Despite recent progress toward sexual equality, it's still a man's world in many ways. But numerous studies show that when it comes to language, girls start off with better skills than boys. Now, scientists studying a gene linked to the evolution of vocalizations and language have for the first time found clear sex differences in its activity in both rodents and humans, with the gene making more of its protein in girls. But some researchers caution against drawing too many conclusions about the gene's role in human and animal communication from this study.
Back in 2001, the world of language research was rocked by the discovery that a gene called FOXP2 appeared to be essential for the production of speech. Researchers cautioned that FOXP2 is probably only one of many genes involved in human communication, but later discoveries seemed to underscore its importance. For example, the human version of the protein produced by the gene differs by two amino acids from that of chimpanzees, and seems to have undergone natural selection since the human and chimp lineages split between 5 million and 7 million years ago. (Neandertals were found to have the same version as Homo sapiens, fueling speculation that our evolutionary cousins also had language). In the years since, FOXP2 has been implicated in the vocalizations of other animals, including mice, singing birds, and even bats.
During this same time period, a number of studies have confirmed past research suggesting that young girls learn language faster and earlier than boys, producing their first words and sentences sooner and accumulating larger vocabularies faster. But the reasons behind such findings are highly controversial because it is difficult to separate the effects of nature versus nurture, and the differences gradually disappear as children get older.
Wondering if FOXP2 played a role in these possible sex differences, a team led by psychologist J. Michael Bowers and neuroscientist Margaret McCarthy of the University of Maryland School of Medicine in Baltimore started off by looking at young rat pups, which emit cries in the ultrasonic range—frequencies higher than humans can hear—when separated from their mothers. The team acoustically recorded the cries over 5 minutes in groups of 4-day old male and female rats that had been separated from their mothers. Both male and female pups made hundreds of cries, but males emitted twice as many as females, the researchers report today in The Journal of Neuroscience. When the pups returned to their cages, mothers retrieved the males before the females, which additional experiments suggested was a response to the higher number of distress calls. The team then killed eight male and eight female 4-day-old pups and examined how much FOXP2 protein was in their brains. Male pups had up to twice as much of the protein in regions of the brain known to be involved in vocalization and other cognitive functions, such as the amygdala, cerebral cortex, and cerebellum, but showed no difference in brain areas not linked to vocalization, such as the hypothalamus. To confirm that FOXP2 drove the extra cries in males, the team next injected a strand of RNA designed to partly block the activity of the gene into the brains of young male rats. Those males emitted fewer cries, behaving more like the females.
Finally, the researchers conducted a small study on human children aged 4 to 5 years who had died in accidents less than 24 hours previously. In the brains of five boys and five girls, they analyzed the amount of FOXP2 protein in part of the left frontal cortex called Brodmann area 44, which has been linked to language in humans. In direct contrast to the rats, the researchers found 30% more FOXP2 protein in the brains of the girls as compared with the boys.
The team concludes that in both rats and humans, what it calls "the more communicative sex" has higher FOXP2 levels, as might be expected if the gene and its protein do play a major role in the ability to vocalize. "We are highlighting a previously unreported sex difference in a gene that is generating great excitement for its potential role in the evolution of language," McCarthy tells ScienceNOW.
Dwayne Hamson, a neuroendocrinologist at the University of British Columbia in Vancouver, Canada, calls the paper "very exciting" and "convincing and compelling evidence that FOXP2 is a key molecule for communication in mammals." But Simon Fisher, a co-discoverer of the FOXP2 gene who is now at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands, cautions against drawing "big conclusions about human sex differences" from such a small sample of human children, which focused on only one brain area and a narrow age range. And he argues against any notions that there are simple parallels between rat pup calls and human language. He says the cry of a human baby might be analogous to a rat pup vocalization, but that "speech and language … are so much more complex and interesting than simple innate vocalizations."