Mama Bats Literally Nudge Their Babies Out of the Roost

Getting kicked out of the nest is an idiomatic way of describing one’s abrupt jump from their childhood abode and parental payroll into fledgling independence. But for one species of bat, the phrase might be literal.

In Gamboa, Panama, researchers studied hours and hours of footage of Uroderma bilobatum, better known as Peters’ tent-making bats. The team found that in the weeks leading up to the moment these furry flyers leave for good, their mothers start poking and prodding them to perhaps not-so-subtly hint that it’s time for the pups to hit the road.

Bats occupy a unique place in evolutionary history. The winged mammals fly like birds, yet they give birth to live young and nurse them. As a result, baby bats face a daunting enterprise no other living thing does: simultaneously weaning off their mother’s milk and fledging, or learning to fly. That’s a hefty dose of independence for one little bat.

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Across the world, there are more than 1,300 species of Chiroptera, the only mammals capable of flight, according to Bat Conservation International. Bats account for about one-fifth of all mammals, making them the second largest order of mammals after rodents. But unlike rodents, scientists know surprisingly little about bats. There hasn’t been a lot of research done on the lifecycle of individual bat species from birth to death, primarily because such studies are huge undertakings and fieldwork can be messy and unpredictable.

But as bat species of all kinds face increasing threats to their existence, understanding how the animals behave from their first day of life to their last is invaluable.

“Knowing more about how these bats reproduce is important for their conservation,” says Mike Smotherman, a bat expert and biologist at Texas A&M who was not involved in the new study. “Knowing how far mothers carry their babies while they forage, and how the babies then learn to feed themselves, will be important in coming years to conserving not just this species but all bat species.”

Bat biologist Jenna Kohles, lead author of the paper published recently in the journal Plos One, began observing and filming tent-making bats in Gamboa at the Smithsonian Tropical Research Institute while she was still a student at Clemson University. She was able to capture live births on video—a rare feat in the field—and study the bats’ long journey to fledging, which can take more than 40 days to complete.

Peters’ tent-making bats get their name from a behavior that wild populations exhibit, altering the middle vein-like ribbing of banana tree leaves in order to make them flop into an A-frame, just like a tent. The population that Kohles studied in Gamboa likes to take up residence in the eaves of people’s homes. She observed more than 30 homes, with each house representing one roost. Each roost had anywhere from 2 to 73 individuals, usually with between 1 to 29 pups.

Most bats, including the bats Kohles studied, are born weighing nearly a third of their adult bodyweight. Mother bats only have one pup at a time because until their baby is ready to fly, the young one stays latched on to its mother’s body. Understandably, these strong moms are likely eager to ditch the extra weight as fast as they can.

Starting around day 25, Kohles noticed a strange and repetitive behavior. Around 30 minutes before the mothers were ready to take flight into the night and forage, they would start tapping their babies with their forearms repeatedly. When the mothers first started this nudging behavior, the babies would briefly stop nursing, perhaps flap about a bit, but then quickly latch back onto mom—sometimes detaching and reattaching several times during the half hour period of prodding.

“After analyzing all the video, the most exciting thing we saw was this nudging behavior,” says Kohles, who is now completing her masters at the Max Planck Institute for Ornithology. “It was something that hadn’t been described before. There’s not a lot of information about tactile communication in bats in general and certainly nothing like this about bat pups and their mothers.”

As the faithful day of fledging grew closer, the babies started to get the hint. The mothers didn’t need to nudge their young as much to signal that it was time to stop suckling, and the pups would detach and reattach fewer and fewer times until they finally flew the coop for good.

All in all, the nudging makes sense. As the pups start to reach adult size, they become harder and harder to carry. Before the babies were fully fledged, the moms appeared to take breaks from lugging their young around—behavior that became apparent when Kohles observed mothers foraging without their pups with them. Wherever the mothers left their young, they were clearly safe, because the adults always had their offspring with them when they returned to the roost for the day.

“I’m interested to know where the moms take the pups,” Smotherman says. “Really I’m dying to know where that is.”

Smotherman notes that the researchers were likely able to get a glimpse at what these bats are up to because the animals interact with human-made environments. The Peters’ tent-making bat is already adapted to coexist with humans, but as human influence and development spreads, the flying mammals' food source could be threatened.

“These bats, like many, will be affected by habitat loss,” Smotherman says. “This study is right at the crux of that because they’re studying roosting bats and their babies and how far they go for food. Ten years from now or so, there will be less food and they’ll have to travel farther to get that.”

Kohles says that in recent years, scientists have witnessed the population of Peters’ tent-making bats mysteriously shrinking in size. It could be that the bats simply got fed up with people who don’t respond kindly to bat droppings on their houses, Kohles jokes. Tent-making bats dine exclusively on figs, exacerbating the excrement problem, but they also spit out the seeds and play a crucial role as seed dispersers for the proliferation of the fig trees.

Kohles hopes to see the population bounce back in Gamboa, if only to improve the relationship between humans and their batty neighbors.

“Working in Gamboa gave me an opportunity to talk to people about why bats are not all bad, and they’re doing a service for us, and we do need them,” she says. “They make up one-fifth of all mammal species—it’s no wonder that they play such an important role in our ecosystems."

Rachael Lallensack | READ MORE

Rachael Lallensack is the former assistant web editor for science and innovation at Smithsonian.