Rats may spread disease (or maybe not) and make unwelcome houseguests, but you have to give them credit for being a lot nicer to members of their own species than we are. A recent study has found just how much one Rattus norvegicus will sacrifice to save another from suffering.

One study that was testing how helpful rats will be found that most will apply pressure to a door to open it if there is a trapped cage-mate inside. In fact, if presented with a trapped fellow rat and some chocolate in a similar device, half the rats will free their friend first and share the chocolate, rather than eating it all themselves. Awww.

Not all rats are saints, however. In one study, 100% of female rats freed their species-mate, but 30% of males wouldn't lend a helping paw.

Now, a new study has tested their altrusim by putting rats into a box split by a transparent partition. On one side, a rat was forced to swim in a pool of water. A ledge was provided to remove the danger of drowning, but they clearly didn't like the experience. There was no way for the wet rat to free itself, but a dry rat on the other side of the partition could come to its aid by pushing open a small door.

Nobuya Sato of Kwansei University, Japan, reveals in Animal Cognition that the dry rats “quickly learned” to open the door. Sometimes, however, they needed a spur to their altruism. “Rats that had previously experienced a soaking were quicker to learn how to help a cagemate than those that had never been soaked,” the paper reports.

After the previous study, doubters suggested the helpful rats freed the trapped ones because they wanted companionship, rather than through a sense of altruism. Sato tested for this by having both rats on dry land. The rats were far less likely to open the door, indicating their actions were inspired by efforts to help a distressed member of their species, rather than a search for a playmate. Notably the rats were from separate litters, ruling out the possibility that their motivation was based around a desire to help those with similar genes. However, the rats had been housed together for at least two weeks prior to the trial.

Finally, Sato made it hard. Rats were given the choice of saving their wet colleague, or opening a door to receive chocolate. Most chose friendship, although the proportion was influenced by the process through which they learned to open the door.

Rats are extremely social animals, to the extent that studies on addictive behavior are now called into question as possibly representing the psychological effects of being cut off from their fellows. Yet, the idea that they could engage in prosocial behavior towards those they are not closely related to, or feel a form of empathy, has taken many scientists by surprise, with claims made as recently as two years ago that this sort of reaction was limited to humans or our nearest relatives.

H/T: Science.