Vampire bats (Desmodus rotundus) do drink blood, which can be off-putting. However, vampire bats are also very cuddly, at least with one another. Female bats cluster together for warmth, share food, and groom their cuddle-mates by licking each other’s fur. Being groomed can reduce stress, lower heart rate, and promote cooperation.
It’s clear that bats benefit from being groomed, but not why others volunteer to be the groomer. Bat grooming often prompts animal behavior researchers to ask: why do animals do nice things, like grooming?
To study when bats are willing to groom one another, researchers at the Smithsonian Tropical Research Institute set up two tests for a colony of female vampire bat. In the first test, scientists rubbed water on the fur of each bats’ back, and then measured the amount of time each wet bat spent being groomed by other bats.
As expected, bats were groomed more after getting wet. So, it appears that vampire bats will help out a friend in need. However, the second test showed that groomers weren’t only motivated by helping one another.
In the second test, scientists grouped a few female bats in clear cages for 30 minutes at a time, watching for each moment when one bat groomed another. The scientists noted what was happening just before each grooming event to measure how often the bats were grooming themselves before leaning over to spare a lick for a friend.
The researchers found that one bat is most likely to groom another if the groomer has just been grooming herself. In short, some bats agree to groom others just because they like to lick. Perhaps the act of grooming reduces anxiety, just as being groomed does.
Studying the details of how animals cooperate is useful for understanding how social groups stay together in nature. For vampire bats, grooming isn’t all about utilitarianism or the fear of punishment. Sometimes, bats groom because they want to.