Directionality of Drinking Passes by Bats at Water Holes: is there Cooperation?

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Abstract
In 2000 and 2001, we used an infrared imaging system to film the drinking behaviors of bats at high-use water holes outside Boulder, Colorado. We recorded for two hours on each of four nights at two water hole sites, Stockton Cabin (SC) and Bear Creek (BC), known to be high in bat visitation and small enough to allow filming of the entire hole from a single position. A total of 855 drinking passes was observed: 417 and 438 in SC and BC, respectively. Of these, 814, or 95.2%, of all drinking passes occurred from a particular directional pathway (dominant approach pathway) at each site, with a mere 1.3% occurring from the immediately opposite direction, and 3.5% occurring from a direction convergent with, but not opposite to, the dominant approach path. At both sites, the direction of the dominant approach path was against stream-flow. The strict directionality of drinking passes portrayed at the water's surface was in stark contrast to activity above the water hole where no directionality of flight could be discerned, even when dozens of bats were circling together. We hypothesize that bats use unidirectional coordination of drinking passes to lessen the chance of collisions and/or to avoid the energetic expense of collision avoidance.