Vampire Bats Groom One Another, Reveal Complex Social Order

Vampires bats (Desmodus rotundus) spend a lot of time grooming each other. Wikimedia Commons

Vampire bats, which live in Latin America and feed at night on the blood of large animals like cows and deer, are surprisingly brainy and social creatures, engaging in certain behaviors such as social grooming that aren't common outside of primates.

Gerald Carter, who studies vampires bats at the Smithsonian Tropical Research Institute in Panama, wanted to see how their grooming habits stacked up against other bats. In a study published this month in the journal PLOS ONE, Carter and co-author Lauren Leffer found that the bats spend as much as 6 percent of their waking hours grooming one another—14 times more than that of other bat species.

Carter performed the study at the Organization for Bat Conservation, outside Detroit, where he observed the grooming habits of six different species. In this environment, the bats have no external parasites, so the time they spend grooming one another isn't changed by random differences in parasite quantity, Carter adds.

The results provide more evidence that vampire bats (Desmodus rotundus) are creatures with well-developed social lives, at least compared to other bats. Vampires, for example, are unique in their habit of sharing blood-meals with other adults to whom they are not related. To do this, they have the adorable habit of regurgitating blood into one another's mouths.

Vampires, you see, don't have the easiest lives, as it's not infrequent for them to fail to find a blood meal at night. Blood is relatively nutrient-poor to begin with. Going without blood—which serves as their sole source of food and water—for a short period of time could lead to starvation. Under such circumstances, sharing is necessary to stay alive, Carter says.

Grooming helps maintain the social order that facilitates that sharing. It helps strengthen bonds, and time spent grooming another can serve at a "social investment," Carter says. Likewise, when one bat pukes up blood into another's maw, that's a type of "social capital"—the puker is more likely to be shared with in the future.

Grooming is used to maintain social relationships, and species in larger, more complex groups spend more time grooming one another, says Gerald Wilkinson, Carter's former advisor at the University of Maryland, who wasn't involved in the work. "The vampire bat data is consistent with that pattern," meaning that vampire bats are more socially complex than the other bats tested.

Wilkinson and Carter also just published a study this week in the journal Hormones and Behavior, showing that vampire bats shared more food (i.e., blood) with each other after being given intranasal oxytocin. Females given the hormone likewise spent more time grooming one another. Oxytocin plays an important role in bonding in many animals, including humans, and, as the duo has shown, in vampire bats as well.