It pays to share widely, because you never know when you might need a friend to go to bat for you.
At least that’s the strategy employed by the common vampire bat, according to new research. A study published Tuesday in the journal Biology Letters found that female members of the species who had previously shared their meals with a greater number of non-relatives tended to fare better during hard times than those who invested in smaller social networks.
These findings add to a growing body of evidence that humans are far from alone in forming friendships, that is, preferential associations with non-relatives, social bonds that appear to run deeper than straightforward tit-for-tat exchanges. The bat’s strategy, which the researchers call “social bet hedging,” may play a role in shaping cooperative behavior in other species, including our own.
“Understanding how individuals make cooperative investments based on the returns in more ‘simple’ social bonds, like in food-sharing vampire bats can help us understand the foundations of more complex relationships like those among humans,” says lead author Gerald Carter, a postdoctoral researcher at the Smithsonian Tropical Research Institute in Panama.
Dr. Carter describes Desmodus rotundus, the scientific name for blood-slurping bat native to Central America and South America, as “a great organism to study for insights into cooperative relationships.” Wild females and their young will often roost in groups of eight to 12 in caves or hollow trees, which they leave each night in search of a meal, one that typically dribbles from the bites they inflict on chickens, pigs, dogs, and other animals (human blood is rarely on the menu). Because their blood-only diet contains so little fat, the bats cannot store energy for very long: Those who go two or three nights without feeding starve to death.
When a female bat fails to secure a meal for herself, as happens with about a third of juvenile bats and about 7 percent of adults each night, she will groom her roost-mates in the hopes that they will regurgitate some of their partially digested meal into her mouth. Help often comes from mothers, daughters, or other kin, but bats will also often share their food with unrelated individuals – their friends.
In research conducted in Costa Rica in the 1970s and 1980s, biologist Jerry Wilkinson observed bats refusing to feed roost-mates that had previously snubbed them, and he quantified the costs and benefits of such sharing. These insights, combined with the relative ease with which researchers can replicate the conditions that promote this behavior, have made the bats a model species for studying what biologists call reciprocal altruism, a behavior in which one organism makes a sacrifice to help another, with the expectation that the favor will be repaid.
“The most frequent assertion is that vampire bats are exhibiting tit-for-tat,” says Dr. Wilkinson, a professor at the University of Maryland and a co-author on this newest paper. “The more recent work that Gerry Carter did for his PhD under my supervision indicates that the bats are not engaging in strict tit-for-tat, but they are reciprocating over a longer time interval.”
In 2015, Carter and Wilkinson found that the bats would sometimes appear to forgive roost-mates who declined to help because they didn’t have enough food to share, and bats who had previously been unable to help would be especially generous later on, almost as though they were compensating for past stinginess.
In other words, each bat appears to navigate a complex social environment, one where she must to keep track of who snubbed her – and for what reason – while also working to repair relationships that have been strained.
This most recent finding, based on a four-year study of about 30 captive bats, builds on this research. Some bats feed more non-relatives than others, and those who cultivated weaker ties with a larger number of friends would usually be fed as often as those who forged stronger bonds with a few friends.
But when the researchers separated a bat from her primary donor, typically her mother or daughter, the benefits of having a bigger social network became apparent. Bats who had invested in quantity instead of quality had an easier time finding donors. Their friends helped them cope with the loss.
“When I very first plotted the data, I was overwhelmed with surprise and joy that it looked exactly how I thought it should,” says Carter. “That is a very rare thing in science.”
“I was surprised to see such clear evidence for the value of having backup partners. We’ve not seen that before,” says Joan Silk, an anthropologist at Arizona State University who has studied social relationships among baboons.
“The great contribution of this paper is that it provides evidence about a completely new way in which having relationships matter,” Dr. Silk says. “It makes sense that if relationships are important for individuals, then strategies to deal with the loss of partners may also be very important.”
Like female vampire bats and humans, female baboons are known to form close ties with non-relatives, and they tend to spend more time grooming more partners following the death of a female relative. Humans tend to report greater happiness from having a small number of close friends as opposed to a larger network of weaker ties, but in environments where friends are likely to leave, quantity matters more than quality.
Among humans, baboons, and bats, these strategies likely operate outside of conscious awareness: pursuing and maintaining friendships does not require any deliberate calculation of the costs and benefits. “In many situations, our first impulse is often cooperative,” says Carter. “We feel emotionally compelled to help others,” he says, “because natural selection has done the calculations for us.”
“You don’t ask your friends to exchange 25 minutes of emotional support for two dinners at your house,” he says. “That’s not how friendships work at all.”