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Vampire bats establish friendships by sharing regurgitated blood with their neighbours in a "kind of horrifying French kiss", a new study says.

Researchers observing the mammals said their sharing behaviours appeared to be an important aspect of their bonding.

If bats go three days without eating, they can die of starvation, so sharing the blood can be a life-saving act.

The study, published in the journal Current Biology, aimed to determine how the species developed relationships.

It found that when the vampire bats became isolated in a roost, pairs unfamiliar with one another - but in close proximity - would begin grooming, then "mouth-licking" before swapping food.

"We go from bats starting as strangers from different colonies to groupmates that act to save each other's life," said Prof Gerald Carter, author of the study and behavioural ecologist at Ohio State University.

"They have this 'boom and bust' foraging experience, so they either hit it big and get a large blood meal or they're starved for that night.

". But what's special with vampire bats is they do this for other adults," Prof Carter said.

He added that the bats would groom even after their fur had been cleansed, suggesting that the behaviour was not just an issue of maintaining hygiene.

Vampire bats are the only mammals to feed entirely on blood, which they get by biting larger animals such as cattle.

The flying creatures can drink up to half their weight in blood a day, unlike their other bat relatives, which generally dine on fruit, nectar or insects.

In November, a scientific study discovered that bats that form bonds while in captivity often continue their relationships when released back into the wild.