COLUMBUS, Ohio - The death of a vampire bat 19 days after giving birth presented scientists studying the animals in 2019 with an unexpected chance to observe a rare event: a female bat's adoption of an unrelated baby.
The researchers had captured common vampire bats in Panama as part of ongoing studies of the formation of cooperative relationships among strangers. The team used infrared surveillance cameras to observe six hours of vampire bat activity spaced over the span of each day.
Two unrelated and unfamiliar female bats were observed forming a social bond based on mutual grooming and food sharing that increased over time. The researchers had named them BD and Lilith.
Lilith gave birth to a female pup about five weeks after the bats first met in captivity. As Lilith grew ill and spent less time caring for her pup, BD picked up the slack and even appeared to start nursing the baby. After Lilith died, BD adopted the pup, consistently nursing, grooming and sharing blood meals with the baby with almost no assistance from other adults in the colony.
"It's really cool that the little pup was adopted by the mother's closest social partner," said lead author Imran Razik, a graduate student in evolution, ecology and organismal biology at The Ohio State University.
"Even though it's a single observation, this method of recording social interactions every day for many hours allows us to contextualize it and better understand these social relationships. It's all about the social relationships."
The study is published today (Feb. 10, 2021) in Royal Society Open Science.
Razik works in the lab of senior author Gerald Carter, assistant professor of evolution, ecology and organismal biology at Ohio State, who studies vampire bat social behavior.
For this study, the team combined 23 adult and three juvenile common vampire bats captured from three distantly located roosts in the wild into a single captive colony, with plans to track how strangers developed social relationships over time.
Over four months, three surveillance cameras captured 652 hours of sampling periods, recording any cooperative behaviors that lasted at least five seconds. Vampire bats commonly groom each other and regurgitate their meals to feed roostmates that have been unsuccessful at getting their own meal of live animal blood. Carter's group fasts adult bats to induce costly investments of food sharing between recently introduced individuals.
The surveillance showed several clear trends. BD and Lilith increasingly groomed each other over time on an almost equal basis, and BD shared food with Lilith up until her death, even though Lilith did not frequently share food with BD. Lilith's grooming of her pup sharply declined shortly after birth, and her food-sharing interactions with the pup barely got off the ground. After Lilith died (of what the researchers believed was a gastrointestinal illness), BD provided steady grooming support to the pup, and her food-sharing with the baby noticeably increased. BD was still providing care to the pup at the end of the experiment.
BD's interactions with the baby actually began before Lilith died.
"I was noticing that sometimes the pup would lean over and attach to BD every now and then. And as Lilith was getting sick, I noticed the pup would spend more time near BD. I suppose Lilith didn't have the energy to raise the pup as she normally would have," Razik said.
Though the researchers don't know exactly when or by what mechanism, BD began to lactate. On the day Lilith died, Razik manually expressed milk from BD to confirm she would be able to nurse the pup that was "still in a stage of full dependency."
The researchers don't know why BD adopted Lilith's pup, but the adult vampire bats' social connection may provide clues.
"Both of these females had grooming relationships with several other bats in the colony, but those weren't as strong as the relationship they had with each other," Razik said. "This is only one observation, but it's interesting to speculate about what's going on."
This work was supported by the Smithsonian Tropical Research Institute, with which the authors are affiliated; the Animal Behavior Society; a graduate enrichment fellowship from Ohio State; a student research grant from Sigma Xi; and a Critical Difference for Women Professional Development Grant from Ohio State. Additional co-authors are Bridget Brown and Rachel Page, both from Ohio State.
Written by Emily Caldwell,
Royal Society Open Science