Parents of newborn infants invariably gush: “There’s nothing like the smell of my baby!” But this is no mere boast. In fact, researchers at the Weizmann Institute of Science in Rehovot have found that the scent emanating from the scalps of newborn mammals, both humans and mammals, has a purpose – survival of the species.
Prof. Noam Sobel of the Department of Brain Sciences and Dr. Eva Mishor, in cooperation with the Azrieli National Institute for Human Brain Imaging and Research, recently published a paper in the peer-reviewed journal Science Advances. They found that in terrestrial mammals, volatile chemicals that emanate from the body can effectively trigger or block aggression in the same species. The research was also presented in the Weizmann Institute’s Hebrew-language magazine Hamachon.
They tested whether hexadecanal (HEX), a human body volatile compound implicated as a mammalian-wide social chemosignal, affects human aggression. They found that sniffing HEX, whose scent cannot be identified by the human nose, blocked aggression in men but triggered aggression in women.
Next, using functional brain imaging, they uncovered a pattern of brain activity mirroring behavior. In both men and women, HEX increased activity in the left angular gyrus, a brain area involved in the perception of social cues.
They created a computer game that was divided into two: a provocative stage meant to arouse aggression among the 130 men and women involved in the study; and a reaction stage in which they were allowed to off-load the aggression they accumulated. Half of the participants were exposed to the HEX molecule, while the other half served as a control group.
Everyone was asked to play a game versus a computer that was presented as another human being. In the first half of the game, they were asked to share among themselves an amount of money, while the computer insisted on keeping most for itself. Each participant had the opportunity to “punish” the computerized opponent by sounding a noisy explosion sound. The loudness of the sound represented how aggressive the participant was.
Mishor, who led the experiment, said the participants reacted differently than the control group, but at first, the results did not seem consistent. Only when the researchers registered the gender of each participant did the picture become clear. Men who were exposed to the HEX molecule scent became less aggressive, while the women who smelled the “baby scent” became more aggressive.
They then put 50 participants who had been exposed to the molecule into a functional MRI (fMRI) scanner, which measures brain activity by detecting changes associated with blood flow. The participants could not know whether they had been exposed to HEX.
In both men and women, the molecule activated the left angular gyrus, which is involved in processing social signals. But communication between this brain region and others was clearly different among men and women. In women, HEX reduced the neural connections between the social, emotional and decision-making parts of the brain; in men, it increased the neural connections.
The reason for this evolution was that male aggression in mammals is often expressed with violence toward the offspring, but aggression in females is expressed by protecting the baby, Mishor said. Researchers estimate that a quarter of male mammals, including rodents and primates, kill some of their offspring, especially when they live in large groups.
“Because infants can’t communicate verbally with their mothers, they have the possibility of communicating with chemicals,” Sobel said.
The team contacted a Japanese scientist who is investigating odor molecules of babies. They decided to collaborate on their data and found that HEX is one of the most common chemicals released by infants’ scalps.
It has been known that animals release chemical signals from their sniffing organs and affect social behaviors, but it was not known before the Weizmann study that they also influence humans.
“Humans smell each other all the time,” Sobel concluded. “Now we apparently understand what happens when we smell babies, how our brain processes this information and what could be the evolutionary role of this effect.”