A hungry vampire bat can borrow a blood meal from a friend, and is expected to repay it
Vampire bats can starve within about three days without a blood meal, so a bat that comes home empty will often beg a mouthful from a roost-mate, who regurgitates blood to share it. Researchers found these relationships build gradually, starting with grooming, and are strongly reciprocal: bats preferentially feed those who have fed them before, and some stingy bats start giving more after being cut off, apparently to repair the relationship.