Bloody great mates

Every human has a hatful of stories about the ways they formed friendships over the years – with university friends, house-share mates, lovers and co-workers. 

But until now, scientists haven’t known how friendly connections form among strangers in nature. That’s all changed thanks to a fascinating new study of vampire bats, from the subfamily Desmodontinae, living in captivity.

For vampires, sharing blood with a roost-mate is the mark of a true bond. These relationships aren’t common, but when they occur they demonstrate behaviour akin to what some might call friendship. 

The research – published in the journal Current Biology – shows how social grooming and food sharing can build up trust among unrelated vampire bats to form life-saving bonds that can last a lifetime.

“We go from bats starting as strangers from different colonies to groupmates that act to save each other’s life,” says behavioural ecologist Gerald Carter from Ohio State University, in the US.

“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 feed only on blood. If they are unable to feed for three days they run the risk of starving. 

“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,” says Carter.

“If they starve three nights in a row there is a high chance they’ll die.” 

Because of this, vampire bats with close social ties can rescue their weakened partners from the brink.

To test how these bonds emerge, Carter and his colleagues at the Smithsonian Tropical Research Institute collected bats from Las Pavas and Tolé, Panamá, two geographically distant sites. 

The flying mammals were then placed either in pairs – one from each location – or in small mixed cohorts.

For each group, the research team withheld food from one of the bats and observed how it interacted with others in its cage. Several bats, particularly those in the pairs, began grooming one another more over time. 

In some cases, this eventually led to sharing blood with hungry companions.

“Even if you remove all ectoparasites from their fur, they still groom each other more than necessary for just hygiene,” says Carter.

“We think of social grooming as a kind of a currency – a way to gain tolerance and bond with another individual.”

Carter suggests the increased grooming over time demonstrates “raising the stakes” of friendship or “testing the waters” to build cooperative relationships, a notion that was first proposed in 1998, in a paper in the journal Nature, but has been difficult to demonstrate in animals. 

“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,” Carter says.

“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.”

If vampire pairs raise the stakes enough, one might provide food by regurgitating blood into the mouth of a hungry partner – an action that visually resembles a French kiss, of sorts.

“Food sharing in vampire bats is like how a lot of birds regurgitate food for their offspring,” says Carter.

“But what’s special with vampire bats is they do this for other adults, eventually even with some previous strangers.”

These social interactions could be a fundamental step in creating bonds that last years in vampire bats – some unrelated pairs have been found to travel together for more than a decade.

“The idea of using low-cost behaviours to build up to higher-cost investments can be something of much more general importance outside just food sharing in vampire bats,” says Carter. 

The idea could be applied to other social animals such as primates and could provide insight into how humans build relationships.

Research has shown that people don’t like to think of their friendships as strategic, but Carter says his recent findings suggest that human relationships might be more conditional than we want to admit.

“I don’t think you can understand human relationships that well by just asking people to reflect on them,” he says. 

“It may often be subconscious, but I think both human and nonhuman relationships have a lot of little conflicts that are negotiated and navigated in a subtle way.”

Carter now wants to better understand how bats assess and choose their partners. 

“When two bats are unfamiliar, we have the opportunity to make a good partner or a bad partner and really see how that affects how the relationships form.”