Vampire bats (Desmodus rotundus) form enduring relationships much like humans and other social primates, according to new research.
“That is, the bats take those bonds with them when released into the wild, where they can associate with anyone and go anywhere,” says Gerald Carter from The Ohio State University in the US.
Carter and co-author Simon Ripperger, from the Leibniz-Institute for Evolution and Biodiversity Science in Berlin, Germany, were curious that the bats show reciprocal cooperative behaviours: they groom each other and share the blood they suck from other animals.
Such bonds have evolutionary significance, says Carter, because they lead to repeated interactions that can help regulate cooperation. He has been watching captive vampire bats help others for years, filming females regurgitate their blood meals to feed fasting bats kept in a separate cage.
“They do this for kin and non-kin. It’s pretty rare outside of humans to have behaviours where I’m paying an obvious cost to help you and you’re not related to me.”
But social bonds are hard to study using observation alone, Carter notes. “Two animals that are always together might just be attracted to the same resource, or one might follow another around without any mutual benefit.”
“If stable bonds exist,” the authors write, “they should persist even if the individuals are moved to a dramatically different physical and social environment.”
Putting this to the test, they first housed a colony of female vampire bats captured from a hollow tree roost in Panama, and their captive-born offspring, in a laboratory setting for 22 months.
To elicit bonding behaviours, they periodically withheld food from selected bats and observed the resulting relationships that formed.
They then fitted custom-made proximity sensors to 23 captive bats before returning them to their natural outdoor home roost. They placed the same sensors on a control group of 27 wild bats living in the same roost.
Ripperger had worked for eight years with engineers to develop the tiny computers – which weigh less than a penny – so they could be attached to the back of each teacup-sized bat with glue.
“The major challenge was to miniaturise the sensors in a way that we can fit them on bats,” he says. “And the other thing we wanted was information on association between bats.”
The sensors in the network sampled the proximity of all the animals every two seconds, inferred from signal intensity, over eight days.
They found that the test bats roosted together more than control bats, and that their bonds to each other were stronger than those of their wild counterparts.
Even with about 200 potential partners in the roost, many of the bats that had bonded in the lab stuck together in the wild, suggesting they had formed genuine social connections.
The findings confirm that vampire bats form similar relationship bonds that are seen in other social mammals like whales, dolphins and humans.
The researchers want to continue exploring individual differences in cooperation among vampire bats to learn how they go from being strangers to cooperative partners.