Fairer sex: Colonies of female-only vampire bats live in equal societies

Are vampire bats operating one of the most coherent societies on Earth? Recent research revealed the species socially distances when they are ill, and a separate study recorded an incident where a baby vampire was adopted by another female after its own mother died.

Now, astonishing new research suggests colonies made up of female vampires share food equally and groom each other without regard to any strict social hierarchy.

The research team studied how the bats, whose only food source is blood, interacted in the presence of food over a three-month period, using a captive colony of common vampire bats at the Smithsonian Tropical Research Institute (STRI) in Panama.

The colony consisted of 24 adult females captured from two distant sites as well as nine young bats – four males and five females. The bats were kept in a large cage, and drops of blood were deposited on the floor where infrared video cameras captured the interactions between the bats as they accessed the food.

In total, over a thousand interactions between the bats were observed.

The scientists found that unlike in other mammal societies, the biggest and strongest bats didn’t necessarily dominate every interaction over food availability, and there was a randomness to the ranking order – no specific quality they measured gave a bat a better chance at dominance, so any adult female had an equal opportunity to rank very high or very low on a scale of dominance in the roost.

Previous research on gregarious mammals, especially primates, has revealed how a dominance structure impacts survival, longevity and the health of offspring, while companionship is a less important factor.

Lead author of the study, Gerald Carter, an assistant professor at Ohio State University, said that in contrast, vampire bats make “friends” through a gradual buildup of trust and show signs of maintaining those friendships in the wild.

As well as grooming and allowing equal access to food, another key behaviour to help maintain the health of the overall colony is regurgitating their blood meals for other members.

Dr Carter told The Independent: “We’ve known for many years that female vampire bats develop cooperative grooming and food-sharing relationships, but we did not know what to expect about female competition.

“We knew that female social rank is very important in many well-studied primate groups and in cooperative breeding birds and mammals, and we suspected these bats had some kind of social rank, but we had no strong prediction.”

The research team ultimately found that the hierarchy among female bats is “weakly linear and shallow, suggesting egalitarian access to resources among familiar bats”.

“They groom each other a lot,” Dr Carter said. “They spend about 5 per cent of their time social grooming. They will also regurgitate food not only to their offspring, but also to unrelated adults with whom they closely associate.”

But life is different for the males. “The male vampire bats defend territories and compete over access to females,” Dr Carter said.

“They are not as social as the females, but we also know much less about their social lives. They are more likely to be solitary. If you see a lone vampire bat roosting somewhere, it’s probably a male.”

He said the bats’ behaviour could remind humans how balancing competition with magnanimity was a path to better outcomes.

He said: “We can learn a lot about complexity and cooperation from the similarities and differences in other species. So many well-studied species – humans, chimps, baboons, meerkats, vampire bats, over 300 species of cooperative breeding birds, the eusocial insects like honeybees and paper wasps, and even microbes – they have all independently evolved traits that allow them to cooperate, coordinate, and maintain stable cooperative societies.

“They often do this by actively suppressing behaviours that are selfish and competitive. In nature, there is never pure voluntary altruism between all individuals, because that scenario is inherently unstable due to free-riding.

“So as humans we should always see cooperation and competition as two sides of the same coin. If we want to foster cooperation, we must understand competition too.”

The research is published in the journal Royal Society Open Science.