Are friendships formed with those we truly like? Or do we settle for whoever happens to be around?

This question is hard to answer in humans, and even harder in other animals. But a new study of vampire bats suggests that bat “friendships” go beyond mere convenience. Many social bonds built between captive bats persist when the bats are released into the wild, researchers report October 31 in Current Biology.

“This study convincingly shows that vampire bats can form stable bonds,” says Joan Silk, an anthropologist at Arizona State University in Tempe, who wasn’t involved in the study. While she cautions against assuming that other animals’ friendships are anything like our own, she says that this study adds to a growing body of research that critters can form friendshiplike bonds.

As their name suggests, vampire bats (Desmodus rotundus) drink nothing but the blood of other animals. If the bats are lucky, they’ll get a tablespoon each night. “It’s pretty difficult for bats to extract blood from an animal, so they often go without a meal,” says Gerald Carter, an evolutionary biologist at Ohio State University in Columbus. Three straight nights without a meal, though, and the bats can die.

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“But there’s a social safety net,” Carter says. “Other bats will regurgitate portions of their blood meal to feed bats who didn’t get their own meal.” Previous lab work has revealed that bats that aren’t related to one another can form long-term cooperative bonds that loosely resemble friendships (SN: 11/19/15).

The nature of these relationships, however, is a topic of debate. Do bats — or other animals — pair up with individuals they prefer? Or are those bonds transient, transactional relations between two individuals seeking the best deal they can get on any given day?

Carter and his colleagues devised a way of distinguishing between these two extremes in vampire bats. If social bonds are like friendships, those bonds should persist across radically different contexts — captivity and the wild. But if bats form bonds strictly out of convenience, then friendships forged in captivity should dissolve in the wild.

The researchers took 23 vampire bats from a colony of about 200 in a tree hollow in western Panama and brought them to a lab at the Smithsonian Tropical Research Institute in the central Panamanian city of Gamboa. As would happen in nature, only some bats were fed blood meals on certain nights. Then, as some bats stepped up to feed their hungry friends, researchers could witness the formation of social bonds. Over time, certain bonds grew stronger, as measured by time spent grooming each other.

“There were definitely bats that liked each other more or less,” Carter says.

Researchers fitted vampire bats with tiny proximity sensors, each lighter than 2 grams, to track social networks in a colony. Sherri ad Brock Fenton

After 22 months, the researchers drove five hours back to the tree hollow and released the bats. Each bat was fitted with a tiny computer sensor, glued to the animal’s back, that measured its proximity to other sensors — giving the researchers an unprecedented look at behavior inside the colony. The team caught an additional 27 bats and fitted them with sensors too, and then tracked which bats spent more time together over the next eight days.

It turned out that bat friends stayed bat friends.

Captive bats spent more time hanging out with other captive bats, while control bats showed no such association. “Most importantly, the bats who had stronger bonds in captivity had stronger bonds in the wild,” Carter says, though he notes the bats were tracked for only eight days. The researchers aren’t sure if social bonds endured beyond that.

Persistent social bonds have been seen before in other animals, including baboons, crows and whales (SN: 7/1/10). But this study shows these bonds aren’t just circumstantial, says Damien Farine, a biologist at the Max Planck Institute of Animal Behavior in Konstanz, Germany, who was not involved in the research. “There’s a benefit to maintaining them, or a cost to breaking them,” he says.

Carter says that benefit could come down to trust. It may make sense for a bat to stick with a partner it knows, rather than find a new one that may not reciprocate with as much grooming or food.

To study how social bonds between vampire bats persist across radically different contexts, researchers collected 23 wild bats from western Panama and brought them back to the lab. The researchers measured the strength of social bonds in the lab over 22 months and then released the captive bats into the wild, each fitted with a special proximity sensor. The team found that bonds formed in captivity were maintained, at least for a short period of time, in the wild.

The researchers also looked at bonds between mother and offspring, which surprisingly did not persist for bats born in captivity in the study. Carter says that in the wild, the mother-offspring relationship is even stronger than other cooperative bonds. While the six bats born in captivity bonded normally with their mothers in the lab, they all left the site before the study ended. “We aren’t sure why this happened,” Carter says. The bats could have flown back to where they were born, a behavior common in vampire bats, or they could’ve been forced out by other wild bats in the colony.

Carter says the study shows that animal sociality is complicated. Animals can form durable, social bonds. But those bonds may be broken when circumstances change — perhaps not so differently from human relationships.