Sociality across species: spatial proximity of newborn bats promotes heterospecific social bonding

Mixed-species groups occur in a variety of social animals and have been widely investigated in many different orders of mammals. The advantages of mixed-species groups include improved information transfer, dilution of predation risk, and social thermoregulation. We hypothesized that interspecific associations may be facilitated by the development of heterospecific "social" preferences. We manipulated the early social environment of captive Kuhl's pipistrelles (Pipistrellus kuhlii) and Savi's bats (Hypsugo savii), raising newborns in artificial mixed-species groups to simulate conditions in natural mixed-species nurseries. We then measured association patterns among all bats and analyzed the occurrence of social interactions by behavioral observation protocols and social network analyses. Bats preferentially interacted and affiliated with former group members regardless of species, showing that social bonding may occur between different species and that it develops after close contact with newborn heterospecifics, possibly through imprinting-like mechanisms. To our best knowledge, this is the first time such a phenomenon is documented for mammals. Although thermal preferences are often advocated to explain mixed-species associations among bats, individual experience may facilitate heterospecific groups in bats and other taxa.