Vampire bats' diet exclusively consists of blood, and going even three days without it can be fatal for them. Luckily, and somewhat grossly, the animals will share regurgitated blood—notably, even among bats who aren't related. In a study published Thursday in Current Biology, researchers explain that they set out to understand the process by which these non-familial relationships formed. They got there by studying two populations of vampire bats that were captured in Panama, "repeatedly fasted [the animals] to induce food-sharing," and introduced them to each other in various groupings over 15 months. They found that before sharing food, the bats would "trade favors gradually increasing in size," as the New York Times puts it, essentially slowly developing "friendships" much like humans do.
Bats would first engage in light and then heavier grooming, following by "mouth-licking"—sharing blood in what the BBC terms a "kind of horrifying French kiss." Lead author Gerald Carter of Ohio State University explains the behavior is similar to "how a lot of birds regurgitate food for their offspring," with one big exception: the bats "do this for other adults." Carter notes that sharing blood outside one bat's family doesn't confer any benefits right away. But "individuals that only have a food trading relationship with a mother, they don’t have backup partners. So we think that’s what these weaker non-kin relationships are about. It's basically an insurance policy." National Geographic notes it's a two-way street: Bats were more likely to share their blood with a bat that had already shared with them. (Some vampire bats now drink human blood.)
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