Female vampire bats like to meet up with friends for dinner, according to a new study. They also seem to use a special call, previously unknown to scientists, during their nightly hunting trips. The findings indicate that bonds forged in the roost could help the animals save time and energy when grabbing a blood meal.
The new work “opens up a whole new avenue of vampire bat research,” says Mirjam Knörnschild, a behavioral ecologist at the Berlin Museum of Natural History, who was not involved in the study. Efforts like this, she adds, are “crucial” for understanding social cooperation in bats.
Vampire bats live in groups ranging from a dozen to thousands of individuals. Colonies roost in caves or hollow trees and typically consist of mother-daughter pairs, young males, and transient males that fly in to mate. The bats are a great model for analyzing animal behavior, says Gerald Carter, a behavioral ecologist at Ohio State University, Columbus, who has been studying the social lives of these bats for a decade. That’s because the pug-nosed mammals spend most of their time in the small, enclosed space of the roost, where they form close relationships with relatives and unrelated “friends.”
Carter’s past work has shown female vampire bats spend time cozied up with their best buddies while they’re roosting. The animals groom each other to strengthen social bonds, and—once they’re close—they’ll even help a hungry friend in need by regurgitating a blood meal. Although the roost is essentially the bats’ whole world—they spend more than 21 hours per day squished inside—Carter wanted to know whether their cooperation extended to the world outside, and their nightly search for blood. To find out, he needed to observe bats on the wing in the wild—a much more challenging feat.
He teamed up with Simon Ripperger, a bat researcher at the Smithsonian Tropical Research Institute. Ripperger recently helped develop a sensor to monitor social behavior in bats, and after talking to Carter, he realized the vampire bat was the perfect species to test the new technology on. The custom devices, which track how close bats are to one another, are “cutting edge,” says Dina Dechmann, a bat behavioral ecologist at the Max Planck Institute of Animal Behavior, who wasn’t involved with the work.
Carter and Ripperger captured 50 female bats from a 200-bat roost in a hollow tree in Tolé, Panama. Twenty-three of the bats had previously been captured—and observed—by the researchers, and 27 were completely wild. They caught the bats with a large mesh net placed outside the hollow—a process that required an all-night stakeout. Then, they carefully attached the sensors, which looked like little backpacks, to the bats using surgical glue.
The sensors “talked” to each other and broadcast data on the bats’ locations every 2 seconds, allowing Ripperger and Carter to measure how long the tagged bats spent in each other’s company. Over the course of several nights in September 2017, they recorded 586 foraging encounters.
Their data indicated that although closely bonded bats typically left the roost separately to go foraging for food, they often met their friends later for a meal, before parting ways again, the team reports today in PLOS Biology. Bats with more friends in the roost also met up with more bats when foraging. “This was not what we initially expected,” Ripperger says. “I thought that if they did social foraging, they would go on a trip together and maybe come back together.”
What they saw instead—bats leaving alone and then somehow finding their favorite roost mates—was more complex, Ripperger says. “It requires a higher degree of [social] coordination.”
Because of these nuanced data, the researchers wanted to supplement the automated observations with observations collected “the old-fashioned way,” Ripperger says. That’s how he found himself in Panama, 2 years later, walking among a herd of cattle—the vampire bat’s favorite source of blood—in the dead of night without a flashlight.
After several fruitless efforts to see bats socializing with his own eyes—including trying to corral a herd of cattle to entice them—Ripperger decided to spend his last night in the country wandering around with an infrared camera and a microphone—and a free-ranging herd of cattle. “It was smelly,” he says. But when he finally saw a bat, “I literally had goosebumps.”
Ripperger’s videos from that night, which captured 14 bat interactions, show different sorts of encounters: a few short, antagonistic ones, and several longer, friendlier looking ones as they fed on the cattle. The observations added crucial context to the sensor data, which couldn’t tell the researchers what the bats were actually doing when they were in close proximity. “All of a sudden it made more sense in our heads.”
Ripperger also recorded well-known social calls and a foraging-specific call that had never been heard before. It was similar to the social calls, but whereas those started high and finished low, this new call started low and rose high before dipping back to its initial frequency. Such calls might help bats recognize each other after leaving the roost separately—and even signal their friends for a meal. But further studies are needed to figure out exactly how the bats are managing to meet up.
Carter says this work could help researchers better understand interactions not just between individual bats from the same colony, but also between different bat colonies when they meet outside the roost. “I see this as laying the groundwork for a whole other aspect of [vampire bats’] social lives which we’ve basically been ignoring up until now,” he says. “We’re just scratching the surface.”