Forced proximity promotes the formation of enduring cooperative relationships in vampire bats

Spatial assortment can be both a cause and consequence of cooperation. Proximity promotes cooperation when individuals preferentially help nearby partners, and conversely, cooperation drives proximity when individuals move towards more cooperative partners. However, these two causal directions are difficult to distinguish with observational data. Here, we experimentally test if forcing randomly selected pairs of equally familiar female common vampire bats (Desmodus rotundus) into close spatial proximity promotes the formation of enduring cooperative relationships. Over 114 days, we sampled 682 hours of interactions among 21 females captured from three distant sites to track daily allogrooming rates over time. We compared these rates before, during, and after a one-week period, during which we caged random triads of previously unfamiliar and unrelated vampire bats in close proximity. After the week of close proximity when all bats could again freely associate, the allogrooming rates of pairs forced into proximity increased more than those of the 126 control pairs. This work is the first to experimentally demonstrate the causal effect of repeated interactions on cooperative investments in vampire bats. Future work should determine the relative importance of mere association versus interactions (e.g. reciprocal allogrooming) in shaping social preferences.