Two prairie voles, life-long mates, sit in separate cages. One hears a distinctive tone, and receives a small electric shock. When the voles are reunited, the second vole quickly starts licking and grooming its stressed mate, continuing for around 10 minutes. It looks a lot like what we’d call consolation.

Larry Young from Emory University, who studies prairie voles, has seen this behavior again and again. To him, it's a sign that the rodents are showing empathy.

Such claims have proven controversial in the past. For example, in 2012, scientists at the University of Chicago showed that rats will free trapped cage-mates, even if they have to sacrifice a bit of chocolate to do so. The researchers billed these rescues as evidence of empathy—that “rats free their cagemate in order to end distress.” But others argued that the rescuers could simply have been behaving selfishly, in an attempt to get social contact.

The challenge with these kinds of studies is that they’re looking at subjective states. You can’t measure empathy, only its external manifestations. That’s what Young’s team tried to do, as thoroughly as possible. “We all worked together to figure out what empathy in voles would look like,” he says.

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.”

Besides humans, several animals have been seen consoling each other during times of distress, including great apes, crows and their relatives, elephants, and wolves (although not macaque monkeys). These are all relatively intelligent creatures, and some have argued that smarts are necessary for consolation behavior. The prairie voles suggest otherwise; that they also console each other is “a very beautiful demonstration that we’re not talking about something that depends on fancy primate processing,” says Mason.

Young’s team showed that the consoling behavior depends on a hormone called oxytocin, acting on a particular part of the brain called the anterior cingulate cortex (ACC). If they injected this region with a drug that blocks oxytocin, the voles no longer consoled each other.

Oxytocin has been linked to empathy in humans, pair-bonds in prairie voles, and maternal care in sheep. It’s not a “love hormone” though; it's a general social spotlight that draws an animal’s attention to social cues. For example, it makes mouse mothers focus on the distress cries of lost pups. Young thinks that prairie voles have simply redirected this maternal instinct towards other adults, by deploying oxytocin to a different part of the brain—the ACC.

“That’s what evolution does: it takes circuits and molecules that are already there and tweaks them a little,” says Young. “It takes the behaviors that already exist, and makes them emerge under different contexts. You can get a lot of species-specific behaviors that way.” (Hence why meadow voles don’t console but prairie voles do.)

“It's really credible to me that this behavior could be shared across rodents and primates due to a deep heritage of these mechanisms,” says Steve Phelps from the University of Texas at Austin, who also studies prairie voles. “The basic phenomenon may well be very ancient.”