Vampire bats that share food and groom each other in captivity are more likely to stick together when they’re released back into the wild, find researchers in a study reported on October 31 in the journal Current Biology.
While most previous evidence of “friendship” in animals comes from research in primates, these findings suggest that vampire bats can also form cooperative, friendship-like social relationships.
“The social relationships in vampire bats that we have been observing in captivity are pretty robust to changes in the social and physical environment–even when our captive groups consist of a fairly random sample of bats from a wild colony,” said Simon Ripperger of the Museum für Naturkunde, Leibniz-Institute for Evolution and Biodiversity Science in Berlin.
“When we released these bats back into their wild colony, they chose to associate with the same individuals that were their cooperation partners during their time in captivity.”
He and study co-lead author Gerald Carter of The Ohio State University say their findings show that repeated social interactions they’ve observed in the lab aren’t just an artifact of captivity.
Not all relationships survived the transition from the lab back into the wild.
But, similar to human experience, cooperative relationships or friendships among vampire bats appear to result from a combination of social preferences together with external environment influences or circumstances.
Carter has been studying vampire bat social relationships in captivity since 2010.
For the new study, he wondered whether the same relationships and networks he’d been manipulating in the lab would persist or break down after their release in the wild, where the bats could go anywhere and associate with hundreds of other individuals.
Studying social networks in wild bats at very high resolution hadn’t been possible until now. To do it, Simon Ripperger and his colleagues in electrical engineering and computer sciences developed novel proximity sensors.
These tiny sensors, which are lighter than a penny, allowed them to capture social networks of entire social groups of bats and update them every few seconds.
By linking what they knew about the bats’ relationships in captivity to what they observed in the wild, they were able to make this leap toward better understanding social bonds in vampire bats.
This picture shows a tagged desmodus rotundus bat in the wild. The image is credited to Sherri ad Brock Fenton.
The researchers found that shared grooming and food sharing among female bats in captivity over 22 months predicted whom they’d interact with in the wild.
While not all relationships survived, the findings suggest that the bonds made in captivity weren’t just a byproduct of confinement and limited options. The researcher report that the findings are consistent with the idea that both partner fidelity and partner switching play a role in regulating the bats’ relationships.
“Our finding adds to a growing body of evidence that vampire bats form social bonds that are similar to the friendships we see in some primates,” Carter said.
“Studying animal relationships can be a source of inspiration and insight for understanding the stability of human friendships.”
The researchers say they’ll continue to work on individual differences in cooperativeness among vampire bats and exploring how individuals go from being strangers to cooperation partners. Taking advantage of their newfound abilities to measure relationships in the wild, they’re also looking into social foraging and whether bats that cooperate within their day roost also go hunting together at night.
Funding: This work was supported by the Deutsche Forschungsgemeinschaft, a Smithsonian Scholarly Studies Awards grant, and a National Geographic Society Research Grant.
Long-term cooperative relationships are most evident in primates [1–6], but evidence for similar social relationships has been accumulating for several other social vertebrate groups [3, 7, 8], including cetaceans [9, 10], bats , elephants , hyenas [13–15] and ravens [16–20]. The functional importance of these complex social relationships across different species may have led to similar cognitive or behavioral mechanisms for manipulating social bonds [19–24].
A prime example of such a mechanism is social grooming—the cleaning of the body by a partner. Experimental and observational studies show that primate social grooming can be ‘exchanged’ for multiple social benefits, including reciprocal grooming, social tolerance, access to food, and agonistic support [1, 25–37].
Although most of what is known about social grooming comes from studies of primates, evidence for a role of social grooming in maintaining social ties is emerging from several other mammals (marsupials , deer , cows , horses , voles , mice , meerkats [48, 49], coati [50, 51], lions ) and group-living birds [53, 54].
In bats, adult social grooming is female-biased in species with female philopatry [55–58], and has been most studied in the common vampire bat (Desmodus rotundus) [55, 59–60]. Kerth et al.  compared social grooming rates of vampire bats with the temperate and insectivorous Bechstein’s bat (Myotis bechsteinii).
These two species both have long lifespans and demonstrate fission-fusion social dynamics, where individuals maintain long-term social associations while moving between several roost trees [61–63]. In both species, social grooming rates among individuals were not predicted by self-grooming or numbers of parasites [55, 57].
Bechstein’s bats spent more time grooming themselves (38% of their time in roosts) compared with vampires (23% of their roosting time), but wild vampire bats spent about 5% of their roosting time grooming others, which is 2–4 times higher than Bechstein’s bats [57, 64].
Patterns of social grooming among categories of individuals also differed between the two species. In the Bechstein’s bat, adult female social grooming was not detectably symmetrical, and was predicted by kinship, occurring mostly between adult mothers and daughters, sometimes between sisters, and only rarely between non-kin .
It is not entirely clear if vampire bat social grooming is typical or exceptional when compared to other bats or non-primate mammals. One hypothesis is that social grooming in vampire bats is exceptional in quantity and quality, because it is related to their uniquely cooperative food sharing behavior .
Like many primates, reciprocal patterns of vampire bat food sharing and social grooming extend beyond mother-offspring bonds, suggesting they may provide both direct and indirect fitness benefits [11, 60, 65].
Among bats, the common vampire has an extraordinarily large brain and neocortex for its body size [66, 67]. In primates, increased neocortex size has been linked to higher metrics of social complexity, such as social grooming network size  and strategic deception .
ike many primates, reciprocal patterns of vampire bat food sharing and social grooming extend beyond mother-offspring bonds, suggesting they may provide both direct and indirect fitness benefits [11, 60, 65]. Among bats, the common vampire has an extraordinarily large brain and neocortex for its body size [66, 67]. In primates, increased neocortex size has been linked to higher metrics of social complexity, such as social grooming network size  and strategic deception .
Alternatively, the apparent distinctiveness of vampire bat social grooming might stem from purely ecological factors. Social grooming may be more obvious in vampire bats due to higher levels of ectoparasite infestation.
Bat fly density has been linked to species-level grooming rates  and the two vampire species that were observed ranked 5th and 6th place out of 53 neotropical bats for average number of parasitic streblid flies per bat . A sampling bias could also over-emphasize social grooming in vampire bats, because there is much effort focused on studying vampire bat social behavior  and a lack of data on social grooming in other bats.
Comparing social grooming data across studies can be difficult due to study differences in ectoparasite density, temperature, sampling method, visibility, and level of human disturbance.
In this study, we took advantage of an opportunity to compare captive vampire bats with four other captive group-living bat species housed at the same facility under the same light/dark schedule, temperature, humidity, and levels of human disturbance. We compared adult-to-adult social grooming in vampire bats Desmodus rotundus, two frugivorous bats Carollia perspicallata and Artibeus jamaicensis (Family: Phyllostomidae) and two Paleotropical fruit bats, Rousettus aegyptiacus and Eidolon helvum (Family: Pteropodidae). Importantly, the adult bats we compared have fixed levels of social association (stable group composition) and no insect ectoparasites.
Carly Britton – Cell Press
The image is credited to Sherri ad Brock Fenton.
Original Research: Open access
“Vampire Bats that Cooperate in the Lab Maintain Their Social Networks in the Wild”. Simon P. Ripperger, Gerald G. Carter, Niklas Duda, Alexander Koelpin, Björn Cassens, Rüdiger Kapitza, Darija Josic, Jineth Berrío-Martínez, and others.
Current Biology doi:10.1016/j.cub.2019.10.024.