Vampire bats bond when randomly paired, like college roommates

Being forced to bond with your randomly assigned college roommate isn’t just a human phenomenon.

Vampire bat pairs forced to live together for just one week sustained their friendly relationships for more than two months after they were released into a larger bat community, according to a new study that provides rare findings about how social bonds form.

Researchers found that unfamiliar vampire bats forced into close proximity with each other had an increased rate of social grooming, compared to already familiar bats and those allowed more freedom, forming lasting social bonds.

“It was a striking pattern,” says senior author Gerald Carter, assistant professor of evolution, ecology and organismal biology at Ohio State University, US.

“One thing you might imagine is that, after these bats are in their ‘college dorm room’ together, they stay together for a little while afterward but that quickly goes away – but we didn’t see that.

“The test bats were still grooming each other more than the control bats even at the end of the experiment, nine weeks later.”

The team of researchers captured seven adult female vampire bats (Desmodus rotundus) from each of three distantly located roosts to assemble a colony of 21 bats for the study. For the first six weeks they were allowed to mingle freely together among familiar roost mates and strangers from other groups.

The researchers then split the bats up into groups of three – consisting of one bat from each of the three sites – and forced the strangers to live together as a trio for seven days.

After rooming together in trios, all of the vampire bats were then allowed to live freely together again for nine weeks.

During all three phases of the study, the team measured the grooming interactions between the pairs of vampire bats, capturing interactions of five seconds or longer using three infrared surveillance cameras that operated for six hours a day.

By comparing the grooming behaviour of each unfamiliar pair against control vampire bats that were not forced into close proximity and familiar bats caught from the same roost, they found that there was an increase in the average change in social grooming rates for previously unfamiliar bats forced into close quarters.

“During the forced proximity phase, each bat had two partners with whom they could interact, and in the post-treatment phase they had at least 20 other available partners – even some they knew beforehand that had been captured from the same site,” explains Imran Razik, co-lead author and graduate student from Ohio State.

“So the fact that the preference was visible and clear and continued throughout the entire nine weeks is a meaningful result.”

While this study doesn’t settle the question of how social bonds form in the wild, it does tell us that there is a causal relationship between being forced into the same space and preferring each other’s company later.

“Which is why the college dorm room is a perfect example: you get randomly paired with somebody and, because of that, you continue to seek that person out later,” says Carter. “A relationship has formed.

“It might be really obvious for humans, but we don’t know to what extent this is happening in other animals.”

The study was published in Biology Letters.

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