Rat catchers may need to up their game. The distress shown by a trapped rat will encourage another rat to spring the trap and free the rodent. The finding suggests the common pest shows a level of empathic behaviour previously thought unique to primates.
Inbal Ben-Ami Bartal and Peggy Mason at the University of Chicago housed 60 rats in pairs. Two weeks later, one of each pair was placed in a plastic trap that could be opened if the other nudged the door with its snout.
The free rats showed signs of distress at their cage-mates’ predicament, says Bartal. After 12 days of practice, 77 per cent of them learned how to open the door and liberate the trapped rat. In control experiments featuring an empty trap, or a trap containing a toy mouse, just 12 per cent of rats learned how to open the door. “They weren’t interested in these restrainers at all, so they didn’t learn how to open them,” says Bartal.
The rescuers did not seem to have an ulterior motive for freeing their trapped cage-mate: they continued to do so even when the experimental set-up was changed so that the two rats would not be able to benefit from touching and interacting after the liberation. Moreover, the plaintive calls of the trapped rat were too infrequent to suggest that the free rat acted simply to get some peace and quiet.
The experiments suggest not only that rats can share in the distress of another, but that they dampen their natural anxiety in the face of that distress enough to offer assistance. “If you share somebody else’s anxiety and you are unable to down-regulate that, you will be paralysed,” says Bartal.
Friendship vs chocolate
To see how far they could push this empathy, the researchers gave the free rats a tempting alternative. “We pitted chocolate against liberating a trapped rat,” says Bartal. The free rat was confronted with a trap restraining their cage-mate and another harbouring a tasty chocolate treat. Which would they open first?
On average, the free rats were as quick to free their cage-mate as to liberate the chocolate. In a control experiment involving the chocolate trap and an empty trap, free rats opened the chocolate trap more swiftly, on average.
Most surprisingly, says Bartal, although the liberator could choose to eat all the chocolate before freeing their cage-mate, they were more likely to share. “They were very generous. It is really impressive for rats,” says Bartal. “It would be impressive for people too.”
The study is “truly ground-breaking”, says Frans de Waal at Emory University in Atlanta, Georgia, who was not involved in the work. He says it shows that empathy motivates altruism, rather than altruism being merely the result of a cost/benefit analysis.
Bartal adds that the results suggest that empathy is a basic mammalian function – one that doesn’t require the ability to plan ahead, for example.
It may have evolved from a mother’s ability to sense hunger, distress, fear and pain in her offspring. “Once these neural structures are in place, all evolution would need to do is expand on the stimuli,” says Jeffrey Mogil at McGill University in Montreal, Canada. “It would almost be inconceivable if this behaviour is not found in other mammals.”