The Unbreakable Bond of Vomiting Blood into Your Friends' Mouths

There are few things worse than a friend vomiting blood into your mouth. That's a rough night by anyone's standards—unless you're a female vampire bat who hasn't lapped up a cow vein in twenty-four hours. A new study says that female vampire bats regurgitate blood to feed hungry friends, revealing a more complex social structure of female animal behavior that extends past caring for their families.

It doesn't take an expert to understand why that bat is looking for a blood burp: If a vampire bat doesn't eat for two days, it will die. Most vampire bats are known for flying, hopping, and running around herds of cattle for a taste of that sweet hemoglobin. White-winged vampire bats are particularly sneaky and cuddle up to mother hens like a baby chick until they're close enough to slurp.

When the bat can't find a blood source, they hungrily lick the mouths of well-fed bats. It's this well-fed bat that's more interesting from a scientific perspective. When another bat licks your mouth, you have a choice: give up your snacks to help a thirsty pal or ignore the tongue.

Why a well-fed vampire bat give up her blood? That's the question Gerald Carter, a researcher at the Smithsonian Tropical Research Institute in Panama, set out to discover when he fasted some female vampire bats and then exposed them to some of their recently-fed female peers.

The majority of animal interactions are selfish. It doesn't suit evolutionary biology to share, especially with non-relatives. Wolves drag meat to the rest of their pack, and bonobos, one of the closest relatives to humans, share fruit to make new friends, but that's largely the extent of animals giving away food to friends. This makes the vampire bat's altruism rare; they're one of the few animals that will figuratively lend a cup of sugar to their neighbor.

"Social behavior can be very complex, meaning that many different factors can play a role in who we decide to help and how much," said Carter via email. "It is easy to just say, 'animals help each other because they are related,' or 'they are in the same group' or 'animals follow a rule like, I'll scratch your back if you scratch mine.'"

There are multiple factors at play when a vampire bat decides whether she should puke in her friend's mouth. She's simultaneously evaluating supply and demand, the social value of her hungry friend and the relative cost to herself versus the benefit to another bat. "Just because a problem seems complex to us—in this case how to cooperate the optimal amount without being exploited—doesn't mean that animals can't solve it. The brains of social animals (including humans) might be integrating way more kinds of social information than we realize," said Carter.

The study discovered that female vampire bats won't reward selfish behavior—a bat is more likely to feed another bat that shared with the group previously. Similarly, cheaters don't get blood. If a bat got the vomit treatment but didn't give it later, she can lick her fellow bats' mouths until the cows come home and no one will puke for her.

Furthermore, bats keep tabs on which bats they feed. When they can't feed a friend—because hey, you can't feed everyone all the time—they give that friend even more blood the next time. In their report on the study, National Geographic explains, "female vampire bats constantly keep track of whom they can turn to in a time of need—and actively work to repair relationships that have gone off-track."

Gerald Wilkinson, the study's co-author and Carter's doctoral advisor, explained via email that research on vampire bat social behavior focuses on females because "female vampire bats form long-term relationships, whereas males simply fight to gain access to female groups for mating. In the wild, males never share food with each other and never live to be older than 8 years. Females, on the other hand, often feed each other and live to be at least eighteen years of age in the wild."

It's been well documented that, when it comes to same-sex friendship, the relationships between females—both human and animal—are different. A UCLA study revealed that stress in females causes a "tend and befriend" reaction: When females are stressed, they tend to relationships and create new ones. The brain chemicals responsible for this behavior are enhanced by estrogen but diminished by testosterone. Men, conversely, tend to experience a "fight or flight" reaction during times of stress.

Does the social complexity of reciprocal altruism in vampire bats further translate to females at large? Should we think about this study the next time your girl wants to snag a fry off your plate? "I could provide some insights," Wilkinson said, "but humans often don't follow the same rules that animals do, perhaps because the consequences for survival and reproduction are not as direct."

Although vampire bats are punching above their weight (or batting above their average) when it comes to exhibiting intricacies of female friendship behavior, Carter reminds us that human friendships are still more complex.

"When you share your fries with a friend, there is a lot going on subconsciously that makes you decide to offer fries at a certain moment to a certain somebody," he said. "You might have subconsciously noticed your friend glance at your fries (a cue that they want them). You might think that you are not 'keeping track' of things like French fries, but later, you have an argument with your friend, and you find yourself suddenly remembering and bringing up how you shared your fries (and why didn't they do XYZ for you).

"This is not to say that humans are Machiavellian. I think humans are often genuinely altruistic in the psychological sense, but we also have a lot of instinctual moral emotions that allow us to consciously or unconsciously manipulate others and that keep us from being socially exploited."