Vampire Bats Form Bonds By Regurgitating Blood Into Each Other's Mouths

Vampire bats form friendships by regurgitating blood into each other's mouths, according to a study. A team of scientists observed that when pairs or small groups of unfamiliar bats were isolated, they would start to initiate relationships, first by grooming one another, then by sharing blood with any malnourished roost mates.

In a study published in the journal Current Biology, the researchers demonstrate how behaviors such as these can help to build trust among unrelated vampire bats, forming bonds which can last a lifetime.

These long-term cooperative relationships between unrelated bats appear to be "analogous in form and function to human friendship," the authors wrote in the study.

"We go from bats starting as strangers from different colonies to groupmates that act to save each other's life," Gerald Carter an author of the study from Ohio State University, said in a statement. "This is the first animal study to look carefully at how a new cooperative relationship forms and can be maintained between complete strangers of the same species."

Vampire bats, which are native to the Americas, survive only on blood meals, using their razor-sharp teeth to bite into the veins of their victim — usually livestock like horses or cows — before lapping up the red liquid with their tongue. However, the animals ideally need to feed every night; if they don't for three days in a row, they may starve to death.

"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. If they starve three nights in a row there is a high chance they'll die," Carter said.

This is where forming bonds with other bats is useful for survival, because friends can provide a their companions with a blood meal in cases of malnourishment. How these bonds initially form has long been something of a mystery — an issue the authors wanted to investigate in the latest study.

To do this, the researchers collected female vampire bats, which unlike males are known to share regurgitated blood meals with both family and non-relatives, from two different sites in Panamá.

They then placed them in pairs, one from each location, or in small mixed groups inside cages. In each group, the team withheld food from one of the bats and observed how they interacted with the others. The scientists noticed that several bats, especially those in pairs, began grooming one another more over time, an indication of bonding behavior.

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

According to Carter, this increase in grooming is an example of the animals "raising the stakes" during the formation of social bonds. This idea, which was first proposed in 1998, has been difficult to demonstrate in animals.

vampire bats
Vampire bats in the study. Rachel Moon/Ohio State University

"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," he said. "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."

According to the researchers, as the bats raised the stakes further in the process of bonding, they became comfortable enough, in some cases, to share regurgitated blood meals with their malnourished companions.

"Food sharing in vampire bats is like how a lot of birds regurgitate food for their offspring," Carter said. "But what's special with vampire bats is they do this for other adults, eventually even with some previous strangers. The idea of using low-cost behaviors to build up to higher-cost investments can be something of much more general importance outside just food sharing in vampire bats."

The researchers say that the findings could shed light on how other social mammals like primates—including humans — form bonds with each other.