Ancient Temple Reveals Secrets of Mexico’s Meat-Eating Bats

They hang from the cold, stone ceiling of an ancient Maya temple like a bunch of fuzzy grayfruits, staring down at us witheyes that shine golden in the red glow of our headlamps.

The toothy, hungry bats have long, nearly transparent bunny-like ears and wrinkled, wolfy faces with a lance-shaped nose leaf—an appendage that bats use for echolocation—on top of their snouts.

These mysterious bats just outside the Calakmul Biosphere Reserve, in the Yucatán Peninsula, don’t roost in the thousands like some other species, said Rodrigo Medellín, the country’s leading expert on the flying mammals and a professor at the National Autonomous University in Mexico City. “They’re always in these small groups—they’re very protective of one another.”

Medellín swung his butterfly net at one of the six bats, catching it. He held the animal in a leather-gloved hand so we could examine the bat’s thick, woolly fur, which made it look sweet and cuddly, and its protruding snout and sharp teeth, which made it look anything but. This was a female, and she chattered her jaws at us, protesting. Medellín gently stretched out one of her wings and pointed to the bat’s thumb, which curved out from the edge. It was armed with a pronounced claw, shaped like a saber and just as sharp.

“That’s what they use to lock their prey,” he said. From the size of this weapon, you know these hunters of the night aren’t slashing mosquitoes—they’re after rodents, songbirds, even other bats.

Carnivorous bats, known collectively as false vampire bats because they don’t drink the blood of animals, as true vampire bats do, are found throughout the tropics, although in low numbers: Less than one percent of all bat species eat other vertebrates.

Two species are found from southern Mexico to Bolivia and Brazil, with one even extending farther south into parts of Paraguay and Argentina: the woolly false vampire bat (the ones in the Maya temple) and the spectral bat, also called Linnaeus’s false vampire bat. The latter are the largest of the New World bats, some with wings spanning up to three feet. Threats to their rain forest habitats are putting pressure on Mexico’s bats—and adding urgency to efforts to understand their ways.

Little is known about woolly false vampire bats, so Medellín caught all six and brought them outside the temple in soft, white cotton bags to weigh and measure them. Four were male, and two female, one pregnant. To learn more about the bats’ relationships, the researchers punched out a tiny skin sample from one wing of each bat to send to a laboratory for genetic analysis. Small GPS tracking devices were attached to the backs of three of the bats (including the pregnant female) to provide information about their hunting habits.

“Look,” Medellín said, pulling a male from a bag and turning him on his back. “The males have tiny testicles—that correlates with monogamy.” In the animal world, monogamy is rare. “It could be that two of these males are the females’ mates, and the other two are their offspring from last year and are in the process of becoming independent,” he explained. One female was nursing a four-month-old pup, and the pregnant one’s swollen belly indicated that she would give birth soon.

On the floor of the bats’ room was the partially eaten carcass of a spiny pocket mouse. “Look at that—a fresh meal, and probably the last prey of the night,” Medellín said. “We think they eat some prey outside, and after they’re full, they bring the last one back.” The catch often goes to bats that may not be able to hunt, such as nursing mothers. “We want to know what time they leave their roost, where they go, who feeds whom, and who they travel with,” he said.

Clues to the bats’ diets were gleaned from dinner leftovers and feces in the temple. “Look,” Medellín exclaimed again, holding up the tiny, decaying carcass of the juvenile mouse. “I’m surprised they didn’t eat it all, because usually they eat every bit—even the bones, toenails, and tail, sometimes.” They’d also eaten a yellow butterfly, forgoing the wings; a cicada; and a bird, possibly a woodcreeper, judging from a cluster of russet feathers. “Usually we find the wings of other bats.” They eat a lot of one uncommon species that is difficult to find. Medellín is now studying why this species is highly favored and how the woolly false vampires find them.

According to Medellín, woolly and spectral bats “are likely the bats described in the Maya origin myth, Popol Vuh.” In that tale the Maya hero twins are placed inside a “bat house”—a cave filled with death bats, called Camazotz by the Maya. The bats had snouts like blades, which they used to kill people and animals. To escape, the twins crawled inside their blowguns, and all night long the bats terrorized them. Toward dawn, one of the twins said he would check to see if it was safe to leave. He raised his head out of his gun—and promptly had it cut off by a Camazotz.

“These bats do the same thing,” Medellín said. “They stalk their prey, land on them with half-spread wings, locking them with the thumb claws, and deliver a death bite to the back or top of the head. Camazotz was not an invention.”

