During the night’s darkest hours, somewhere between 3 and 4 a.m., vampire bats that dwell in the agricultural regions of Panama leave their roosts for their blood meal. One by one, they come out of the hollow trees they live in and fly into the night looking for grazing cattle. The bats, which have wingspans of about a foot, flutter around until they spot their victim. After a vampire bat eyes its target it will land on its back, crawl up and down and then pick a spot to bite. The flying mammals use the heat sensors in their nose to locate the cow’s blood vessels. They prep the wound site, shaving the hairs off with their rough-textured tongues. Finally, they sink their razor-sharp teeth into the cow’s flesh, creating a deep wound, and as the blood flows, they lap it with their tongues.
And while they feast, they may shout to their bat friends—albeit not in sounds humans can hear—to join them for dinner, according to a study published today in PLOS Biology.
Scientists know that bats are social creatures, particular the females. They groom each other inside their roosts and often regurgitate their meals to feed others who weren’t successful in their nightly hunts. Similarly to humans and other animals, they seem to have social preferences. In other words, bats have friends within the roosts—those they rest next to or groom—and others they may not be so close with. But how far these friendships go is less clear.
Researchers at the Ohio State University wanted to find out whether bats forage with friends or alone. Their study found that while bats almost always embarked on their nightly trips solo, they often joined others to share meals. Moreover, female bats that were friends, and spent a lot of time of time together within the roost were more likely to share their meals, says Ohio State University’s behavioral ecologist Gerald Cater. His team recorded three distinct calls at dining locations that suggests friends communicate with each other about available food.
Compared to other animals, such as primates or birds that are easy to watch in nature, bats are harder to observe. They tend to live inside trees, caves and other secluded areas, which they leave only for a few hours and often at night when humans can’t see. However, understanding bats’ behavior is important. Vampire bats in particular can pose a real threat to cattle because they may carry rabies. As scientists learn how bats interact with each other and their hosts they can better understand how rabies spreads.
The team studied two colonies of Desmodus rotundus—common vampire bats living near cattle ranchers in rural areas of Panama. In the first spot, named Tolé, the team focused on a fairly large colony of about 200 to 250 individuals. Using nets stretched across the bats’ flying path, researchers captured and tagged 50 females with tracking devices and then monitored their movement for several days.
Catching bats and outfitting them with tracking devices was a complex feat. “They are slippery and very difficult to handle,” says Carter. His team wore sturdy leather gloves bats can’t bite through.
The tracking devices informed researchers about the bats’ locations inside the roost and helped determine which bats were friends. The devices also let researchers know which of the bats then met up later at feeding sites. Bats that had more friends in the roost also met up with more of their friends during foraging flights, the team found.
In the second location, called La Chorrera, researchers observed the bats as they flew to a cowherd, where the scientists recorded their feeding interactions. Studying the bats at night in La Chorrera was both an exciting and eerie endeavor that required befriending cows. “At first, the cows would move away from me, but after awhile, they got used to me, so I basically became part of the herd,” says Simon Ripperger, a study author and biologist at The Ohio State University.
To observe and record the bats, he carried an infrared camera and an ultrasonic microphone that could capture the bats’ audio, which is outside the sound range humans can hear. The microphone was connected to a computer inside his backpack and would automatically record the sound waves bats were emitting. He couldn’t use any lights because that would scare the bats, so he observed the animals through the infrared camera.
"I could see them moving around on cows and locating the spot where they could bite,” Ripperger recalls. “Then they would bite and I could see the blood running down cows’ necks. I was so close, it literally gave me goose bumps.”
Watching the bats social interactions was fascinating, says Ripperger. As the mammals fed, they clearly made various calls—either to attract other bats or to keep them away. Ripperger noticed the microphone signal change as behavior changed. “I could see the bat’s mouth opening and closing,” he says—and then other bats would show up. Sometimes they would lap the blood together and sometimes they would fight over the wound. “I could tell there was a lot of communication going on while feeding.”
While cows weren’t part of the study, Ripperger learned something about them too. Some cows didn't react to bites very much. Some tried to slap the bats with their ears. One started running and shook the creatures off. And on two occasions when a couple of bats crawled on a cow’s back, another cow came and knocked them off.
But the study focused on bats, and specifically female bats because they are more social than males and keep friends within the colony. Males are far more territorial, researchers say. Inside the roost, they tend to stay alone and defend their spots against other males of the same colony, sometimes fighting with each other. Males don’t develop friends, except when they mate with females.
The ultrasonic recordings collected at the second study site revealed three distinct call types vampire bats used to communicate. One was the social call that researchers described as ‘downward sweeping’ they think may be used to recognize or alert friends, while the second was an antagonistic “buzz” which the team interpreted as “stay away.” A third call fluctuated from low frequency to high and back to low—shaped like the letter “n”—which has never been recorded before. “We think it’s a call they use to coordinate or compete over food,” Carter says.
These food communications may give notified bats some advantages. For example, prepping a wound takes time and work, and that leaves bats more vulnerable to predators like owls. The longer they sit on the cows’ backs feeding, the greater are their chances of becoming food themselves, Ripperger says. Bats notified of a prepped meal are able to fly in quickly and feed, leaving them less likely to be eaten.
Brian Bird at the University of California, Davis, who also studies bats but was not involved in this research, says the study advances scientists’ understanding of bats. “It shows a greater complexity of how bats live their lives and how they have their social structure, and what you call friends,” he says.
Even more interestingly it reveals that vampire bats—which often get a bad rep for their bloodthirstiness—are not unlike us. “It shows that the way bats behave in some ways is so similar to how we, humans, behave,” Bird says. “They preferentially take care of family members and friends.”