The Nose Knows Fear

This has actually happened to us…You’re walking down the street with your dog, and some guy walks by apparently minding his own business. Your dog then begins to uncharacteristically act out of sorts, and instantly the guy jumps out of his skin – really triggering your dog now.

Clearly, your dog knew from the start the guy is afraid of dogs. But how did your dog know this when the guy wasn’t overtly acting afraid?

Mice — even those that have never before encountered other species
— will act fearfully when exposed to the odor of many different kinds
of predators, including cats, rats, snakes, ferrets, weasels, and foxes
. This is the first time that scientists have been able to identify the
distinct chemical signal evoking this response and understand how mice
are able to detect this signal “The Smell of Fear.”

Identifying the chemical pathway of signals that make their way
through the neurological system is not easy. One of the challenges for
scientists studying brain circuits is that the brain is constantly
changing through learning and memory. But certain behaviors tend to be the same each time they are
triggered, suggesting a steady pathway through neurological circuits.

In the current study, Stowers and her colleagues investigated the
fearful response that mice were known to have to odors emitted by
predators.Could this trigger predators, just as your dog might be triggered by the guy on the street who is afraid of dogs?

Mostly how is the fear response triggered – it’s all in the nose,
actually the vomeronasal organ. So scientists compared  the behavior of
normal
(wild type) mice with mice with a genetic mutation that left them
without functioning vomeronasal organs which has the extraordinary
ability to detect the most minute of scents.

When the regular mice were put in a cage with a cotton ball swabbed
with rat, cat, or snake odor, the mice reacted fearfully — staying
away from the cotton ball, repeatedly striking a cautious posture (nose
in the air and ready to run), and exhibiting elevated levels of stress
hormones. 

In contrast, the mice without functioning vomeronasal organs
appeared to be curious about the cotton balls and demonstrated few
signs of fear. In fact, the behavior of the mutant mice was comparable
to that of normal mice presented with a cotton ball swabbed with a
control odor (such as that from a rabbit or another mouse). From these
experiments, the scientists were able to determine that the vomeronasal
organ was essential for detecting chemical cues from predators.

To see just how important the
olfactory system was in determining this behavior, the team put a
series of mice in a cage with an anesthetized live rat. In contrast to
normal mice, those without vomeronasal organs showed few signs of fear.