Scientists manipulated bats' social relationships — and watched them go from strangers to friends

On 29th August 2019, Lilith, a vampire bat housed in the Smithsonian Tropical Research Institute in Panama, died, leaving behind a pup. Curiously, for Gerry Carter and Imran Razik — the scientists studying these bats — BD, another bat in the colony, whom Lilith had met only two months ago, adopted her pup. In that two-month span, BD and Lilith had evidently formed a close bond. While this is not the first instance of adoption exhibited by vampire bats (or in the animal kingdom), it does raise a peculiar question: if natural selection is about maximizing one's genetic legacy, what does a bat get out of raising the children of an unrelated bats?

Vampire bats are highly social animals, for whom "blood relative" takes on a whole new meaning. Roosting in groups, they spend a large portion of their waking time grooming each other, and even regurgitating food — i.e., blood — for the benefit of a few special friends. Gerry Carter, an assistant professor at Ohio State University, is trying to unravel the mysteries of vampire bat friendships.  Carter's findings get us closer to understanding how and why cooperative friendships form in social animals, including humans.

Carter's journey with bats began with a fragment of a memory. 

"It had a rubber band on its mouth," he said, describing the fruit bat he saw at age two on a trip with his mother to her native Philippines. "My mother is from an indigenous tribe in the Philippines, and she used to hunt flying foxes when she was a kid. Her family ate them." Carter explained. This memory re-emerged when, in elementary school, he saw the same bat in an encyclopedia. "From then on, I was obsessed with bats," Carter mused. After twenty years of studying vampire bats, Carter continues to be fascinated by these animals.

Common vampire bats – the species Carter works with — live in colonies of 8-12 in a tree or a cave. Typically, Carter says, the female bats stick around the colonies, while males leave within the first year. This leads to an interesting social structure. "

"You get these female social networks that form," Carter says, "and males fight for territory and access to females.  Even the most dominant male doesn't monopolize paternity." Apart from the maternal lines of mother-daughters-granddaughters, members of a colony are mostly unrelated to each other. 

Sometimes, an "unrelated" female may join the colony and start forming her social network. This would be an ideal situation for Carter to study the formation of new relationships forming in the wild but that would be hard, he says, "you would have to be there the moment a new individual joins a group."

So, they brought common vampire bats into a flight cage on the edge of forest at the Smithsonian Tropical Research Institute, where Gerry's student Imran Razik has spent most of his PhD manipulating the social milieu of bats. Their captive colony usually has 20 to 40 bats, all of whom have names. Thanks to Razik's manipulations, Lilith and BD met; began grooming each other; and, soon enough, were throwing up bits of food for one another.

Lilith and BD weren't an anomaly. By observing hundreds of manipulated interactions, Razik and Carter learned that vampire bats go from being strangers to friends by mutual grooming. In rare instances, this turns into a food-sharing relationship. Every bat has a set of food sharing partners and a set of grooming partners.

In another set of experiments, Razik forced bats from three different sites in Panama to spend time together in a small cage for 114 days. Not only did these bats begin grooming each other, they continued to do so after Razik released them back into the larger cage where they had the option of spending time with more familiar bats. Considering bats can't go more than two days without food, having  food-sharing partners has an obvious advantage. So why bother with grooming partners at all? 

To explain this, Carter analogizes their situation to human relationships.  

"If you're a person living in a small town where everybody is born and dies [there], you may want to have strong relationships at the expense of diversifying your network. But if you were in a situation like in a college or an academic conference, where you have no idea which individuals are going to stick around, you may want to diversify. If you have only two relationships and those individuals die or leave, you are left with nobody."

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Bats in a colony have different kinds of networks that comprise a mix of relatives and non-relatives, food-sharing partners and grooming partners.  When Carter's group took away the key food donors of some of their captive bats, those who had previously fed non-relatives did much better because they had a larger support network to which they could turn. According to Carter's hypothesis, friendships with non-relatives act as an insurance against tough times (or, in this case, insurance against manipulative scientists). He calls it the social bet-hedging hypothesis, and it may explain why natural selection has favored relationships that don't directly maximize an individual's genetic legacy, i.e., offspring. In the extreme case of BD and Lilith, the latter directly benefited from having a non-relative as a food-sharing partner. 

While the social bet-hedging hypothesis is intriguing, it doesn't necessarily explain how a bat goes about forming its own social network or even why BD adopted Lilith's pup. Carter, who has spent hundreds of hours watching bats and several more poring over data collected by his students, is determined to get to the bottom of these mysteries.

In the future, he hopes to understand how bats ensure cooperation and suppress conflict. "I think that's interesting to think about both in vampire bats and human relationships," Carter says.  "We have amazing skills for social networking, manipulation and cooperation. I don't think we're doing most of that in a conscious way."  

Loaded as we are with political baggage, we struggle to understand our own, human propensity to cooperate and compete. Studying highly cooperative winged mammals could provide insight into our own social dynamics.