Bats are nocturnal hunters and use echolocation to orientate themselves by emitting high-frequency ultrasonic sounds in rapid succession and evaluating the calls’ reflections. Yet, they have retained a functional vision for light in the spectrum visible to humans. Scientists have now used miniature light, ultrasound and motion sensors to show that common noctule bats emit fewer echolocation calls when hunting insects in lit environments, but at the same time catch their prey 50 percent faster than in dark surroundings. This suggests that bats combine acoustic and visual information to catch escaping prey during split-second interactions when, the team elaborates in the scientific journal “PNAS”.

In the brief report article in the “Proceedings of the National Academy of Sciences of the United States of America”, the team led by Laura Stidsholt from Aarhus University and Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) describes the experiment in which they equipped 21 common noctules (Nyctalus noctula) with high-tech miniature sensors. The sensors recorded high-resolution data on the light conditions, and the ultrasonic calls and movements of the bats. This enabled the scientists to analyse how the bats’ behaviour varies under different light conditions and between different behaviours (commuting and foraging). To do this, they analysed the brightness of the environment, intervals and pitch of the ultrasonic calls, the frequency and strength of the wing beats and the flight speed of the bats.

Common noctules reduce echolocation when hunting in brighter surroundings
While the analyses show that the bats did not change their echolocation and flight behaviour under changing ambient brightness when commuting between two locations, the scientists found strong differences in foraging flights between lit and dark environments. In brighter environments, the common noctules reduced the rate of their echolocation calls by half compared to dark environments. At the same time, these calls were up to 7 decibels louder. “We show that the bats reduce the acoustic tracking of their prey when enough light is available as complementary sensory information”, says first author Laura Stidsholt, formerly a postdoc in the Leibniz-IZW Department of Evolutionary Ecology and now an assistant professor at Aarhus University. “At the same time, we were able to reveal that this happens not at the expense of hunting efficiency, because the bats flapped their wings more vigorously in light environments and approached at the insects significantly faster than in dark environments.” In the dark, the average foraging speed was 5.2 metres per second, whereas under lit conditions it was 7.9 metres per second.

According to the scientists, half as many echolocation calls at more than 50 percent higher flight speeds imply that additional information may be used for hunting under lit conditions. “We observe that insectivorous bats are very active in the early hours of the night, i.e. at dusk, and can presumably use the combination of echolocation and vision most efficiently then”, concludes Christian Voigt, head of the Leibniz-IZW Department of Evolutionary Ecology. These findings challenge previous assumptions from experts that bats’ vision, especially at dusk, is too limited for precise prey tracking. “Our study provides the first direct evidence that bats rely on multisensory information when foraging, highlighting the remarkable flexibility of their sensory systems”, Voigt concludes. With this investigation, the team scientifically documents the first example of bats combining sensory information to catch escaping prey during split-second interactions.