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