Human fear is contagious; rats can also catch it, which is observable at the neuronal level

Polish scientists have discovered that information about danger can be transmitted across species, possibly through scent. This was demonstrated by the following experiment: even if a human was behaving normally but had previously experienced negative emotions, rats playing with the person could sense that something was amiss and would become afraid themselves. This could be observed in the activity of their brains.


The ability to read the emotions of others, to understand what they are feeling at a given moment, known as empathy, is not a purely human trait. It exists in many animals, even in aquarium fish. However, until now, among scientists, the prevailing view was that these were automatic responses based on mimicry – if one individual froze without moving, the other did the same.

The latest research by scientists from the Laboratory of Emotion Neurobiology led by Prof. Ewelina Knapska and the Laboratory of Brain Imaging led by Prof. Artur Marchewka at the Nencki Institute of Experimental Biology of the Polish Academy of Sciences, suggests that it might be different. The team suspects that the carrier of this information may be a scent, as reported in the latest issue of the prestigious journal "Proceedings of the National Academy of Sciences" (PNAS).

"Previously, there were reports that some domesticated animals, such as dogs, goats, or horses, react to human emotions, especially negative ones, like fear. However, no one had previously studied this in rats, so we came up with the idea to investigate whether rats also exhibit such reactions at a behavioral level and, for the first time, to examine what happens in the brain of these animals," says Prof. Knapska, adding that previously studied species "did not offer such opportunities."

For the experiment, researchers recruited a group of volunteers. These individuals were first conditioned to experience negative emotions. During the study, the volunteers sat in front of a computer and learned that the display of squares of a specific color was accompanied by a mildly unpleasant electrical stimulus. Some of the participants undergoing this conditioning were observed by their peers while, at the same time, researchers from the Laboratory of Brain Imaging at the Nencki Institute, recorded the brain activity of the observers. It turned out that watching another person's discomfort activated the amygdala in the observer's brain. The amygdala is a very ancient part of the brain responsible for experiencing fear.

"The final experiment involved volunteers - authorized to work with laboratory animals - coming to the laboratory daily for several days to acclimatize the rats to them. This is a standard procedure where animals are picked up and petted. Rats learn who is touching them and that it is safe," explains Dr. Anna Kaźmierowska, who conducted the study.

After a few days, a control test was conducted. Volunteers underwent a procedure similar to the final one, but instead of an electric shock, they felt mild vibrations on their forearms. Nothing unpleasant happened to them, and right after that, they went to play with the rats. "On the last day, however, the volunteers experienced the fear conditioning, during which they were treated with an electric shock. It was unpleasant, which we confirmed by measuring the electrical conductivity of their skin - a nervous or generally emotionally negative person sweats," says Prof. Knapska.

Then, the volunteers went to a room with a cage containing the rats, placed their hands on it, and played with the animals. "In the control condition - without the electric shock - the rats spent most of their time near the hands of the volunteers, sniffed them, and climbed on them. However, on the last day when the same people came after the fear conditioning, the rats avoided contact with them. They ran around the cage and sniffed it. This is characteristic behavior for rats when they don't feel secure in their environment; they explore it to find the source of danger," Prof. Knapska explains.

Their disorientation was even more evident when volunteers picked them up. "They weren't as eager to sniff around; instead, they tucked their heads under the volunteer's shoulder, a sign that they were a little afraid. We saw a change in behavior between the day when the volunteers experienced vibrations and the situation when they were shocked and became nervous," comments Dr. Kaźmierowska.

The most interesting results, however, came from the study of the activity of different parts of the rat's brain in the control and experimental situations. In the latter, the amygdala was activated in exactly the same areas where it had been previously recorded in humans who watched their colleagues' discomfort.

In summary, animals did not witness the procedures that humans underwent, yet they sensed their fear. The humans did not show any signs of experiencing something unpleasant. So, what was the carrier of information about potential danger? "Probably scent. Perhaps we emit substances that are universal across different species. They may be found in sweat," says Prof. Knapska.

The results of the experiments conducted by her team have fundamental significance for the theory of the origin of empathy. "The prevailing view in science is that empathic behaviors, especially the ability to respond to the emotions of another individual, evolved from caring for offspring. A mother who can better read a child's emotions and respond to them can take better care of the child, and that offspring has a better chance of survival."

"The results of the research, both mine and those of other teams, confirm, however, that the ability to read the emotions of others may be an adaptation that evolved because it allowed individuals to avoid danger. The studies described here show that reading fear in other species may use the same biological mechanisms as reading fear in one's own species," adds the researcher.

08 November 2023