In 1970 child welfare authorities in Los Angeles discovered that a 14-year-old girl referred to as “Genie” had been living in nearly total social isolation from birth. An unfortunate participant in an unintended experiment, Genie proved interesting to psychologists and linguists, who wondered whether she could still acquire language despite her lack of exposure to it.
Genie did help researchers better define the critical period for learning speech—she quickly acquired a vocabulary but did not gain proficiency with grammar—but thankfully, that kind of case study comes along rarely. So scientists have turned to surrogates for isolation experiments. The approach is used extensively with parrots, songbirds and hummingbirds, which, like us, learn how to verbally communicate over time; those abilities are not innate.
Studying most vocal-learning mammals—for example, elephants, whales, sea lions—is not practical, so Tel Aviv University zoologists Yosef Prat, Mor Taub and Yossi Yovel turned to the Egyptian fruit bat, a vocal-learning species that babbles before mastering communication, as a child does. The results of their study, the first to raise bats in a vocal vacuum, were published this spring in the journal Science Advances.
Five bat pups were reared by their respective mothers in isolation, so the pups heard no adult conversations. After weaning, the juveniles were grouped together and exposed to adult bat chatter through a speaker. A second group of five bats was raised in a colony, hearing their species' vocal interactions from birth. Whereas the group-raised bats eventually swapped early babbling for adult communication, the isolated bats stuck with their immature vocalizations well into adolescence. They figured out how to produce adult vocalizations but could not discriminate infantile sounds from mature acoustics. Once the two groups of adolescents were combined, the secluded bats caught up with their peers. Yovel points out that bat communication is more comparable to human language than is birdsong. “Fruit bat vocalizations are emitted in a conversational context,” he says. “They are not singing to advertise their status, as birds do. This is much more similar to the context in which humans use speech.”
Duke University neurobiologist Erich Jarvis, who studies vocal learning in birds, agrees that bats are poised as our fellow mammals to reveal more of the details of human language acquisition. But he also notes that the study's bats might have received nonauditory feedback from their mothers, possibly affecting their vocal learning. For now the researchers want to understand just what the bats are talking about, both in the laboratory and in the wild. Perhaps insights from their vocabulary will echo through our own language arts.