Put an adult female common vampire bat (Desmodus rotundus) in a cage with two bats she has never met, seal the three of them in together for seven days, then release all three into a shared space packed with more than twenty other bats. Nine weeks later, she is still grooming those same two strangers more than anyone else in the room. That is the finding of a study run by Ohio State University researchers Imran Razik and Bridget Brown, with senior author Gerald Carter, published in Biology Letters in 2022 under the title “Forced proximity promotes the formation of enduring cooperative relationships in vampire bats.”
The Fact That Doesn’t Fit What We Assume About Friendship
The team worked with 21 wild-caught adult female common vampire bats, a small, blood-feeding bat weighing about 0.9 to 1.4 ounces (25 to 40 grams), pulled seven apiece from three roosts in Panama that sat 75 to 217 miles (120 to 350 km) apart. For six weeks, all 21 bats mixed freely in an outdoor flight cage at the Smithsonian Tropical Research Institute’s field station in Gamboa, Panama, choosing their own company. Then the researchers split them into seven trios, one bat from each of the three roosts per group, and locked each trio together for exactly seven days. After that, everyone went back to living together in the open cage, free to groom, feed, or ignore whoever they wanted. The researchers kept watching for nine more weeks. The bats that had been forced into a trio kept grooming their two assigned trio-mates specifically, far more than they groomed any of the other twenty-plus bats around them, and the bond outlasted the week that built it by more than nine times as long.
How Researchers Told a Roommate From a Stranger
Three infrared cameras ran six hours a day, and over 114 days the team logged 682 hours of footage, counting only grooming bouts of five seconds or longer, long enough to count as a real interaction and not a passing brush. They then split every possible pair of bats into three categories: “test pairs,” the bats forced into a trio together; “control pairs,” bats that had never lived together at all; and “familiar pairs,” bats pulled from the same original roost who had spent six weeks mingling freely and voluntarily before the experiment even began. Only the test pairs showed a grooming rate that stayed elevated across the full nine weeks. The control pairs never bonded, which is the expected result. The more telling comparison is the familiar pairs: bats that had known each other, shared a roost, and chosen to interact for six unforced weeks groomed each other no more than strangers did. The forced trio-mates, who had never chosen each other at all, out-bonded them.
Why Proximity Alone Was Enough
Carter’s reading of the result is blunt. “What this experiment tells us is there is a causal relationship between being forced into the same space and actually having a preference for each other later,” he said. Bats, it turns out, do not seem to run a compatibility check before they bond. Seven days of enforced closeness produced a preference that looked, on the grooming logs, exactly like the bonds built on kinship or months of free choice. Carter compares it to human college roommates: assigned at random by a housing office, with no say in who they end up living with, and yet plenty of them stay close for years afterward. Vampire bats are known to share regurgitated blood meals with roost-mates, a behavior that depends on having a partner willing to help when a night’s hunt comes up empty. Seen against that backdrop, a bond that can be manufactured by circumstance rather than earned through history matters more than it might for a less socially dependent animal.
The Myth This Kills
The standing assumption about animal social bonds is that they need a foundation: relatedness, a track record of voluntary interaction, some prior sign of compatibility. This experiment argues against that. Bats from the same roost, with six weeks of free contact and every reason to bond on their own terms, groomed each other at roughly the same rate as total strangers. Bats trapped together for a single week, who never chose one another and shared no history at all, kept preferring each other for more than two months. Proximity alone outperformed months of freedom.
What Nobody Knows Yet
Carter is explicit that this does not explain how bonds form inside a wild vampire bat colony generally: the experiment shows forced proximity can build a lasting preference, not that this is how bonds usually form outside a locked cage. Whether closeness causes cooperation or cooperation causes closeness is itself an open question in the field, since bats that already help each other also tend to seek each other out, and the two directions are hard to separate just by watching a colony. And, in Carter’s own words, “we don’t know to what extent this is happening in other animals.” Nobody has run the trio experiment on elephants, wolves, or parrots yet.