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A female vampire bat has adopted an orphaned baby bat and begun nursing it, after creating a close social bond with the baby’s mother before she died.

Although female bats live in “maternity colonies”, sometimes composed of hundreds of bats, they seem to raise their young individually rather than as a community. This rare observation of a bat adopting an unrelated infant gives further insight into the complexity of social relationships between individual bats, says Imran Razik at The Ohio State University.

“The cool thing about vampire bats is they form these long-term social relationships with one another, which are in some ways comparable to human friendships,” says Razik. “With this adoption, something we don’t usually see, we’ve had the chance to contextualise this event very easily by having a recorded, long-term history of the social interactions [leading up to the adoption].”

Razik and his colleagues at the Smithsonian Tropical Research Institute in Panama were studying the social behaviour of common vampire bats (Desmodus rotundus) at the time of the adoption. They captured 23 adult female bats from three wild colonies located hundreds of kilometres from each other, then put them together in a single captive colony to see how they developed relationships with strangers. Female vampire bats can form social bonds with other bats, grooming each other and sharing food by licking each other’s mouths.

One bat fell ill during the study and died within weeks of giving birth. To the researchers’ surprise – and relief – another bat adopted the infant, says Razik.

By viewing hundreds of hours of video recordings of their social interactions, the researchers could fully trace the development of the relationship between the sick mother bat (“Lilith”) and the adoptive bat (“BD”), who wasn’t pregnant or nursing. They had initially shown a lot of mutual grooming, suggesting they were bonding, says Razik.

The healthy bat shared food much more often than Lilith did, a trend that increased as Lilith became sicker with what the researchers believe was gastrointestinal illness.

BD also helped take care of Lilith’s baby, grooming it and even nursing it, although she herself didn’t have a baby.

When Lilith died 19 days after giving birth, BD fully adopted the baby and raised it as her own. “I manually expressed milk from BD’s nipple, and I thought, ‘wow, she’s actually nursing this pup’, and I knew this was a really great story,” says Razik.

The other 21 females in the colony didn’t help Lilith or her pup to nearly the same extent.

“We set out to study partner choice, and why bats invest in one other and how they form these relationships,” says Razik. “This adoption has really been an interesting contribution to that.”