It’s long been known that girls, on average, tend to develop language skills earlier and faster than their male counterparts. The reason behind that particular sex difference has been a source of great debate—a consistent clash of biology and culture.
“FOXP2 is one of the first genes directly linked with language in human beings,” said J. Michael Bowers, formerly a postdoctoral researcher in Margaret McCarthy’s lab at the University of Maryland School of Medicine and author of the new study published in the Journal of Neuroscience (1). “My interest is in understanding how communication has evolved in human beings and animals, and how sex differences in the brain may influence that; so FOXP2 seemed the most logical place to start looking.”
Bowers, McCarthy, and their colleagues examined the levels of FOXP2 proteins in four-day-old rat pups using Western blot analysis and then compared those levels with the ultrasonic vocalizations the pups made while in distress. They found that male pups not only had higher levels of FOXP2 in the cerebellum, amygdala, cortex, and thalamus, but they also produced nearly double the amount of vocalizations of female pups when separated from their mother, leading the dams to preferentially retrieve them over their female siblings.
The researchers wondered what might happen if they reduced the levels of FOXP2 in the pups. Using an intracerebroventricular injection, the researchers directly targeted the protein using a small interfering RNA (siRNA) that stops brain cells from producing FOXP2.
By decreasing FOXP2 in the pups, the researchers eliminated the sex difference. As expected, siRNA-treated males showed a decreased number of vocalizations two days after treatment. But the number of calls in females actually increased, equaling the number emitted by treated males. The researchers hypothesized that suppressing what little FOXP2 the females had caused their brains to overcorrect for the loss and produce enough FOXP2 to raise levels of vocalization.
While males are the more verbose of rat pups, human children show an opposite pattern. So the researchers looked at levels of FOXP2 in post-mortem tissue from five male and five female children with a mean age of approximately four years. They found human girls, unlike the rats, had a higher level of FOXP2 in Brodmann’s area 44, a brain area associated with language, than human boys.
“These results could speak towards the evolutionary development of language. We see FOXP2 expressed differently in male and female rat pups as well as in a small number of human subjects,” said Bowers.
Moving forward, Bowers hopes to extend these findings. He and his colleagues are currently trying to determine the role sex hormones may play in the expression of FOXP2 in the brain and how that may affect vocalization. “We were surprised by the results, but were pleased with the great outcome. Still, we want to be cautious about the interpretation,” Bowers said.
1. Bowers, J. M., M. Perez-Pouchoulen, N. S. Edwards, and M. M. McCarthy. 2013. Foxp2 mediates sex differences in ultrasonic vocalization by rat pups and directs order of maternal retrieval. The Journal of Neuroscience 33(8):3276-3283.