General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms Running title: Effects of artificial illumination on drinking bats AbstractArtificial illumination at night (ALAN) alters many aspects of animal behaviour. Commuting and foraging bats have been found to be affected by ALAN, but no study has yet addressed the impact of lighting on drinking activity, despite its critical importance for bats. We experimentally illuminated cattle troughs used by drinking bats at four forest sites in Italy, and compared drinking activity and foraging activity under lit and dark conditions. We predicted that 1) the number of bat 2 species and drinking events will be lower under illumination and 2) forest bat species will be more affected than edge specialists. We recorded 2549 drinking events from 12 species or species groups, most of which decreased drinking activity under illumination. The effects of ALAN on drinking were stronger than on foraging. Forest species never drank when the light was on. Edge-foraging species reduced drinking activity while also increasing foraging under lit conditions. We highlight a previously overlooked negative effect of ALAN on bats, whose implications may be locally catastrophic. Given the importance of water sites for both bat foraging and drinking, their illumination should be forbidden, appropriately mitigated or, if necessary, compensated for with the creation of alternative drinking sites.

Bats show pronounced and often-adverse reactions to artificial illumination at night (ALAN) when commuting, roosting or foraging. ALAN also affects bat drinking activity, at least when lighting occurs over short intervals. We tested whether continuous illumination of drinking sites over 4-h periods would lead bats to tolerate ALAN and resume drinking in the course of the night. We conducted our experiments in forest (Italy) and desert (Israel) sites to test whether in the latter habitat, where water is scarce, a greater motivation to drink might lead to less adverse bat reactions. We recorded 6853 drinking buzzes and 1647 feeding buzzes from 17 species and one species group. In the forest sites, species that hunt in open spaces or along forest edges showed little (P. pipistrellus and H. savii) or no (P. kuhlii and N. leisleri) drinking activity decrease, while those associated with forest interiors (Barbastella barbastellus, Plecotus auritus and bats in the genus Myotis) exhibited a strong negative response. In the desert sites, all studied species reduced drinking activity, yet in the desert populations of P. kuhlii we recorded stronger adverse reactions only far from human settlements. The harsh reactions that the desert bat species showed towards ALAN rule out any effect of a greater motivation to drink. Illumination had no effect on foraging by most species, except in the forest sites, where Pipistrellus kuhlii and Nyctalus leisleri increased foraging when the light was on, and in the desert sites, where Hypsugo bodenheimeri decreased foraging in such situations. The progressive human encroachment that is taking place in many world regions on both forests and especially deserts, where few sites for drinking are available, may jeopardize bat populations also through increased exposure to ALAN.