The Smell of Fear: how baby rats learn fear from their mothers

Infants learning a variety of behaviours from their parents is an evolutionary advantage.  After all, if mom hid from that huge animal and survived, it's probably a pretty good thing to learn if you want to survive.  But apparently, learning is not only done through observation - especially since often, infant animals are born without a fully functional sense of sight or hearing.  A new study published in PNAS suggests that some behaviours that are absolutely crucial to survival, like fear, can be transmitted intergenerationally through smell.  Mothers release pheromones when they feel fear, and their offspring pick up on it, thus transferring the fear. 



Top panel: social transmission of fear
Bottom panel: olfactory transmission of fear in the
absence of mother

The mothers were exposed to an odor and conditioned either before breeding or during lactation.  The researchers delivered a mild electric foot shock to the female mice while exposing them to the smell of peppermint, which was new to them - in this way, the mothers were trained to fear the smell of peppermint.  Once the pups were born, their mothers' fear was transmitted either socially (through physical proximity to their frightened mother) or through odor (in an isolated chamber with the pheromones of their frightened mother, but no mother in sight).  Then the pups were tested to see if they had internalized their mothers' fear using a Y-maze.
When the baby rats were in the presence of their mother and no stimulus, they were calm.  Only when the mother smelled peppermint and expressed fear did the pups get stressed.  To take this one step further, the researchers exposed pups to either the smell of peppermint and their mother, or the smell of peppermint and the smell of their frightened mother.  They found that cortisol levels in the pups (an indicator of stress level) was significantly higher in the baby rats that smelled their frightened mother.  Observations of amygdalae - an area of the brain that processes emotions - in these pups also showed elevated activity. 

This type of phenomenon is also seen in humans: often, mothers with strong phobias or PTSD can transmit these fears to their children.  Dr. Jacek Debiec, who led the research, treats mothers and children with anxiety, and hopes that this research will help in his work by exploring the roots of fear in infancy and how maternal trauma can affect subsequent generations.  He hopes that this research will inform the development of better preventative and therapeutic methods.