If a new acquaintance leaned over and puked blood into your mouth, you might be less inclined to hang out with them again.
Unless, that is, you’re a common vampire bat (Desmodus rotundus).
These teacup-sized mammals, which are native to the Americas, maintain super-specific diets: They feed exclusively on the blood of other wild animals, slurping up an ounce or so of liquid at a time, and can die if they go just two or three days without food. So when a nighttime hunt goes poorly, vampire bats will rely on friends and family to share their sanguine snacks—even if they’ve already swallowed them.
During these feeding events, the bats get surprisingly cuddly, says Gerald Carter, a behavioral ecologist at Ohio State University. “They get really close, belly to belly,” he says. “Then they just look like they’re kissing.”
These ritual regurgitations are actually an important way for vampire bats to form and strengthen social bonds—ones built on a wholesome cycle of generosity and reciprocity. It’s a system, Carter says, not unlike human friendship.
Carter has built a career playing platonic matchmaker to these oft-maligned bats. In his latest study, he and his colleagues report that vampire bat relationships are remarkably enduring—strong enough to survive even the stressful transition from captivity to the wild. The team’s findings, published today in the journal Current Biology, suggest that it’s more than circumstance that brings (and keeps) these bitty blood-sippers together.
“For a long time, people assumed that there are certain characteristics limited to humans or primates,” says Christina Riehl, an evolutionary biologist studying social behavior at Princeton University who wasn’t involved in the study. “It’s now emerging that social bonds occur in everything from fish to birds to bats...it’s more general than people originally thought.”
If their name is any indication, vampire bats get a pretty bad rap. Perhaps it’s no surprise, given their unconventional diet and toothsome smile—but they’re actually one of the more social species out there, Carter says. Though males leave their mothers early on to live somewhat solitary lives, females roost together, and can often be found grooming and swapping spit (and blood) with family and nonrelatives alike.
Building and maintaining this network gives bats something of a social safety net when times get tough, Carter says. The biggest predictor of whether Bat A will share food with Bat B is if Bat B shared food with Bat A. But there’s more to it than simple quid pro quo, he says. Gifts and favors can be doled out without the expectation for immediate payback; relationships appear to strengthen and evolve over time. What happens between vampire bats, Carter says, isn’t a cold, calculated business transaction. It’s genuine companionship, warm fuzzies and all.
This new study, led by Simon Ripperger of the Museum for Natural History in Berlin, is the latest to expand researchers’ understanding of vampire bat friendship. Years of observations had shown Carter and his team that captive bats seem to cozy up to the same individuals time and time again. But it wasn’t clear if these relationships, forged in an environment where bats don’t have the luxury to pick and choose their bunkmates, were born out of affection or mere convenience.
Riehl compares it to the social dynamics of a college dorm. Some will go out of their way to hang out with people they like; others find it easiest to just pal around with the folks next door.
To test the limits of these batty bonds, Ripperger and Carter’s team put the residents of a vampire dormitory to the ultimate test: move-out day. After housing 23 wild-caught female vampire bats in an outdoor cage for 22 months, the researchers released them back into the Panamanian tree roost they’d originally been captured from.
During their stint in captivity, the bats were provided with regular meals of blood. But every month or so, the scientists would withhold food from one bat, giving them a chance to see who swooped in to help when the hungry animal rejoined the group.
After nearly two years of observations, it was clear where allegiances lay. To see if the sentiments would stick, the researchers returned the bats to their roost and set them free.
For the next eight days, the team monitored the bats’ behavior through small, ultra-sensitive sensors tethered to their backs—inventions of Ripperger’s. Every two seconds, the backpacks would log how close two tagged bats were to each other. An encounter within 20 inches was a “meeting”; less than one inch denoted “close contact.”
Not all friendships are built to last. The once-captive colony experienced its fair share of fractures in the week post-rewilding, Ripperger says. But on average, the more bats had partnered up in the lab, the more likely they were to rendezvous at the roost. That suggests that the bonds they’d built in captivity were genuine relationships—ones resilient enough to weather an abrupt change in scenery, Carter says, and not just byproducts of being in “prison” for almost two years.
For bat buffs, the finding might not be a huge surprise, Riehl says. Among researchers, vampire bats are already renowned for their reciprocity. That said, tiny, blood-guzzling vampire bats still stand out among other animals known to engage in bona fide friendships—a list that includes big-brained, charismatic megafauna like elephants, chimpanzees, and dolphins. To manage such a filial feat, she says, “you need to remember what your cooperative partners have done before, and tell individuals apart. That requires a lot of mental hardware.”
Studies like this might help fight the stigma that’s often attached to vampire bats, which can carry diseases that have the potential to hop into humans, says Marina Escalera Zamudio, an ecologist at the University of Oxford who has studied vampire bats in the context of infection, but was not involved in the study. Bats come with a bevy of benefits, too, she points out. They’re important pollinators, and nosh on insect pests like mosquitoes.
And for researchers like Carter, bats represent an untapped resource for the study of complex social structure. They’re reminiscent of primates, he says, only much easier to study in their natural environment.
Diet aside, we humans aren’t all that different from these blood-loving buddies, Riehl says. “Vampire bats have complex social behaviors, just like humans do,” she says. “Maybe thinking about these bats as having friends can make people realize that they’re not as awful as just the ‘vampire’ part would imply.”