Fear that was associated with the smell of peppermint was shown to be transferred to offspring in a study recently reported in the journal Proceedings of the National Academy of Sciences. While fear has been shown to be transmitted across generations in clinical situations, the authors of this report wanted to identify a mechanism for this phenomenon.
The study was carried out in rats. Female rats were conditioned to fear the smell of peppermint by linking the smell to mild electric shocks. Then the rats were allowed to become pregnant. After giving birth, the researchers set up an experiment to see if the newly born pups could learn to feel the fear that their mothers experienced when smelling peppermint.
In the next step of the experiment, the rat pups from mothers that were conditioned to the smell of peppermint were tested to see if they learned to feel fear as well. The smell of peppermint was introduced to the pups with the mother present, who of course expressed fear, and the pups showed fear as well. Then the researchers presented the smell of peppermint to the pups again but with the mother absent and only her “alarm odor” present. In both cases, the pups were shown to have a higher level of the stress hormone corticosterone and an area of the brain called the amygdala was activated, which indicated the pups were experiencing stress when sensing the fear of the mother. Showing that the pups experienced the stress of fear even when their mother was not present, but with her stress odor present, meant that the pups learned to feel fear in association with peppermint through this stress odor.
A control group of rats was designated as well and these rats also were subjected to the smell of peppermint without a shock. As expected, they did not exhibit fear in relation to the peppermint smell and did not transmit any fear behavior to their pups. Also, to confirm the effect was mediated by the amygdala, some pups had the amygdala suppressed and, in these pups, the stress response was blocked when exposed to mothers in a state of fear or their stress odors.
The results of the study showed that the pups learned the fear of the mother. The mother’s state of fear was acquired by the pups during the first few days of their life. When the pups were exposed to the peppermint smell without the mother present but in the presence of her stress odor, they still elicited the fear stress response. Just the sense that their mother was in a state of fear due to the peppermint smell was enough for the pups to learn to be in fear.
The authors of the study report did not offer any information on what the stress odor was that the mother produced when in a state of fear, however, the likelihood is that it is a pheromone. Pheromones are hormone-like chemicals that are emitted from the body and become airborne. The airborne pheromones are then sniffed into the nose of other animals and can affect social behaviors, such as bonding behaviors. The stress odor produced by the mother in a state of fear was likely a pheromone that the pups “smelled” and set off a fear response in them.
The researchers suggest that their study may have meaning for understanding emotional trauma that can be transmitted across generations. If a human infant experiences the stress odors from their mother, they will associate the stressful condition with fear as well. The hope is that this will be information that can be beneficial in clinical settings.
By Margaret Lutze