Why Do Animals Adopt? The Case Of A Baby Bat Adopted By Its Mother’s Best Friend Sheds Some Light On One Of Biology’s Mysteries.

Adoption is one of the great mysteries of the animal kingdom. 

In the world of natural selection, caring for an infant that isn’t related to you doesn’t carry obvious benefits for furthering your own genetic code. Despite this, non-kin adoption has been reported in hundreds of species ranging from finches to primates. 

There are a number of theories about why animals adopt. One is that adoptive parents get to practice their parenting skills by raising another animal’s offspring. Another theory is that adoption is misdirected parental care where the adoptive parent doesn’t realize they are caring for a baby that isn’t biologically theirs. 

Now, a new study published in Royal Society Open Science suggests that vampire bats are more likely to take on an orphaned infant if they were friends with the infant’s biological mother. Researchers at Ohio State University wrote a paper about a baby bat in a captive colony that was adopted by its mother’s best friend when the mother died.

First author Imran Razik was in Panama in 2019 studying vampire bats at the Smithsonian Tropical Research Institute. His research included tracking relationships between bats by monitoring how often they groomed and shared food with one another, using video footage of the colony.

Two female bats named Lilith and BD interacted a lot.

“They were each other’s number one grooming partner,” said Razik.

Lilith and BD were originally from different wild colonies and met in captivity at the research institute.

Unfortunately, their friendship came to an end when Lilith began to get sick, possibly with a gastrointestinal issue. BD fed Lilith by regurgitating food for her as she grew ill. However, Razik went to check on the bats one day and found Lilith struggling to fly in the bats’ cage. He tried to help her, but she died shortly after. 

This was particularly alarming since Lilith had a pup. Razik wasn’t sure what to do.

“At the time I was very worried about what was going to happen to this pup,” he said. “It was very young and very small. It could not care for itself.”

But things worked out just fine. BD took over the pup’s care. She started lactating despite not having her own pup. Razik said it is possible that BD’s interactions with the pup before Lilith’s death caused a hormonal reaction that led to lactation.

Razik said BD’s response was a surprise. There have been other examples of non-kin adoption in bats, but it isn’t commonly reported. Since he already had long-term data tracking interactions between Lilith and BD, he decided to write a paper about the adoption.

Female vampire bats form strong grooming and food-sharing relationships, mostly with kin but also with non-kin, according to Razik. He said it is less common to see relationships between pups and adult bats that aren’t their parents.

There isn’t research on adoption in wild bats, but the new study suggests that social relationships might play a role in non-kin adoption, according to Razik.

Stacy Rosenbaum, a biological anthropologist at the University of Michigan who studies the evolutionary consequences of social behavior, said it is difficult to draw conclusions from the paper given the anecdotal nature of the case it describes. She said she would need to see a lot more well-documented cases like the one presented in the new paper before deciding that prior interactions are an important determinant for adoption. Unfortunately, data on adoption is rare since adoption is a rare behavior.

Still, she said the paper provides an intriguing case study and said its conclusion isn’t implausible. Adopting an infant from a friendly parent could be evolutionarily advantageous if that infant also grows up to be friendly.

“If the parent was a cooperative individual, and their offspring inherit those cooperative genes, then the adoptee might grow up to be a cooperative individual who will themselves eventually cooperate with the adopter someday,” she said. 

Mirjam Knörnschild, a scientist leading bat research at the Museum of Natural History Berlin, said the paper’s evidence supports the hypothesis that non-kin adoption in bats is influenced by interactions between an adopting female and a deceased mother. Vampire bat adoption could thus have parallels to adoption behavior seen in chimpanzees and humans.

She said the paper is an excellent example of how detailed behavioral observations of bats can provide scientists with valuable information on rare social behaviors. She said the field needs more behavioral studies.

“A female vampire bat adopting an unrelated pup is a truly fascinating behavior and a testimony of the high sociality these bats exhibit,” she said. “The more I learn about vampire bats, the more they amaze me.”

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