(CNN)For vampire bats, regurgitating blood into a roostmate's mouth is a sign of ultimate trust.
How vampire bats make friends before sharing meals of blood
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 form social bonds has been unclear till now, as these developments are difficult to study, Carter said, who devised a way to watch how bat's behavior changes as they go from a first encounter to a cooperative relationship.
They found that vampire bats in Panama began grooming each other, a low-cost behavior, before developing a higher-cost, food-sharing relationship.
This "raising the stakes" behavior" -- the idea of testing the waters to build cooperative relationships -- is common in the development of long-term relationships among humans and other animals, he added.
To observe how new food-sharing relationships form between adult vampire bats, the researchers captured female bats from two distant sites in Panama and divided them into different sized groups.
In each group, the researchers alternated which bats weren't given any food and watched how it interacted with its mates. Several bats, especially those in pairs, began testing the waters by grooming each other more over time, which led to sharing blood, or food, with their hungry companions.
"Grooming each other is helpful," said Merlin Tuttle, an ecologist, conservationist and wildlife photographer specializing in bat ecology and conservation, who also founded Merlin Tuttle's Bat Conservation. Tuttle was not involved in the study.
"They may be ridding them of parasites or helping them stay clean and that is certainly beneficial and not very costly to the donor," he said.
The ability of vampire bats to go from strangers to feeding each other could create years-long bonds that help them to survive, Tuttle said.