Vampire bats don’t have the best reputation. For one thing, they are bats, and despite the value of bats and their great variety — about a quarter of all mammal species — not everyone loves them.
It might have to do with the viruses they host, which wouldn’t be much of a problem if we left them alone. Or it might just be that the fluttering of bat wings at night strikes some anxious chord in most humans. But these are not the bats thought to be the source of the new coronavirus).
And then there’s the vampire thing. But bats are not Dracula, or any of his animated fellow blood fiends in shows, such as Castlevania. Vampire bats are, in fact, the soul of cooperation, with a complex social structure. Like good toddlers, they have learned to share. For the bats their lives depend on it.
If a vampire bat drinks from a cow’s ankle one night, it is likely to share that meal with another bat. They do it via regurgitation, but that’s just a matter of style. Blood meals are hard to find, and they don’t keep you going for very long. By one estimate, a bat needs to feed every 48 hours to survive. That’s a good evolutionary incentive to develop food sharing.
Vampire bats usually share what they’ve drunk with other members of their female-led families. But they also extend the blood sharing habit to other bats that they create close bonds with, even though they have nothing in common genetically.
Gerald Carter, a behavioral ecologist at Ohio State University who studies cooperation in vampire bats, reported Thursday in Current Biology that an experiment in his lab showed that bats build friendships gradually, as other bats show they are reliable.
Dr. Carter, who recorded the behavior of bats in his lab over more than a year, said that they follow predictions from a mathematical model called “raising-the-stakes,” in which traded favors gradually increase in size.
The model has been demonstrated to work in tests with humans playing cooperation games. But, Dr. Carter said, his experiment in a situation that mimics natural conditions shows “how animals go from being strangers to forming a natural cooperative relationship.”
“This is a great study,” said Dina Dechmann, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany, who also studies bats and was not involved in the research. Actions that cost the giver, like food sharing were though to be related to kinship by preserving ones own genes. So sharing with friends, was a puzzle.
“But we find increasingly that groups, and cooperating individuals are not related — here is an explanation of how this could happen,” Dr. Dechmann said.
Dr. Carter is a bat aficionado from childhood. Fortunately, vampire bats turned out to be extremely useful for the study of animal cooperation, his area of focus. They are easy to raise in a small space, like rats, but socially complicated, like primates.
In nature, Dr. Carter said, “Vampire bats spend almost all of the time, like 23 hours of the 24-hour-day, inside a space that’s maybe the size of a small closet.” He experimented with female colonies, which also contain young males.
“When we put them in captivity, they’re doing all the things they would be doing inside of a hollow tree,” he said.
The cooperative behavior comes naturally, and experimenters can trigger it, simply by imposing a fast of the sort that vampire bats experience in the wild.
Dr. Carter said that extending a blood donor network to nonfamily doesn’t offer any immediate benefit. But, in a pinch, if Mom and Sis disappear, having a backup can be valuable.
“Individuals that only have a food trading relationship with a mother, they don’t have backup partners. So we think that’s what these weaker non-kin relationships are about,” he said. “It’s basically an insurance policy.”
Dr. Carter took vampire bats from the wild and put them together in the lab as strangers with cameras pointed at them.
Some were in pairs. In other cases, one stranger was introduced to a group of three bats that had lived together. And in other cases two groups of three were put together.
The isolated pairs formed relationships fastest; the groups were slower to bond, like college roommates thrown together, Dr. Carter said.
But what Dr. Carter’s group was studying was not time, but different behaviors and their cost to a bat. Grooming, in the form of licking each others fur, doesn’t cost much in terms of energy, so it’s a small investment in a social bond. But food sharing costs a lot, particularly when you only have 48 hours to find a meal.
As predicted, grooming behavior emerged first and grooming relationships were more common than food sharing relationships. And the amount of grooming was an indication of later food sharing. The more grooming the more likely food sharing was. They predicted that grooming would occur first and later graduate to sharing blood.
What Dr. Carter would like to study next is what happens if one bat doesn’t hold up her end of the relationship.
Dr. Dechmann, in email comments on the paper, couldn’t help putting in a plug for bats and bat lovers everywhere: “I also like that in today’s bat-phobic times, such a study comes out, showing what a complex social system (among other remarkable things) these highly intelligent animals have.”