Like wolves, lions, most bird species, and some cetaceans, insects, and primates—including us—woolly and spectral bats share food, but only with their roost mates. Scientists consider food sharing to be a sign of altruism, although the behavior is most often found among animals that are closely related to one another—and then it’s more indicative of a close genetic relationship than a kind heart. Giving food to individuals outside of one’s genetic relatives, as humans often do, is seldom seen in other species. That’s another reason Medellín and his students want to clarify the genetic relationships among the woolly bats. They know the bats are bringing prey back to their temple roost (“It’s the soft side of their nature,” Medellín said), but they’re not yet sure which ones are sharing and which receiving. They expect to get answers from video recorders deployed in the roost by Ivar Vleut, Medellín’s postdoctoral researcher, to capture footage of feedings.

Vleut played some food-sharing clips for me on his laptop. “You can see the mother with her pup hanging here, and now another bat has arrived with a mouse,” he said. The bat, most likely one of the males, flies in and roosts close to her, grabbing an overhead beam with his feet. Hanging upside down, she shakes her furry forearms at him much as a baby bird does when begging from its parents. She bites the prey and removes it from the hunter; the mouse is already missing its head, which the male probably ate. Grasping the mouse’s body by its neck and chewing vigorously, she devours everything but the tail, which drops to the floor. The hunter bat hangs quietly beside her, grooming his wings by licking them. “That’s the usual behavior,” Vleut said. “They often seem to be full, satiated, when they return, which is why we think they’ve been hunting for a while and then bring back their last catch to share.”

The scientists have yet to see the bats hunting in the wild, but they have a good idea of their technique because Medellín kept two woolly false vampire males captive for two weeks in his hotel room and let them hunt for mice he released. He scattered leaves for the rodents to rustle through, sounds the bats quickly detected as they rotated their giant ears like satellite dishes to target their prey. “The bats flew inside a cage and hung from its wire sides or a branch,” Medellín said, “and just stayed there, perfectly still. But as soon as they heard the tiniest sound from the mice, they pounced.” The bats were easy to train, he added, quickly learning that a certain sound on his cell phone meant they’d soon have a chance to hunt. Medellín suspects that woolly bats in the wild hang out on tree trunks when hunting, listening for the scuttling footsteps of rodents or reptiles, or the sound of another bat’s wings.

One day we trekked through the rain forest hoping to find spectral bats. Vleut had first encountered one in 2009. “They’re very rare, and I knew nothing about them,” he said as we hiked along a trail. Once he trapped a spectral in a net while doing a bat survey. “I smelled it before I saw it—I thought something had died in our net. I was completely confounded, without words, when I saw this giant creature chewing his way out of our net. I was so surprised and a bit scared because of its size.” After removing the bat from the net, Vleut put on two pairs of leather gloves to handle it.

Fascinated, Vleut read everything he could find about the bats—which was very little—and discovered that no one had looked into their ecology. He contacted Medellín, and the two launched their study. “Right now we’re trying to determine how many of these bats can live in an area,” Vleut said. “We give rewards to people who find their roosts and tell us.”

That was how they’d located several roosts. They found another one after netting a spectral bat and attaching a transmitter to its back. Our group followed the signal to a hollow tree, where Medellín and Vleut hoped to be able to observe spectral bats at home. The scientists attached a GoPro camera to a cable. Vleut fixed a set of ropes and carried the camera to the uppermost part of the trunk, where the hollow opened. He slipped the camera inside and inched it farther down.

For several minutes the computer monitor on the ground was dark. Then: “We see them! There they are! Look, look! Right there!” Three pairs of eyes shone in the camera’s light, an adult female with a juvenile and an adult male. Surprised by the strange intruder in their home, they were protesting. We couldn’t hear their sounds, but the bats’ faces were contorted, their teeth chattering furiously, their long ears vibrating.

“They do that when they make the burrrr sound, which tells you to back off,” said Medellín, who kept his eyes fixed on the screen. “Now what’s that down there?” he said, looking past the bats at what appeared to be the floor of the tree’s interior. “That’s a bat’s wing!” one of his students replied. It was the remains of a recent meal, perhaps, lying there like part of a discarded Halloween costume. The bats continued to glare and chatter at the GoPro, and finally in what was surely a desperate protective gesture, one of the adults turned and spread its wings around the pup.

“Ohhh,” we said in unison, and then: “Take the camera out—get it out of there!” Vleut pulled on the cable, and the bats disappeared from the screen. An animated discussion ensued about how the scientists might put an unobtrusive camera into the tree to get images of the lives of bats without disturbing them.

Undoubtedly these carnivorous bats were terrifying to the Maya, just as they must be today to the animals they hunt. But we saw their softer, vulnerable side when they were huddled in their roosts or being handled by the scientists. Strange-looking creatures—a winged mix of mouse and wolf—as gentle and caring of their own as any other mammal.