Empathy is the ability to feel others' pain or distress; we feel bad when someone else feels bad. It’s what motivates us to give a few dollars to the homeless man on the corner, donate our time to a worthy cause, or hug a friend who has just been dumped. Scientists used to believe that empathy was unique to humans and was one of the traits that actually distinguished us from other species. Recently, however, there is increasing evidence for empathy in several species, most notably other primates.

Now, new research in Science suggests that rats are capable of empathy. The study tested how rats responded when their fellow rats were trapped, and found that they would not only spend time and energy deliberately helping their trapped companions, but they would even share food after liberating them.

The experiments were conducted in a square arena with a small cage at its center. In some trials, the center cage had a rat trapped inside, and in some trials it was empty. Another rat was released into the arena, and it could decide whether or not to free the trapped rat by rotating the door on its cage. This situation was repeated for 12 days, and the researchers noted not only whether or not the free rat opened the cage, but also its activity patterns during the entire trial.

The free rats were noticeably agitated when there was a rat trapped in the cage, and tended to circle the center of the arena, dig at the cage, and call to the trapped rat; when the cage was empty, these behaviors were absent. Over the course of the experiment, the rats learned to open the cage, and became much faster at doing so. A much higher percentage of the rats that were inside the arena with a trapped rat opened the cage (23 out of 30), compared to the rats that were in the arena with an empty cage (5 out of 40). Clearly, the rats were much more motivated to open the cage when there was a companion stuck inside.

Interestingly, female rats were more likely to open the cage with a trapped rat than males were; during the 12 trials, 100 percent of the females became door-openers, while just over 70 percent of the males did. The females were also quicker to open the door than males were, and were more active than males before liberating the other rat. While there certainly are other explanations, it’s possible that in rats, as in humans, females tend to be more empathetic than males.

The researchers then added another closed cage into the arena; this one was filled with chocolate chips. The rats were just as likely to open the cage with the rat as they were to open the cage with the chocolate, suggesting that the motivation to free a trapped companion is about as strong as the motivation to eat the chocolate (for those of you who don’t know, rats really like chocolate). Additionally, in more than half of the trials, the free rats shared the chocolate with the trapped rat after freeing it. The free rats actually ate fewer chocolate chips when there was another rat in the arena than they did when they were alone, indicating a willingness to share the bounty.

It’s important to note that in these trials, we don’t know whether the rats were trying to alleviate the trapped rats' distress, or to make themselves feel better about the situation. A comparison to human behavior provides a similar question: when we give to charity, are we doing so to actually help those less fortunate than us, or to eliminate the negative feelings we have when we think about the plights of others? While this is an important distinction to make, both motivations still fall under the definition of empathy. In either case, the rats were negatively affected by the trapped rats’ situation, and were acting deliberately to eliminate the source of distress.

There is much more work to be done on this topic, and this type of research always raises just as many questions as it answers. But well-designed studies like these contribute greatly to what we know about empathy in other animals. The more we learn, the more we understand that our ability to respond emotionally to others' distress doesn’t separate us from other species; it reminds us how similar we are to them.

Science, 2011. DOI: 10.1126/science.1210789 (About DOIs).