Vampire bats build friendships in the same way people do, by starting slow and deepening over time into potentially life-saving bonds, according to intriguing new research.

These highly social bats, native to Central and South America, were already known to maintain decade-long relationships, but scientists didn’t know how these connections began.

Now, a new study, published March 19 in Current Biology, shows that the world’s only known blood-sucking mammals develop trust with unrelated individuals first by grooming each other, then eventually regurgitating blood to share—an act of altruism for a species that must eat every three days. What’s more, blood-sharing tends to be reciprocal, with bats more likely to provide a meal to a partner that has shared with them in the past. (Read how vampire bats evolved to feed on blood.)

“In vampire bat relationships, we saw that the history of interactions mattered, and the social environment mattered,” says study leader Gerry Carter, a behavioral ecologist at Ohio State University.

The research supports the relatively new “raising the stakes" theory in ecology, which holds that unrelated animals “test the waters” of altruism first with low-cost behaviors, ie. cleaning each other’s fur, and then work up to more costly investments, in this case, sharing food. (Learn more about why female vampire bats in particular help each other out.)

First proposed in 1998, the theory may hold true in other social animals—including humans.

That’s why studying these social sanguivores may also reveal insights into the complexities of human friendships, he says.

Blood brothers

To test how these bonds emerge, Carter and his colleagues captured 27 common vampire bats (Desmodus rotundus)—one of three known species—from two distant sites in Panama. Active at night, the bats gallop along the ground, approaching their prey on all fours. Razor-sharp teeth slice painlessly into the victim's vein—usually cattle or another large animal—allowing them to lap up the trickling blood with their tongues. (Learn more about how vampire bats hunt.)

In the laboratory, the scientists placed the bats either in pairs, with one from each location in Panama, or in small mixed groups. They then withheld food from one of the bats and observed how it interacted with its roostmates.

After 15 months together, some patterns emerged. Many of the strangers eventually formed grooming relationships, but far fewer of the bats shared blood via regurgitation.

Grooming always came before food-sharing among strangers. And in those that did regurgitate food for others, mutual grooming ramped up before the first exchange of blood and then leveled off.

World's Weirdest: Vampire Bats Vampire bats are the only mammals on an all-blood diet—and an unsuspecting cow is the perfect prey.

What’s more, relationships between strangers were more likely to form when familiar bats weren’t around. When bats were introduced as isolated pairs, they formed relationships faster and more frequently than when they were in large groups.

Raising the stakes

The “raise the stakes” theory is simple and intuitive: You don’t want to invest a lot in helping someone who doesn’t help you back, says Tom Sherratt, a co-creator of the model and a biologist at Carleton University in Ottawa, Canada.

“You test the trustworthiness of your potential partner by making a low-level investment first and then seeing if it is reciprocated,” Sherratt says. If not, no relationship forms and it’s no big deal.

“It’s a powerful strategy because it means you can escalate into a trusting relationship, but you also don’t lose out too much if you meet an uncooperative individual.”

But it’s been difficult to demonstrate the theory in animals. Detecting such a pattern would mean introducing random strangers and monitoring what happens over a long period of time—exactly what Carter and his colleagues successfully did here, says Sherratt, who was not involved in the bat research. (Watch a rare video of vampire bats feeding on penguins.)

Tit for tat

Some people may not like to admit it, but reciprocity is a vital part of human relationships, too, Carter adds.

“Human friendships have subtle contingencies and expectations, but it’s not in anyone’s best interest to make that explicit,” he says.

Think of it this way: “What would happen if one of your friends became completely unreliable?” Carter asks.