With vampire bat food sharing, there must be a benefit. It must be that by helping others I'm also helping myself.
Vampire bats help their neighbours in need even if there is no benefit to them - not what one might expect from the blood-sucking creatures.
Scientists found the flying mammals develop social bonds during captivity which they maintained when returned to the wild.
Researchers say vampire bat co-operation is rare in that individuals pay a cost to help others. They were observed regurgitating their food - ingested blood - to feed non-relatives.
Study co-lead author Gerald Carter, assistant professor of evolution, ecology and organismal biology at Ohio State University, said: "What's quite common in animal co-operation is doing something where we both benefit simultaneously: let's live together, let's hunt together. I'm benefiting, you're benefiting.
"Here, there's a kind of risk. If you have a co-operative trait that helps other individuals and costs you, natural selection should wipe that trait out unless the co-operative individual benefits somehow.
"With vampire bat food sharing, there must be a benefit. It must be that by helping others I'm also helping myself."
He added that scientists think the bats must have long-term reciprocal relationships, but it is not yet clear how that works.
Scientists at the Smithsonian Tropical Research Institute (STRI) housed female vampire bats and their captive-born offspring in a closed laboratory colony for 22 months.
This created conditions that prompted social bonding behaviours of food sharing and grooming.
The bats were then returned to their natural outdoor home roost of a hollow tree, equipped with sensors that recorded how close they were to each other every two seconds.
Those same sensors were placed on a control group of wild bats.
Over eight days, researchers collected enough data from the sensors to suggest that relationships between the previously captive bats persisted when they returned to the wild.
According to the study published in the Current Biology journal, from an evolutionary standpoint this suggests that whether bats maintain a relationship can be attributed in part to their shared history.
Researchers returned 23 captive bats wearing sensors to their hollow tree and glued sensors to 27 wild female bats living in the same roost.
According to the data, the test bats had closer bonds with each other than they did with control bats.
The findings suggest that even with about 200 potential partners in the roost, the bats that had connected in the lab stuck together in the wild - a sign that they had formed social bonds.
In order to conduct the research, Simon Ripperger of the Museum fur Naturkunde, Leibniz-Institute for Evolution and Biodiversity Science in Berlin and colleagues had to develop the sensors.
These tiny devices, which are lighter than a penny, were attached to the bats like backpacks and allowed scientists to capture social networks of entire social groups of bats.
Nina Massey is the PA science correspondent.