For example, graduate student James Burkett found that the observing vole would match the behavior of its stressed partner, even though it hadn’t itself experienced any stress. It would groom itself repeatedly. It froze whenever it heard a tone that heralded an incoming shock. And it even developed higher levels of stress hormones.

Given that, you could argue that grooming and licking is just a simple way of alleviating stress, rather than a sign of anything empathetic. But that can’t be right, because the shocked vole—the one that had actually been stressed—didn't groom its partner any more than usual. It was the observer that pulled out the stops.

Skeptics might also argue that the stressed voles were simply releasing pheromones that triggered caring behavior in the observers. That can’t be right either, because the voles only consoled partners and cagemates, but not strangers. Like us, they show biases in their empathy, directing it more towards familiar individuals.

Also, meadow voles, a closely related species that mates promiscuously and doesn’t form lasting bonds, don’t show any signs of consolation. Only the monogamous prairie voles do, and even then only to their own partners. “It seems like a very specific adaptation to social living,” says Young.

Indeed, he found that the prairie voles would console each other the very first time they were tested. That’s crucial; in studies like these, animals often have the chance to learn a behavior or pick it up through conditioning. Not so here: The voles reacted automatically. “It’s very clean,” says Peggy Mason from the University of Chicago, who led the earlier study of rescuing rats. “Somehow, the vole is effectively communicating its stressed affect to another vole very effectively. That’s the umbrella definition of empathy.”

But Young carefully notes that “we’re not saying that these animals experience empathy in the same way as us.” He breaks it up into two kinds. There's cognitive empathy, where individuals put themselves in the shoes of another individual—and there’s no evidence that the voles are doing that. Then, there’s emotional empathy, which Young describes as a “more of a gut, instinctual feeling that we’re not cognizant of.” That’s what he sees in the voles.

Daniel Povinelli from the University of Louisiana at Lafayette is unimpressed with that distinction. “You can call it empathy if you want,” he says, “but if empathy does not mean reasoning about the emotional life of another, it doesn’t have a whole lot of force."

Alex Kacelnik from the University of Oxford, who studies animal cognition, is more enthusiastic. He was skeptical about the rat-rescue study, which he feels didn’t prove that rodents show empathy and overinterpreted the data. “This is not the case here,” he says. “This shows that the study of emotional states in other species is possible and useful.”