Use of the Post-Sunset Glow as an Orientation Cue by Big Brown Bats (Eptesicus fuscus)

Two colonies of big brown bats, Eptesicus fuscus, were studied to determine the importance of the post-sunset glow as a cue for orientation during flight from the roost. Each colony departed in a different, predictable, colony specific direction that did not change during the study. When displaced to an unfamiliar area to eliminate the use of landmarks and individually released at dusk, the bats again flew in their colony specific directions. If released after dark, they flew in random directions. A sample from each colony was transferred to a planetarium displaying an artificial post-sunset glow. When individually placed under a glass dome, nearly all the bats oriented in the colony specific direction relative to this artificial stimulus. Whereas it is probable that landmarks are also important in flights to preferred foraging areas, the glow in the western sky alone is a sufficient cue. This is the first demonstration of the use of a celestial cue by bats and of spontaneous orientation by them in a planetarium.