes, and scientists are still figuring out how the whole system influences vocal communication. Genetic similarities with birds are providing clues. In the zebra finch (Taeniopygia guttata), the most widely studied songbird, FoxP2 is expressed in similar patterns as in the human brain, showing up in the thalamus (an integration and relay station for sensory information), the cerebellum (involved in fine motor coordination), and at especially high levels in the basal ganglia. As in humans, disruptions to FoxP2 activity can have pronounced effects on vocal learning in songbirds. At the Free University of Berlin, neuroscientists led by Constance Scharff have found that suppressing FoxP2 protein levels in a key basal ganglia structure during song learning prevents young zebra finches from imitating their tutors properly. The birds go on to produce abnormally variable songs, dropping some syllables and performing others inaccurately (9). But artificially boosting FoxP2 levels in the same brain region also interferes with song learning, as Stephanie White’s team has discovered at the University of California, Los Angeles (10). Together, these results suggest that precisely regulated FoxP2 levels may be critical to vocal learning, says White. Other genes could have important roles as well. In a 2014 study comparing vocal learning and nonlearning species, led by neuroscientist Erich Jarvis, researchers identified roughly 50 genes with potential links to this special skill (11). These genes showed similar expression patterns in songbird and human brains, patterns not found in vocal nonlearning birds, such as doves, and nonhuman primates, such as macaques. In particular, the genes showed either increased or decreased expression in brain areas that control the vocal organ (the syrinx in birds and the larynx in humans) compared to other, neighboring brain areas. Some of these genes are known to be regulated by FoxP2, including SLIT1, which is involved in guiding neurons to make new connections. Jarvis suspects that SLIT1 could be a major player in vocal learning, as SLIT genes are involved in biological pathways that are disrupted in some forms of autism, dyslexia, and speech and language disorders (12). “Now you can take these genes and study them in the bird,” says Jarvis, a professor at the Rockefeller University in New York. “You couldn’t do that with another animal model.”