Vampire Bats "French Kiss" With Mouthfuls Of Blood To Develop Social Bonds

Common vampire bat (Desmodus rotundus) hanging out in a zoo. belizar/Shutterstock

Are you a hot, single bat looking for a buddy? Well, we suggest you French kiss your neighbors with a mouthful of regurgitated blood to make friends and see where it takes you. 

New research has shown that vampire bats form social bonds by sharing freshly drained blood with unfamiliar members of their roost. It might sound desperately gross, but this behavior is showing scientists that vampire bats are incredibly prosocial animals.

"Food sharing in vampire bats is like how a lot of birds regurgitate food for their offspring. But what's special with vampire bats is they do this for other adults, eventually even with some previous strangers," Gerald Carter, lead author of the new study and assistant professor of evolution, ecology, and organismal biology at Ohio State University, said in a statement.

Reporting their findings in the journal Current Biology, the researchers closely studied a group of vampire bats (Desmodus rotundus) made up of two different roosts from geographically separate locations. This meant many of the bats were unfamiliar with each other, heightening the need for some blood-sharing bonding. 

Over the course of 15 months, the researchers documented how unfamiliar pairs would start to groom one another. After getting a little more comfortable with each other, they would eventually move onto exchanging blood meals via the mouth. In fact, almost 15 percent of the bats were seen indulging in this behavior with a previously unfamiliar partner.

Vampire bats need to feast on blood every three days or they run the risk of starvation, so the food-exchanging can also be a life-saving activity. The same goes for mutual fur grooming, which helps to remove nasty parasites from the bats' skin and slows the spread of disease. However, even beyond practical concerns, the researchers believe this behavior has a much deeper purpose.

"Even if you remove all ectoparasites from their fur, they still groom each other more than necessary for just hygiene," adds Carter. "We think of social grooming as a kind of currency – a way to gain tolerance and bond with another individual."

It perhaps even demonstrates a complex form of social interaction involving cooperative investment. First described in the journal Nature back in 1998, this describes behavior where individuals develop a cooperative relationship through a “you scratch my back I scratch yours” system. Crucially, the relationship only develops if individuals continue to “up the stakes” of their investment with each other. 

“When you make a cooperative investment in another individual, there is a kind of risk, because if you have a bad partner, you can be even worse off than if you had just avoided them altogether," explains Carter. "So, what you could do is invest a little bit to test the waters. Then, if they invest back in you, that's a signal to ramp up your investment, and so on."

Interestingly, Carter suggests this strategic friendship technique may apply subconsciously to humans too, in that human relationships may be more conditional than we'd like to admit. 

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