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."
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