In 2011, scientists placed rats in transparent restrainers; their free companions learned to open the doors and even shared chocolate with them

Picture two rats. One is trapped in a transparent plastic tube and cannot get out by itself. The other rat can move freely in the cage. Would it be willing to help? According to the study by University of Chicago researchers Inbal Ben-Ami Bartal, Jean Decety, and Peggy Mason, published in the journal Science in December 2011, the answer is yes. Over repeated 12-day sessions, the free rats learned how to open the tube and free their trapped companions. Nobody taught them how and nobody rewarded them for it. They simply did it, over and over.

The free rat got upset before it did anything

It was a simple setup. A rat was introduced into a testing arena where its cage mate was confined in a restrainer, a tubular apparatus with an openable door on the outside. The free rat was able to observe and listen to its companion without being forced to do anything.


The study titled ‘Empathy and Pro-Social Behavior in Rats’ reports that the free rat showed signs of increased agitation when its companion was in the restrainer than when the restrainer was empty. Researchers call this emotional contagion: an animal picks up on another’s emotions simply by observing it. At this stage, it is not helping; it is simply catching someone else’s worry.

Learning to open the door, with no training and no teacher

To feel upset is one thing, to act on it is another. As explained by the researchers, the free rats went through the process over the course of that same 12-day span before learning how to push the restrainer open on their own without any training or reward for doing so. One of the authors, Jean Decety, described this as an early demonstration of empathy-driven prosocial behavior in animals.

The team also ran tests to eliminate simpler explanations. The free rats showed little or no interest in the plush rat placed in the restrainer. But when the trapped rat was released into a different compartment, away from the rescuer, the free rats still opened the door, disproving the idea they did it just to socialize afterwards.

Chocolate versus a trapped friend

The most talked-about part of the experiment was to see how far this urge would go. Two restrainers were placed in the same cage, with one containing the captured cagemate while the other had chocolate chips, a favourite treat for rats. The free rat could easily have consumed the whole chocolate before thinking about releasing its cagemate. The study found that rats tended to open both restrainers at roughly equal rates, sharing the chocolate afterwards rather than keeping it for themselves. As Peggy Mason, one of the experimenters, put it, the finding suggests that releasing a partner can be as important as eating chocolate.

The early findings from the same study also suggested that female rats were more likely than males to become “door openers,” but the researchers called this a preliminary observation that needed further study.

Empathy has its limits, even for rats

Rats don’t extend this kind of kindness to just anyone. The same University of Chicago team published a follow-up study in 2014 in the journal eLife, which found that free rats helped same-strain strangers as readily as cagemates, but help didn't extend to strangers of an unfamiliar strain; those were left trapped. This study notes that cohabitation with a rat of a different strain for two weeks was sufficient to alter this. Rats raised from birth by a foster strain, and never exposed to their own strain, helped the foster strain instead. It was familiarity, not shared genes, that seemed to determine who was “one of us.”

What actually drives the helping behavior

Another study titled ‘Anxiolytic Treatment Impairs Helping Behavior in Rats’ published in 2016 in the journal Frontiers in Psychology, looked at what happens when a rat’s own anxiety is dulled with medication. Before the tests, researchers gave some of the rats a mild anti-anxiety drug, midazolam. The research showed that the drugged rats helped their trapped cagemates far less often than untreated rats, but the same drugged rats still opened restrainers containing chocolate. The gap implies that the rats weren't just less likely to move about. Instead, it seemed that the distress of a cagemate stirred up their own inner sense of unease, and did much of the work behind the helping.

A small rodent lesson in kindness

That’s not to say that rats feel things the way humans do, and the scientists involved are clear on the difference. What three studies do show is that noticing the distress of another and acting on it is not unique to humans, or even to primates. It shows itself in a small rodent who has never learned a single lesson about kindness from anybody. It just felt something and acted on it.