How vampire bats make friends before sharing meals of blood

Blood is the diet of vampire bats, leaf-nosed bats native to the Americas.

(CNN)For vampire bats, regurgitating blood into a roostmate's mouth is a sign of ultimate trust.

It's a risky strategy for the creatures, who don't know if donating their food will be reciprocated. Vampire bats sustain themselves mostly on blood from animals, and if a bat is unable to feed for three days, it could starve.
"It's not just sharing in the sense of, 'I have a big chunk of food and I'm going to let you also eat from the same plate," said Gerald Carter, a behavioral ecologist and assistant professor at Ohio State University. "It's actually, 'I'm going to take food that I've already ingested and give it to you as if you were my offspring."
    "We're assuming that food donations are an adaptive trait, meaning it has some benefit for the donor in the long run," Carter continued.
      "Maybe that benefit is that you're creating a relationship that's going to feed back and bring benefits to you. But if you make that investment and you don't get any return, then you are worse off than if you had never made that investment."
      Scientists, in a new study published Thursday in the journal Current Biology, investigated just how bats form these strong relationships.
      How vampire bats initially