Smells like…pain spirit

This post is also available in Dutch.

Emotions can be contagious. We can easily get carried away by the joy of the happy crowd during the Vierdaagse. On the other hand, negative emotions, for instance pain, can also be easily caught— in a somewhat unexpected way.


For mice, pain is contagious no matter where they are – whether they stay together one cage or are caged separately.
Creative Commons Zero (CC0) license.

Mice can feel each other’s pain, discovered researchers from the Oregon Health and Science University last month. A group of healthy mice showed symptoms of pain 24 hours after smelling bedding that injured mice had slept on. Importantly, these healthy mice did not see or make contact with the injured mice beforehand. This suggests that simply odor information can be enough to transmit pain.

What does it have to do with the humans?

Mice and humans share something of great importance with one another: we love cheese We are mammals. This means we both breathe with our lungs, we both have warm blood, and we have a number of other biological traits in common. These commonalities have pushed scientists to question whether mice and humans may function in similar ways. The question that arose from the study was whether pain transfer in humans works in similar ways as it does in mice. Discovering how mice respond to each other’s pain is an important insight on how social pain may be transmitted.

How to measure pain (in a scientific way)
How do we know what the mouse feels? There is no way to know that directly, so scientists do the following. Mice are first given substances like morphine or alcohol, which cause addiction. The withdrawal from these substances results in pain that researchers cannot see directly. However when a researcher agitates a mouse by poking it, the mouse will respond faster when it is injured. This sensitivity level is used to determine how injured the mouse is. The mice that were injured or had pain were therefore more sensitive to poking. If you have ever burnt yourself, you know your burn remains very sensitive afterwards. The same happens to mice: their injury makes them more sensitive to being poked. More interestingly, the mice that had not been injured and also had not been introduced to the other injured mice, also began to show more sensitivity to being poked. Researchers believe that the uninjured mice’s increased sensitivity to pain happened through smell. This is the first time it has been shown that social pain can be transmitted without direct contact or visual observation.

We’re not that different after all.
For mice social pain spreads through the nose. It has not yet been proven if this holds for humans as well. However, people are capable of transmitting part of their pain socially. We call this ability empathy. Moreover, in order to feel empathic pain, we don’t have to be in direct contact with people in pain, just like mice can feel pain when they sniff an unfamiliar mouse’s pain.
You may be surprised but empathy is not a superpower but rather an ancient bonding mechanism that not only holds for humans but many species such as rats, dolphins, and elephants. At the same time empathy works in mysterious ways: it could very well be that odor can trigger empathic responses in humans. Until proven otherwise, have a nice smell!
Written by Lara. Edited by Marpessa.

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